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This is a clinician-written, evidence-based guide aligned to the MCC Examination Objectives. It is structured by clinical presentation — the way the MCCQE tests and the way patients actually present. Management reflects current Canadian guidelines (CMA, CFPC, CPS). Always cross-reference with institutional protocols and clinical judgment.
Key points
- Use a fixed sequence: pH → PaCO2 → HCO3− → expected compensation → anion gap → delta gap if high anion gap metabolic acidosis
- High anion gap metabolic acidosis is the must-not-miss category: DKA, lactic acidosis/shock, renal failure, toxic alcohols, salicylates, and ketoacidosis
- Respiratory acidosis means inadequate ventilation until proven otherwise — COPD/asthma exacerbation, opioid toxicity, neuromuscular disease, or reduced consciousness
- Metabolic alkalosis is most often vomiting/NG suction, diuretics, mineralocorticoid excess, or post-hypercapnic alkalosis; urine chloride helps classify it
- Treat the patient, not the number: shock, hypoxia, coma, severe hyperkalaemia, toxin ingestion, and respiratory failure require immediate resuscitation
Approach to the Presentation
Acid-base abnormalities are usually discovered during assessment of dyspnoea, altered mental status, shock, renal failure, diabetes, poisoning, sepsis, or severe gastrointestinal loss. The first task is to decide whether the patient is unstable, then interpret the blood gas systematically. Venous blood gas is often adequate for pH and bicarbonate screening, but arterial blood gas is preferred when oxygenation, ventilation, shock, or severe respiratory disease is central. The MCCQE1 commonly tests recognition of primary versus compensatory disorders and the immediate management of dangerous patterns rather than complex calculations alone.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Diabetic ketoacidosis / severe ketoacidosis | must-not-miss | Polyuria, polydipsia, dehydration, abdominal pain, vomiting, Kussmaul respirations; often infection or insulin omission | Glucose, serum beta-hydroxybutyrate, high anion gap metabolic acidosis, potassium assessment |
| Lactic acidosis from sepsis, shock, hypoxia, or seizures | must-not-miss | Hypotension, fever, hypoxia, mottling, altered mental status, recent seizure, severe anaemia, or tissue hypoperfusion | Serum lactate, cultures/source evaluation, perfusion markers, blood gas |
| Toxic alcohol or salicylate poisoning | must-not-miss | Ingestion history, intoxication, visual symptoms (methanol), renal failure/crystals (ethylene glycol), tinnitus and tachypnoea (salicylate) | Osmolar gap, anion gap, salicylate level, toxic alcohol levels if available, urine microscopy |
| Severe renal failure / uraemic acidosis | must-not-miss | AKI or advanced CKD, hyperkalaemia, volume overload, uraemic symptoms, reduced urine output | Creatinine/eGFR, potassium, bicarbonate, urinalysis, indications for dialysis |
| Respiratory acidosis from ventilatory failure | must-not-miss | Somnolence, COPD/asthma exacerbation, opioid or sedative exposure, neuromuscular weakness, obesity hypoventilation | ABG/VBG showing elevated CO2 with acidaemia; oxygenation and clinical ventilation assessment |
| Non-anion gap metabolic acidosis | common | Diarrhoea, ileostomy, renal tubular acidosis, saline infusion, acetazolamide; normal anion gap with low bicarbonate | Anion gap, urine anion gap/urine pH when RTA suspected, stool/fluid loss history |
| Metabolic alkalosis from vomiting, NG suction or diuretics | common | Vomiting, nasogastric suction, loop/thiazide use, volume depletion, hypokalaemia, chloride depletion | High bicarbonate, low chloride, urine chloride low unless ongoing diuretic effect |
| Respiratory alkalosis | common | Tachypnoea from sepsis, PE, hypoxaemia, pregnancy, liver disease, pain, anxiety, or salicylate toxicity | Low PaCO2 with alkalemia or mixed disorder; assess oxygenation, D-dimer/CTPA if PE indicated, salicylate level if suspected |
| Mineralocorticoid excess | less common | Hypertension, hypokalaemia, metabolic alkalosis, muscle weakness; primary hyperaldosteronism or Cushing physiology | Blood pressure pattern, renin/aldosterone testing after medication review |
| Mixed acid-base disorder | less common | pH near normal despite severe illness, compensation not appropriate, vomiting plus DKA, COPD plus sepsis, salicylate mixed pattern | Expected compensation calculation and delta gap assessment |
Red Flags & Key History
Symptoms
Altered level of consciousness, seizure, coma, or severe agitation
Shock, hypoxia, severe sepsis, or lactate elevation
Kussmaul respirations with polyuria/polydipsia or abdominal pain — DKA until proven otherwise
Toxin exposure, antifreeze/solvent ingestion, tinnitus, visual symptoms, or unexplained high anion gap
