Renal & Urinary

Metabolic acidosis: anion gap and non-anion gap workup

Clinical Overview and When to Suspect Metabolic Acidosis

— Unexplained tachypnea, Kussmaul breathing, altered mental status

— Sepsis, shock, hypotension, ischemic bowel, post-arrest

— DKA/HHS, salicylate or toxic alcohol ingestion, alcohol use disorder

— Chronic diarrhea, ileostomy high output, ureteral diversion

— CKD stage 4–5, RTAs, post–saline resuscitation

— Drug exposures: metformin (lactate), topiramate/acetazolamide (non-AG), linezolid, propofol infusion syndrome, antiretrovirals

— pH <7.35 with low HCO₃⁻ = metabolic acidosis

— Normal/high pH with low HCO₃⁻ = respiratory alkalosis compensation

Step 3 management: On the CCS, when you see HCO₃⁻ <20, your next three orders should be VBG, lactate, and a repeat BMP with albumin — not bicarbonate therapy. Empiric bicarb without an etiology is a classic distractor.

Board pearl: Always check the delta-delta ratio (ΔAG/ΔHCO₃⁻) in high-AG acidosis to unmask a coexisting non-AG acidosis or metabolic alkalosis.

Definition: Primary process lowering serum bicarbonate (HCO₃⁻ <22 mEq/L) with compensatory fall in PaCO₂; arterial pH typically <7.35 unless mixed disorder.

Why it matters on Step 3: Metabolic acidosis is rarely the chief complaint — it is discovered on a BMP and forces you to backtrack to the cause. The board test is your ability to split anion gap (AG) vs non-AG, then drive to a specific etiology and management plan.

When to suspect:

First step when HCO₃⁻ is low: Confirm it is acidosis (not compensation for chronic respiratory alkalosis) by obtaining an ABG or VBG.

Calculate the anion gap immediately: AG = Na⁻ – (Cl⁻ + HCO₃⁻); normal 8–12 (varies by lab). Correct for albumin: add 2.5 to AG for every 1 g/dL drop in albumin below 4.

Winter's formula for expected respiratory compensation: PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. Deviation = additional respiratory disorder.

Presentation Patterns and Key History

— DKA: polyuria, polydipsia, abdominal pain, fruity breath, weight loss; type 1 DM, missed insulin, infection, new SGLT2 inhibitor (euglycemic DKA)

— Lactic acidosis: sepsis, hypotension, ischemic limb/bowel, seizures, metformin in AKI, linezolid >2 weeks, propofol >48h high-dose, NRTIs

— Uremic acidosis: CKD stage 4–5, missed dialysis, anorexia, pruritus, asterixis

— Toxic alcohols: methanol (vision changes, "snowstorm"), ethylene glycol (flank pain, oliguria, antifreeze ingestion), propylene glycol (lorazepam drip)

— Salicylates: tinnitus, hyperventilation, fever, mixed AG metabolic acidosis + respiratory alkalosis

— Starvation/alcoholic ketoacidosis: binge drinking then abstinence, vomiting, normal/low glucose

— Diarrhea, ileostomy, pancreatic/biliary drainage, ureterosigmoidostomy

— Large-volume NS resuscitation (dilutional/hyperchloremic)

— RTAs: type 1 (kidney stones, autoimmune — Sjögren), type 2 (Fanconi, multiple myeloma, tenofovir, ifosfamide), type 4 (diabetes, hypoaldosteronism, ACEi/ARB, spironolactone, trimethoprim, heparin)

— Carbonic anhydrase inhibitors: acetazolamide, topiramate

— New SGLT2 inhibitor in a thin/type 2 DM patient → euglycemic DKA

— Lorazepam infusion >48h → propylene glycol toxicity

— Antifreeze access (auto mechanic, suicide attempt) → ethylene glycol

— Aspirin bottle at bedside in elderly → chronic salicylism

Key distinction: Vomiting alone causes metabolic alkalosis (loss of HCl). If a vomiting patient has acidosis, look for coexisting diarrhea, AKA, or salicylate toxicity.

Board pearl: A patient on a lorazepam drip in the ICU with rising AG and osmolar gap = propylene glycol toxicity from the diluent — switch to midazolam or intermittent dosing.

Symptom clusters by etiology — let history narrow the AG vs non-AG split:

Non-AG (hyperchloremic) clues:

Medication and exposure review is mandatory:

Physical Exam Findings and Hemodynamic Assessment

— Kussmaul respirations: deep, sighing, rapid — classic for severe metabolic acidosis (DKA, uremia, toxic alcohols)

— Tachypnea with normal-appearing lungs = respiratory compensation

— Fruity (acetone) breath = ketoacidosis

— Lethargy → stupor → coma as pH falls below 7.1

— Acute methanol: blurred vision, photophobia, "looking into a snowstorm," afferent pupillary defect

— Salicylates: agitation, tinnitus, hyperthermia, seizures (late)

— Hypotension + lactic acidosis = shock until proven otherwise; identify type (septic warm extremities, cardiogenic cool/JVD, hypovolemic flat neck veins, obstructive)

— Cold mottled extremity + acidosis = limb ischemia or compartment syndrome

— Distended tender abdomen + lactic acidosis out of proportion to exam = mesenteric ischemia (CT angiography urgently)

— Dry mucosa, flat JVP, orthostasis: DKA, AKA, diarrhea

— Volume overload + acidosis: advanced CKD, cardiogenic shock, ESRD missed dialysis

— Calcium oxalate crystals in urine → ethylene glycol

— Acneiform rash, pancreatitis-like pain on antiretrovirals → NRTI lactic acidosis

— Necrotizing soft tissue infection → source of lactate

— Methanol: optic disc hyperemia, retinal edema

— Papilledema in salicylate-induced cerebral edema (rare, late)

Step 3 management: In the hypotensive patient with lactic acidosis, your CCS order set is two large-bore IVs, balanced crystalloid bolus (LR or Plasma-Lyte preferred over NS to avoid worsening hyperchloremic acidosis), broad-spectrum antibiotics if sepsis suspected within 1 hour, blood cultures × 2, lactate q2h, and central access/vasopressors if MAP <65 after 30 mL/kg.

Board pearl: Balanced crystalloids (LR, Plasma-Lyte) outperform 0.9% saline in large-volume resuscitation by preventing hyperchloremic non-AG acidosis (SMART trial).

General/respiratory:

Mental status:

Hemodynamic assessment (critical for triage):

Volume status:

Skin and extremities:

Eyes/funduscopy:

Diagnostic Workup — Initial Labs

— BMP/CMP: Na, K, Cl, HCO₃⁻, BUN, creatinine, glucose, albumin

— VBG or ABG: pH, PaCO₂, calculated HCO₃⁻

— Serum lactate

— Serum and urine ketones (β-hydroxybutyrate preferred — nitroprusside only detects acetoacetate)

— Serum osmolality (measured) and calculate osmolar gap

— Urinalysis with microscopy

— CBC, magnesium, phosphorus

— Step 1: Confirm acidemia (pH <7.35) and low HCO₃⁻

— Step 2: Calculate anion gap (corrected for albumin)

— Step 3: Check Winter's formula for appropriate respiratory compensation

— Step 4: If AG elevated → calculate delta-delta = (AG − 12) / (24 − HCO₃⁻)

▪ <1: concurrent non-AG acidosis

▪ 1–2: pure AG acidosis

▪ >2: concurrent metabolic alkalosis

— Step 5: Calculate osmolar gap = measured − (2Na + glucose/18 + BUN/2.8 + EtOH/3.7); gap >10 suggests toxic alcohol, mannitol, propylene glycol

— Glycols (ethylene, propylene)

— Oxoproline (chronic acetaminophen, malnourished women)

— L-lactate

— D-lactate (short bowel syndrome)

— Methanol

— Aspirin

— Renal failure (uremia)

— Ketoacidosis (DKA, AKA, starvation)

— Hyperalimentation, Acetazolamide/Addison's, RTA, Diarrhea, Ureteroenterostomy, Pancreatic/biliary fistula, Saline infusion

Board pearl: Always order β-hydroxybutyrate, not just urine ketones, in suspected DKA/AKA — the nitroprusside reaction misses β-OHB, the predominant ketone in severe ketoacidosis, and can falsely "improve" as patients are treated and β-OHB converts to acetoacetate.

Key distinction: Osmolar gap plus AG acidosis = methanol or ethylene glycol; osmolar gap alone (no acidosis) = isopropyl alcohol or early ingestion.

Core panel (order all simultaneously):

Step-by-step interpretation:

Anion gap differential — GOLD MARK:

Non-AG differential — HARDUP/USED CARP:

Diagnostic Workup — Advanced and Confirmatory Studies

— Urine anion gap (UAG) = (UNa + UK) − UCl — to differentiate non-AG acidosis

▪ Negative UAG (<0): appropriate renal NH₄⁺ excretion → GI loss (diarrhea, fistula), or proximal RTA after bicarb load

▪ Positive UAG (>0): impaired renal acidification → distal (type 1) or type 4 RTA

— Urine pH:

▪ >5.5 despite acidemia → type 1 (distal) RTA

▪ <5.5 → type 2 (proximal) or type 4 RTA

— Serum potassium pattern:

▪ Hypokalemia: type 1 and type 2 RTA, diarrhea

▪ Hyperkalemia: type 4 RTA, early CKD acidosis

— Urine glucose, phosphate, amino acids, uric acid: Fanconi syndrome (proximal RTA)

— Methanol, ethylene glycol, salicylate levels (quantitative)

— Calcium oxalate crystals (envelope/needle-shaped) on UA → ethylene glycol

— Wood's lamp fluorescence of urine — unreliable, do not rely on for boards

— Co-oximetry for carboxyhemoglobin if CO suspected

— CT angiography for mesenteric ischemia when lactate is disproportionate to exam

— CT head if altered mental status without clear etiology

— Renal ultrasound if obstructive uropathy contributes to AKI/acidosis

— D-lactate level in short bowel syndrome (standard lactate assay measures only L-lactate)

— Acylcarnitine/organic acids if inborn error suspected in young adult

— Cortisol/ACTH stim if Addisonian picture (hyperkalemia, hyponatremia, hypotension)

— Cosyntropin test if type 4 RTA in non-diabetic without ACEi exposure

Step 3 management: In suspected toxic alcohol ingestion, do not wait for confirmatory levels. Start fomepizole 15 mg/kg IV load + thiamine/folate/pyridoxine and consult nephrology for hemodialysis if pH <7.25, end-organ damage, or level >50 mg/dL.

CCS pearl: Ordering "urine anion gap" and "urine pH" together in a non-AG acidosis case earns points the same way ordering "TSH and free T4" does in thyroid cases.