Severe COPD/asthma, opioid/sedative use, or neuromuscular weakness with CO2 retention
Vomiting, diarrhoea, diuretic use, laxatives, bicarbonate/antacid ingestion
Pregnancy, anxiety, pain, fever, PE risk factors causing respiratory alkalosis
Signs
Hypotension, poor perfusion, mottling, cool extremities
Bradypnoea, shallow breathing, drowsiness, pinpoint pupils — opioid/sedative ventilatory failure
Tachypnoea with deep respirations — metabolic acidosis compensation
Signs of dehydration or volume overload
Focal infection source or abdominal catastrophe signs
Approach to Investigation
First-line
Blood gas (VBG or ABG)Assess pH, CO2, bicarbonate and lactate. ABG is preferred if oxygenation or ventilatory failure is central
Electrolytes, creatinine/eGFR, glucose, anion gapCore acid-base interpretation; correct anion gap for albumin when available
Serum beta-hydroxybutyrate and urinalysisEvaluate DKA or ketoacidosis; urine ketones can underestimate beta-hydroxybutyrate-predominant ketoacidosis
ECGAssess hyperkalaemia, hypokalaemia, hypocalcaemia or toxin-related arrhythmia risk
Second-line
Serum lactate and sepsis work-upCultures and imaging guided by suspected source; lactate supports severity and resuscitation response
Measured osmolality and osmolar gapIf toxic alcohol suspected or unexplained high anion gap metabolic acidosis
Salicylate, acetaminophen and toxicology testingSalicylate classically causes mixed respiratory alkalosis and metabolic acidosis
Urine chloride / urine anion gapUrine chloride helps classify metabolic alkalosis; urine anion gap may help distinguish diarrhoea from renal tubular acidosis
Specialist
Nephrology / medical toxicology / ICUSevere acidosis, dialysis indication, toxic alcohol ingestion, refractory hyperkalaemia, respiratory failure, or severe mixed disorder
Renal replacement therapy planningConsider for refractory acidosis, severe renal failure with complications, toxic alcohols, or salicylate toxicity in severe cases
Management Principles
MCC acid-base objective + KDIGO AKI guidance + Canadian emergency practice1
Immediate stabilization
- ABCs, oxygen/ventilation support, IV access, cardiac monitoring, glucose check, ECG
- Treat shock and sepsis early: fluids when appropriate, vasopressors if persistent shock, antibiotics/source control when infection suspected
- Treat life-threatening hyperkalaemia immediately if present
2
High anion gap metabolic acidosis
- DKA: fluids, insulin after potassium assessment, potassium replacement, identify precipitant
- Lactic acidosis: restore perfusion and oxygen delivery; treat sepsis/shock/hypoxia
- Toxins: early poison centre/toxicology consultation, antidotes when indicated, consider dialysis for severe toxic alcohol or salicylate poisoning
- Renal failure: manage AKI complications and consider dialysis for refractory acidosis or hyperkalaemia
3
Metabolic alkalosis
- Correct volume and chloride depletion with isotonic saline when chloride-responsive
- Replace potassium and magnesium
- Stop or reduce diuretics if possible; evaluate mineralocorticoid excess when hypertensive/hypokalaemic
4
Respiratory disorders
- Respiratory acidosis: improve ventilation; naloxone if opioid toxicity; bronchodilators/steroids/NIV for COPD or asthma when appropriate
- Respiratory alkalosis: treat the driver — sepsis, PE, pain, hypoxaemia, pregnancy physiology, anxiety after exclusion of dangerous causes
Complications & Pitfalls
- Missing mixed disorders: A near-normal pH can hide two serious processes, such as DKA plus vomiting or COPD plus sepsis.
- Ignoring compensation: If compensation is not appropriate, there is a second primary disorder.
- Salicylate pattern: Tinnitus, tachypnoea and mixed respiratory alkalosis/metabolic acidosis should prompt a salicylate level.
- Bicarbonate reflex: Sodium bicarbonate is not routine for most metabolic acidosis; treat the cause and reserve bicarbonate for selected severe cases or specific indications.
- DKA potassium trap: Total body potassium is depleted even when serum potassium is normal or high at presentation.
MCCQE1 Exam Tips
- 1Always interpret in order: pH, CO2, bicarbonate, compensation, anion gap, then delta gap if high anion gap
- 2High anion gap metabolic acidosis on the MCCQE1 is an emergency differential: DKA, lactate/sepsis/shock, renal failure, toxic alcohols, salicylates
- 3Respiratory acidosis means hypoventilation; do not call it anxiety
- 4Salicylate toxicity is a classic mixed disorder: respiratory alkalosis plus metabolic acidosis
- 5Vomiting causes metabolic alkalosis; diarrhoea causes non-anion gap metabolic acidosis
- 6If pH is normal but the patient is very unwell, suspect a mixed disorder rather than reassurance
- 7In acid-base questions, the “next best step” is often stabilization and targeted labs, not a long calculation
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