When initial workup is non-diagnostic, escalate:

Toxicologic confirmation:

Imaging:

Specialized testing:

Risk Stratification and First-Line Management Logic

— Mild: pH 7.30–7.35, HCO₃⁻ 18–22, hemodynamically stable, alert → outpatient or floor workup

— Moderate: pH 7.20–7.30, HCO₃⁻ 10–18 → telemetry/step-down, IV access, hourly reassessment

— Severe: pH <7.20, HCO₃⁻ <10, hemodynamic instability, AMS → ICU

— Two large-bore IVs, continuous telemetry, pulse oximetry

— Balanced crystalloid (LR/Plasma-Lyte) if volume-deplete

— Fingerstick glucose; treat hypoglycemia or hyperglycemia

— Identify and treat the underlying cause — acidosis itself is rarely the target

— DKA: IV fluids → insulin drip 0.1 U/kg/h (hold if K <3.3) → replace K once 3.3–5.2 → dextrose when glucose ~200

— Lactic acidosis from sepsis: source control + antibiotics within 1 hour + fluids + vasopressors

— Toxic alcohol: fomepizole + hemodialysis

— Salicylate: urinary alkalinization with sodium bicarb infusion (target urine pH 7.5–8); hemodialysis if level >100 mg/dL (acute), AMS, pulmonary/cerebral edema, or renal failure

— Uremic acidosis: hemodialysis

— Diarrheal non-AG: isotonic bicarbonate-containing fluid (LR), treat diarrhea

— RTA: oral bicarb or citrate replacement

— Severe acidemia pH <7.1 with hemodynamic instability

— Acute hyperkalemia

— Toxic alcohol or salicylate toxicity (alkalinization)

— Severe non-AG acidosis from diarrhea/RTA

Key distinction: In DKA and lactic acidosis, routine bicarbonate does NOT improve mortality and may worsen intracellular acidosis, hypokalemia, and cerebral edema (especially pediatric DKA). Reserve for pH <6.9 in DKA per ADA.

Board pearl: D5W + 3 amps NaHCO₃ at 150–250 mL/h is the salicylate alkalinization recipe — keep urine pH 7.5–8 and replace potassium aggressively (alkalinization fails if hypokalemic).

Severity tiers drive disposition:

Universal first-hour bundle (CCS-aligned):

Cause-directed first-line therapy:

Bicarbonate therapy — when actually indicated:

Pharmacotherapy — First-Line Drug Regimens by Etiology

— Fluids: NS 15–20 mL/kg/h × 1h, then 0.45% NS based on corrected Na; switch to D5½NS when glucose ~200

— Insulin: regular insulin 0.1 U/kg/h IV drip (no bolus needed); hold if K <3.3

— Potassium: if 3.3–5.2 → add 20–30 mEq/L to fluids; if >5.2 → no K; if <3.3 → hold insulin and replete first

— Bicarbonate: only if pH <6.9

— Transition criteria: AG closed (<12), HCO₃⁻ ≥15, pH >7.3, patient eating → overlap subcut basal insulin 1–2 hours before stopping drip

— Fomepizole: 15 mg/kg IV load, then 10 mg/kg q12h × 4 doses, then 15 mg/kg q12h; redose more frequently during HD

— Ethanol drip if fomepizole unavailable (target level 100–150 mg/dL)

— Cofactors: methanol → folate/folinic acid 50 mg IV q6h; ethylene glycol → thiamine 100 mg + pyridoxine 50 mg IV q6h

— Sodium bicarbonate drip (3 amps in D5W, 150–250 mL/h); urine pH goal 7.5–8

— Avoid intubation if possible — minute ventilation drop will crash pH; if intubated, hyperventilate to match pre-intubation PaCO₂

— Type 1: potassium citrate 1–2 mEq/kg/day (treats acidosis + nephrolithiasis)

— Type 2: higher doses (10–15 mEq/kg/day) of bicarbonate + thiazide to enhance proximal reabsorption

— Type 4: fludrocortisone if hypoaldosteronism, loop diuretic + low-K diet, stop offending agents

— Oral sodium bicarbonate 650 mg TID when HCO₃⁻ <22 (KDIGO) — slows CKD progression

— Alternative: veverimer (investigational HCl binder)

Step 3 management: When transitioning a DKA patient off the drip, never stop the IV insulin before subcut basal is on board — overlap by 1–2 hours or the AG will reopen and the CCS clock punishes you.

Board pearl: Fomepizole is preferred over ethanol — easier dosing, no CNS depression, no glucose monitoring burden, fewer drug interactions.

DKA (ADA-aligned):

Toxic alcohol antidotes:

Salicylate toxicity:

RTAs (chronic):

CKD metabolic acidosis:

Procedures — Hemodialysis Indications and Renal Replacement

— Acidosis: pH <7.1 refractory to medical therapy

— Electrolytes: severe hyperkalemia (>6.5 with ECG changes or refractory)

— Ingestions: methanol, ethylene glycol, salicylates, lithium, metformin-associated lactic acidosis (severe), valproate

— Overload: pulmonary edema unresponsive to diuretics

— Uremia: pericarditis, encephalopathy, bleeding diathesis

— Methanol/ethylene glycol: level >50 mg/dL, severe acidosis, end-organ damage (visual, renal), or unable to maintain therapeutic fomepizole

— Salicylate: acute level >100 mg/dL, chronic >60 mg/dL, AMS, pulmonary/cerebral edema, renal failure, pH <7.2 refractory to alkalinization

— Metformin-associated lactic acidosis (MALA): lactate >20, pH <7.1, hemodynamic instability, AKI

— Lithium: level >4 mEq/L acute, >2.5 chronic with neurologic symptoms

— Intermittent HD preferred for toxin clearance (higher clearance per hour) when hemodynamically tolerant

— CRRT for hemodynamically unstable ICU patients; slower but continuous; rebound common after dialyzable toxins — extend duration

— Temporary internal jugular > femoral > subclavian for tunneled HD line decisions

— Avoid subclavian — stenosis risk if AV fistula needed later

— Source control surgery for ischemic bowel, necrotizing fasciitis, abscess driving lactic acidosis

— Endoscopy/IR for GI bleeding if shock-related lactate

— Revascularization for acute limb ischemia

CCS pearl: Consulting nephrology emergently and ordering "arrange hemodialysis" in toxic alcohol or severe salicylate cases is a scored action — do it within the first simulated hour.

Key distinction: In MALA, lactate level alone does not justify HD — combine with pH, hemodynamics, AKI, and lack of response to resuscitation. Metformin clears slowly; expect prolonged dialysis.

Emergent hemodialysis indications in metabolic acidosis — AEIOU mnemonic:

Specific dialysis thresholds:

CRRT vs intermittent HD:

Vascular access:

Other procedural considerations:

Special Populations — Elderly and Renal/Hepatic Impairment

— Lower baseline GFR — drug-induced acidosis more common (metformin, NSAIDs causing type 4 RTA, ACEi/ARB + spironolactone)

— Chronic salicylate toxicity is a classic elderly trap: confusion, fever, tachypnea mistaken for sepsis or delirium; level may be only modestly elevated but clinically severe

— Diminished thirst → volume depletion → prerenal AKI → uremic acidosis cascade

— Polypharmacy review is mandatory: TMP-SMX, heparin, ACEi/ARB, spironolactone, NSAIDs all drive type 4 RTA

— KDIGO recommends oral bicarb when serum HCO₃⁻ <22 to slow CKD progression and preserve muscle/bone

— Avoid metformin if eGFR <30; reassess at 30–45

— SGLT2 inhibitors: hold during acute illness, surgery, prolonged fasting — risk of euglycemic DKA

— Spironolactone, eplerenone: monitor K and HCO₃⁻

— Impaired lactate clearance → predisposes to lactic acidosis with even mild hypoperfusion

— Acetaminophen in malnourished/alcohol-use patients → 5-oxoproline (pyroglutamic acid) acidosis — high AG, no obvious cause, often missed; treat by stopping APAP, NAC, and supportive care

— Avoid lactated Ringer's in severe hepatic failure? — actually still safe; lactate is rapidly metabolized or excreted; NS not superior

— Fomepizole: same load; redose more frequently during HD (q4h while on dialysis)

— Bicarbonate replacement: titrate to HCO₃⁻ 22–24, avoid overshoot (volume overload, hypokalemia)

— Insulin in DKA: still 0.1 U/kg/h, but monitor glucose more frequently — slower clearance

Step 3 management: Any elderly patient on chronic NSAIDs + ACEi + diuretic ("triple whammy") with new hyperkalemia and mild non-AG acidosis = type 4 RTA / hyporeninemic hypoaldosteronism; stop the offenders before adding fludrocortisone.

Board pearl: Suspect pyroglutamic acidosis in a malnourished/alcoholic woman on chronic therapeutic acetaminophen with unexplained high AG acidosis and normal lactate/ketones/osmolar gap.

Elderly considerations:

CKD-specific:

Hepatic impairment:

Dosing adjustments in renal impairment:

Special Populations — Pregnancy and Pediatrics

— Baseline: chronic respiratory alkalosis from progesterone-driven hyperventilation; serum HCO₃⁻ runs 18–22, PaCO₂ 28–32. A "normal" HCO₃⁻ of 24 in pregnancy may represent relative acidosis.

— DKA in pregnancy: occurs at lower glucose thresholds (euglycemic DKA more common, especially T1DM in 2nd/3rd trimester); fetal mortality 10–25%; treat aggressively with insulin and fluids, continuous fetal monitoring if >24 weeks

— Hyperemesis gravidarum: typically alkalosis from vomiting, but can develop ketoacidosis from starvation — check β-OHB

— Acute fatty liver of pregnancy/HELLP: lactic acidosis + AKI + coagulopathy → expedite delivery

— Avoid fomepizole? — actually category C, but use it in life-threatening toxic alcohol ingestion; teratogenicity risk < maternal/fetal mortality

— DKA cerebral edema: leading cause of DKA mortality in children; risk factors include rapid fluid resuscitation, bicarbonate use, and lower initial pCO₂ — avoid bicarb, use isotonic fluids judiciously, monitor neuro status

— Inborn errors of metabolism: neonate/infant with high AG acidosis + hypoglycemia + hyperammonemia → organic acidemias (methylmalonic, propionic), MCAD, mitochondrial disorders — send acylcarnitine, urine organic acids, ammonia

— Pyloric stenosis: alkalosis, not acidosis (key distinction)

— Diarrheal dehydration: non-AG hyperchloremic acidosis; oral rehydration solution first-line

— Sheehan syndrome can present months later with adrenal insufficiency → mild non-AG acidosis with hyponatremia/hyperkalemia

— Mastitis sepsis → lactic acidosis

Key distinction: In pediatric DKA, fluids should be moderate (10–20 mL/kg bolus, then careful maintenance), and bicarbonate is contraindicated unless pH <6.9 with hemodynamic compromise — cerebral edema risk dominates management.

Step 3 management: A pregnant T1DM patient at 30 weeks with glucose 180 and HCO₃⁻ 10 → euglycemic DKA. Admit, insulin drip, fluids, fetal monitoring; do not be reassured by the "normal" glucose.

Pregnancy:

Pediatrics:

Postpartum/peripartum:

Complications and Adverse Outcomes

— Cardiovascular: decreased contractility, arrhythmias, blunted catecholamine response, vasodilation → refractory hypotension below pH 7.2

— Respiratory: muscle fatigue from sustained hyperventilation → impending respiratory failure

— CNS: confusion, lethargy, coma, seizures (esp. salicylate)

— Hematologic: rightward shift of O₂–Hb curve (acutely beneficial); coagulopathy in severe cases

— Metabolic: hyperkalemia from cellular shift (more pronounced in non-AG/inorganic acidosis), insulin resistance, protein catabolism, bone demineralization (chronic)

— DKA: cerebral edema (pediatric), hypokalemia during insulin therapy, hypoglycemia, ARDS, venous thromboembolism, aspiration

— Toxic alcohols: methanol → permanent blindness, parkinsonism; ethylene glycol → AKI, calcium oxalate nephropathy, hypocalcemic tetany, cranial neuropathies (late)

— Salicylates: noncardiogenic pulmonary edema, cerebral edema, hyperthermia, GI bleeding, ARDS

— Lactic acidosis (sepsis): multi-organ failure, ICU mortality 30–50%

— Chronic CKD acidosis: accelerated CKD progression, muscle wasting (sarcopenia), bone disease, insulin resistance, decreased albumin

— Overzealous saline → hyperchloremic non-AG acidosis and AKI

— Overzealous bicarbonate → metabolic alkalosis, hypokalemia, hypocalcemia (decreased ionized Ca), volume overload, paradoxical CSF acidosis, left-shift O₂ curve

— Insulin without K monitoring → fatal hypokalemia

— Intubating a salicylate patient without matching their high minute ventilation → rapid pH crash and cardiac arrest

Board pearl: Cerebral edema in pediatric DKA classically presents 4–12 hours into treatment with headache, bradycardia, hypertension, AMS — treat with mannitol or hypertonic saline and slow fluid rate; do not wait for imaging.

CCS pearl: In severe salicylate toxicity, if intubation is unavoidable, bag-mask at ~30 breaths/min pre- and post-intubation and set the vent for matched minute ventilation, plus continue bicarbonate drip — failure to do this kills patients on simulated and real boards.

Acidemia itself causes:

Etiology-specific complications:

Iatrogenic complications:

When to Escalate Care — ICU, Consult, Inpatient Triage

— pH <7.20 or HCO₃⁻ <10

— Hemodynamic instability requiring vasopressors

— Altered mental status or airway concern

— Lactate >4 mmol/L with shock physiology

— Salicylate level >50 with symptoms, methanol/ethylene glycol with end-organ effects

— DKA with pH <7.0, K <3.3, AMS, hemodynamic instability, or pregnant

— Need for hemodialysis emergently

— Pediatric DKA with risk for cerebral edema

— Moderate DKA, stable salicylism on alkalinization, chronic acidosis acute worsening

— Need for q1–2h labs but stable vitals

— Nephrology: any patient needing potential HD, RTA workup, severe CKD acidosis

— Toxicology/Poison Control (1-800-222-1222): every toxic alcohol, salicylate, lithium, metformin overdose — document the call

— Endocrinology: recurrent DKA, suspected MODY, pump troubleshooting, transition planning

— GI/Surgery: suspected mesenteric ischemia (lactate disproportionate to exam), ischemic bowel

— OB: pregnant patients with significant acidosis at any gestational age

— Critical care: for vent management of acidotic patient — especially salicylate

— No on-site dialysis for toxic ingestion

— No ECMO when severe shock-related lactic acidosis with reversible cause

— Pediatric DKA with neurologic concerns and no PICU

— Need for liver transplantation evaluation (acetaminophen-induced 5-oxoproline + hepatic failure, ethylene glycol with fulminant AKI/multi-organ)

Step 3 management: A transfer scenario on Step 3 often hinges on calling poison control and accepting facility nephrology before transferring — start fomepizole at the sending hospital while transport is arranged; do not delay antidote for transfer.

Key distinction: A DKA patient with normal glucose on SGLT2 inhibitor belongs in the ICU, not the medicine floor — euglycemic DKA can be more severe than classic DKA because diagnosis is delayed.

ICU admission criteria:

Step-down/telemetry:

Consults to place early:

Transfer criteria (community hospital → tertiary):

Key Differentials — Same-Category (Other Metabolic Acidosis) Causes

— DKA vs AKA vs starvation ketosis: glucose >250 vs normal/low + alcohol use vs low + prolonged fasting

— L-lactate vs D-lactate: standard lactate misses D-lactate; suspect in short bowel syndrome with episodic encephalopathy after high-carb meals

— Type A lactic acidosis (tissue hypoperfusion: shock, ischemia) vs type B (no hypoperfusion: drugs — metformin, linezolid, propofol; toxins — cyanide, CO; mitochondrial; malignancy — Warburg in leukemia/lymphoma)

— 5-oxoprolinic acidosis (chronic APAP, malnutrition, sepsis) — diagnosed when other AG causes excluded; urine organic acids confirm

— Methanol vs ethylene glycol: vision changes + retinal findings vs flank pain + oxalate crystals + hypocalcemia

— Isopropyl alcohol: osmolar gap WITHOUT acidosis (metabolized to acetone, not an acid); ketosis without acidosis

— Negative UAG, hypokalemia: diarrhea, ileostomy, ureteral diversion, post-bicarb-loaded proximal RTA

— Positive UAG, hypokalemia, urine pH >5.5: type 1 (distal) RTA — Sjögren, lupus, lithium, amphotericin, nephrocalcinosis, sickle cell

— Positive UAG, hypokalemia, urine pH <5.5, glucosuria/phosphaturia: type 2 (proximal) RTA — multiple myeloma, Fanconi, tenofovir, ifosfamide, acetazolamide, topiramate

— Positive UAG, hyperkalemia: type 4 RTA — diabetes, ACEi/ARB, spironolactone, TMP-SMX, heparin, Addison's, hyporeninemic hypoaldosteronism

— AG acidosis + non-AG acidosis (sepsis + saline resuscitation)

— AG acidosis + metabolic alkalosis (DKA + vomiting) — AG elevated but HCO₃⁻ normal

— AG acidosis + respiratory alkalosis (salicylate, sepsis, hepatic failure)

— AG acidosis + respiratory acidosis (cardiopulmonary arrest)

Board pearl: A patient with diarrhea, hypokalemia, normal AG acidosis, urine pH 6.0, and positive UAG has distal RTA, not diarrhea — diarrhea's urine pH would acidify appropriately.

Anion gap acidosis mimics — sort by clue:

Non-AG acidosis differentials by urine anion gap and K:

Mixed disorders (delta-delta and Winter's mismatch):

Key Differentials — Other-Category Causes (Mimics of Acidosis Picture)

— Primary respiratory alkalosis (sepsis early, pulmonary embolism, anxiety, pregnancy, salicylate early): tachypnea + low HCO₃⁻, but pH alkaline; HCO₃⁻ drop is compensatory

— Pseudo-acidosis: lab artifact from tourniquet-induced hemolysis or delay → falsely high K and altered values; recheck

— Hyperventilation syndrome: Kussmaul-like breathing in anxious patient with normal BMP

— Asthma/COPD exacerbation with respiratory acidosis — opposite direction

— CHF with hyperventilation → respiratory alkalosis early; lactic acidosis if cardiogenic shock develops

— Pulmonary embolism: respiratory alkalosis early; lactic acidosis with massive PE

— Hypoglycemia (check glucose first)

— Hypothyroid coma (mild non-AG; hypoventilation → respiratory acidosis)

— Adrenal crisis (mild non-AG, hyperkalemia, hyponatremia, hypotension)

— Status epilepticus → transient lactic acidosis that resolves within 1–2 hours; do not anchor on lactate

— Post-arrest: combined lactic + respiratory acidosis

— DKA + alcohol use looks like methanol/ethylene glycol — calculate osmolar gap (ethanol contributes)

— Mannitol, glycerol, sorbitol, IVIG sucrose → elevated osmolar gap without acidosis

— Pseudohyponatremia from hyperlipidemia/paraproteinemia → falsely elevated AG calculation

— Hypoalbuminemia → AG appears falsely "normal"; correct: add 2.5 per 1 g/dL albumin <4

— Multiple myeloma cationic paraprotein → falsely low AG; suspect with HCO₃⁻ drop and AG <6

Key distinction: Post-ictal lactic acidosis from generalized seizure can hit 10–15 mmol/L but resolves within 1–2 hours; do not pursue sepsis workup or start antibiotics if the picture is consistent and lactate clears — but DO recheck.

Board pearl: Low or negative anion gap = bromism, lithium toxicity, multiple myeloma (cationic IgG), severe hyperalbuminemia, lab error — always reconsider when AG <6.

Conditions that mimic metabolic acidosis presentation but are not primary acidoses:

Conditions that mimic the symptom of "shortness of breath + low bicarb":

Conditions that mimic AMS + acidosis:

Conditions that mimic toxic ingestion:

Pseudo-normal AG (when AG looks normal but acidosis is mixed):

Secondary Prevention and Discharge Planning

— Subcutaneous basal–bolus insulin started 1–2h before stopping drip

— Diabetes education: sick-day rules (never hold insulin, check ketones if glucose >250 or ill), glucagon prescription, identify trigger

— Hold SGLT2 inhibitor during illness, fasting, or perioperative period

— Endocrine follow-up within 1–2 weeks; PCP within 1 week

— Continuous glucose monitor referral, especially T1DM

— Address insurance/access to insulin — rationing insulin is a leading DKA cause in the US

— Psychiatric evaluation if intentional ingestion; safety planning before discharge

— Means restriction counseling

— Outpatient ophthalmology follow-up for methanol survivors

— Nephrology if residual AKI

— Substance use disorder referral if alcohol or chronic salicylism

— Type 1: potassium citrate (treats acidosis + prevents stones); urology if recurrent nephrolithiasis; check workup for Sjögren (anti-SSA/SSB), SLE

— Type 2: treat underlying cause (myeloma workup with SPEP/UPEP/free light chains in adults); stop offending drugs (tenofovir, ifosfamide); bicarb + thiazide

— Type 4: stop or minimize ACEi/ARB/spironolactone/TMP-SMX where possible; fludrocortisone if hypoaldosteronism; low-K diet; loop diuretic

— Oral sodium bicarbonate 650 mg TID titrate to HCO₃⁻ 22–24

— Dietary referral: lower acid load (more fruits/vegetables, less animal protein) — comparable efficacy to bicarb in early CKD

— Nephrology q3–6 months

— Medication reconciliation at every visit; eliminate culprit drugs

— Vaccinations (pneumococcal, influenza, COVID) — sepsis prevention reduces lactic acidosis risk

Step 3 management: Before discharging a DKA patient, document on the CCS: insulin teaching completed, ketone meter prescribed, sick-day rules reviewed, endocrine follow-up scheduled within 2 weeks, A1c checked.

Board pearl: A young woman with recurrent renal stones, hypokalemia, urine pH 6.5, and dry eyes/mouth has type 1 distal RTA from Sjögren — order anti-SSA/SSB and minor salivary gland biopsy referral.

DKA discharge bundle:

Toxic alcohol/salicylate post-discharge:

RTA chronic management:

CKD acidosis:

General preventive measures:

Follow-Up, Monitoring, and Counseling

— VBG/ABG q2–4h until pH >7.30 and HCO₃⁻ trending up

— BMP q2–4h in DKA until AG closed; then q6h transitioning

— Lactate q2–4h in sepsis-driven acidosis (until <2 and clinically improved)

— Salicylate levels q2h until trending down (chronic toxicity peaks late)

— Glucose q1h on insulin drip

— Continuous telemetry for K shifts

— Strict I/Os, daily weights

— DKA: HbA1c at admission and q3 months; lipid panel; microalbumin annually; retinal exam annually

— RTA: BMP q3 months once stable; 24-h urine for citrate/calcium (stones); DEXA if chronic (bone disease)

— CKD acidosis: BMP q1–3 months until HCO₃⁻ stable; eGFR trend

— Survivors of toxic alcohol: BMP, GFR, vision testing

— DKA: "Never stop insulin even when not eating; check ketones if glucose >250 or ill; call if vomiting >4h"

— Sick-day rules written and verbally reviewed; teach-back method

— SGLT2 + DKA risk counseling: hold for surgery (3 days pre-op for ertugliflozin/empagliflozin, longer for canagliflozin), prolonged fasting, or significant illness

— RTA: importance of medication adherence; signs of recurrent stones; hydration goal 2.5–3 L/day for type 1

— Chronic alcohol use disorder: treat as a chronic disease; offer naltrexone/acamprosate, AA referral, hepatology if liver disease

— Post-ICU syndrome screening at 1 and 3 months

— Cognitive evaluation after methanol or severe salicylate poisoning

— Dietitian referral in CKD acidosis (plant-based, low acid load)

CCS pearl: Order "sick-day rules education," "diabetes self-management education," and "endocrinology follow-up in 2 weeks" explicitly when discharging DKA — these are scored interventions, not just nice-to-haves.

Board pearl: HbA1c at DKA admission distinguishes new-onset diabetes (often very high, >12) from acute insulin nonadherence — guides outpatient regimen.

Inpatient monitoring during active acidosis:

Post-discharge labs and intervals:

Patient counseling pillars:

Rehab and recovery considerations:

Ethical, Legal, and Patient Safety Considerations

— Hemodialysis for toxic ingestion in an unconscious patient: proceed under emergency exception/implied consent; document attempts to reach family but do not delay life-saving therapy

— DKA with AMS: insulin and fluids fall under emergency consent; once mentation clears, re-consent for ongoing care plan

— Religious refusal of dialysis (e.g., Jehovah's Witness concerns are typically about blood, but dialysis circuit may be debated): use shared decision-making, ethics consult, and document capacity assessment

— A pH <7.2 with confusion = lacks capacity for major medical decisions until corrected

— Use surrogate decision-maker (next-of-kin hierarchy per state law) for non-emergent decisions

— Reassess capacity after correction; document

— Intentional poisoning (methanol, ethylene glycol, salicylate overdose): psychiatric hold (e.g., 5150 in CA) if suicidal; document means restriction

— Suspected child abuse (pediatric ethylene glycol or methanol exposure with implausible history): mandated reporting to CPS

— Munchausen by proxy suspected in recurrent unexplained pediatric AG acidosis

— Poison Control call documented — protects clinician and patient

— Insulin error at handoff: most common cause of recurrent DKA admissions; verify the right insulin, right dose, right time on transfer

— Drip-to-subcut conversion errors: written protocol, double-check K, ensure overlap

— Discharge without insulin access: confirm pharmacy fill before discharge in uninsured patients; manufacturer patient-assistance programs; insulin rationing is a documented driver of recurrent DKA

— High-alert medication double-check for insulin drips, bicarbonate, KCl >10 mEq/h

— Closed-loop communication for critical lab values (pH <7.2, K <3.0 or >6.0, lactate >4)

— Avoid abbreviations ("U" for units → use "units")

Step 3 management: A patient stabilized after intentional ethylene glycol ingestion who now refuses ongoing care: assess capacity — active suicidal ideation impairs capacity for medical refusal. Place a psychiatric hold, continue treatment, obtain ethics/psychiatry consults, and document the rationale.

Board pearl: Documenting Poison Control consultation is both a clinical and medicolegal best practice in every significant toxic ingestion.

Informed consent edge cases:

Capacity assessment in acidosis-related AMS:

Mandatory and recommended reporting:

Transition-of-care risks:

Patient safety bundles:

High-Yield Associations and Rapid-Fire Clinical Facts

— GOLD MARK for AG acidosis (Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis)

— HARDUP for non-AG (Hyperalimentation, Acetazolamide, RTA, Diarrhea, Ureteroenterostomy, Pancreatic/biliary fistula)

— AEIOU dialysis indications

— AG normal: 8–12 (correct +2.5 per 1 g/dL albumin drop)

— Winter's: PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2

— Delta-delta: 1–2 pure AG; <1 mixed AG + non-AG; >2 mixed AG + alkalosis

— Osmolar gap >10 = abnormal

— DKA criteria: glucose >250, pH <7.3, HCO₃⁻ <18, AG >10, ketones present

— Salicylate dialysis: >100 acute, >60 chronic

— Bicarb in DKA: only if pH <6.9

— Metformin → lactic acidosis (esp. AKI)

— Linezolid >2 weeks → lactic acidosis, mitochondrial toxicity

— Propofol >48h, >4 mg/kg/h → propofol infusion syndrome (lactic acidosis, rhabdo, bradycardia)

— Lorazepam drip → propylene glycol acidosis + osmolar gap

— NRTIs (zidovudine, didanosine, stavudine) → lactic acidosis + hepatic steatosis

— Topiramate, acetazolamide → type 2 RTA (non-AG)

— Tenofovir, ifosfamide, aminoglycosides → Fanconi/type 2 RTA

— TMP-SMX, heparin, spironolactone, ACEi/ARB → type 4 RTA

— Amphotericin, lithium → type 1 RTA

— Sjögren + nephrolithiasis + hypokalemia = type 1 RTA

— Multiple myeloma + hypokalemia + glucosuria with normal glucose = type 2 RTA (Fanconi)

— Diabetes + ACEi + hyperkalemia + mild non-AG acidosis = type 4 RTA

— Short bowel + episodic confusion after carb meal = D-lactic acidosis

— Malnourished alcoholic on chronic APAP with unexplained AG = pyroglutamic acidosis

— Antifreeze + flank pain + envelope crystals = ethylene glycol

— Snowstorm vision + acidosis = methanol

— Tinnitus + hyperventilation + fever = salicylate

Board pearl: An isolated osmolar gap WITHOUT acidosis in an obtunded patient = isopropyl alcohol (rubbing alcohol) — supportive care only; no fomepizole, no dialysis unless severe hypotension.

Mnemonics worth memorizing:

Number facts:

Drug–acidosis associations:

Classic associations:

Board Question Stem Patterns

Step 3 management: Recognize that the best next step is usually a diagnostic calculation (AG, osmolar gap, UAG) before therapy — Step 3 rewards rigorous workup over reflexive treatment.

Board pearl: When the stem mentions specific drugs (metformin, topiramate, lithium, TMP-SMX, NRTIs, propofol, lorazepam drip), the drug is the answer — match drug to acidosis pattern.

Stem 1 — Classic DKA: 19-year-old T1DM, missed insulin, abdominal pain, glucose 480, HCO₃⁻ 8, AG 28, K 5.5. Best next step? → IV fluids first, then insulin drip after K confirmed. Trap: starting insulin first when K is 3.2 → hold insulin, replete K.

Stem 2 — Toxic alcohol: Suicidal 40-year-old, garage worker, AMS, AG 26, osmolar gap 35, flank pain, calcium oxalate crystals. → Ethylene glycol; start fomepizole + thiamine + pyridoxine, call nephrology for HD.

Stem 3 — Salicylate: Elderly woman with arthritis on chronic ASA, confusion, fever, tachypnea, ABG pH 7.45 / PaCO₂ 18 / HCO₃⁻ 14, AG 22. → Mixed AG acidosis + respiratory alkalosis = salicylate. Avoid intubation; sodium bicarb drip, urine pH 7.5–8; HD if level >50 with symptoms.

Stem 4 — Non-AG with hypokalemia: Young woman, dry eyes/mouth, recurrent kidney stones, HCO₃⁻ 18, K 3.0, urine pH 6.5, positive UAG. → Type 1 RTA from Sjögren; potassium citrate.

Stem 5 — Non-AG with hyperkalemia: Diabetic on lisinopril and spironolactone, K 5.8, HCO₃⁻ 19, positive UAG, urine pH 5.0. → Type 4 RTA; stop ACEi/spironolactone if possible, low-K diet, fludrocortisone if persistent.

Stem 6 — Euglycemic DKA: T2DM on empagliflozin, gastroenteritis, glucose 180, HCO₃⁻ 9, AG 24, ketones +. → SGLT2-induced euglycemic DKA; hold SGLT2, treat as DKA.

Stem 7 — Lactic acidosis with shock: Elderly with abdominal pain "out of proportion to exam," lactate 8, AG 22, A-fib. → Mesenteric ischemia; CT angiography, surgery consult.

Stem 8 — D-lactic acidosis: Post-gastric bypass with intermittent slurred speech and confusion after meals, normal standard lactate, AG 18. → D-lactic acidosis; oral antibiotics (metronidazole, neomycin), low-carb diet.

Stem 9 — Pyroglutamic acidosis: Malnourished alcoholic woman on chronic therapeutic APAP, AG 22, normal lactate/ketones/osmolar gap. → 5-oxoprolinuria; stop APAP, NAC, supportive.

Stem 10 — Pediatric DKA management trap: 8-year-old DKA, given large fluid bolus + bicarbonate, now headache and bradycardia 4h in. → Cerebral edema; mannitol or 3% saline; slow fluids.

One-Line Recap

Metabolic acidosis is diagnosed by a low pH and HCO₃⁻ on ABG/BMP, classified within 60 seconds by calculating the anion gap (with albumin correction), checking Winter's formula and delta-delta, and then matched to a specific etiology — with management directed at the underlying cause (insulin for DKA, fluids/antibiotics/source control for sepsis-driven lactic acidosis, fomepizole + hemodialysis for toxic alcohols, alkalinization ± dialysis for salicylates, bicarbonate/citrate for RTAs and CKD) rather than at the bicarbonate number itself.

Board pearl: If you can perform the AG, Winter's, delta-delta, osmolar gap, and urine anion gap in under two minutes, you can solve nearly every Step 3 acid–base question regardless of how the stem is dressed up.

Step 3 management: The single most impactful CCS habit — order VBG, lactate, β-OHB, and serum osmolality together the moment HCO₃⁻ comes back low; the answer almost always emerges from that quartet.

The five-step algorithm: confirm acidemia → calculate albumin-corrected AG → apply Winter's → delta-delta → osmolar gap and urine anion gap as needed.

AG acidosis = GOLD MARK; non-AG = HARDUP; urine anion gap and urine pH separate RTA subtypes from GI losses.

Treat the cause, not the number: bicarbonate is reserved for pH <6.9 (DKA), severe hyperkalemia, toxic alcohol/salicylate alkalinization, and severe non-AG losses — not for routine lactic acidosis or DKA.

Emergent HD (AEIOU): toxic alcohols with end-organ damage or level >50, salicylate >100 acute or symptomatic, severe MALA, lithium toxicity, refractory acidemia, uremic complications.

Step 3 differentiators: outpatient bicarb 650 mg TID in CKD when HCO₃⁻ <22 (KDIGO slows progression); SGLT2 hold rules to prevent euglycemic DKA; balanced crystalloids over normal saline in large-volume resuscitation; pediatric DKA avoids bicarb to prevent cerebral edema; chronic salicylism in the elderly mimics sepsis/delirium.