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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
- Hyperkalaemia with ECG changes is an emergency — give IV calcium first to stabilize myocardium
- Symptomatic severe hyponatraemia requires urgent hypertonic saline, but chronic hyponatraemia must be corrected slowly to avoid osmotic demyelination
- Potassium disorders require ECG assessment and magnesium correction where relevant
- Calcium disorders require corrected or ionized calcium, PTH, phosphate, magnesium, vitamin D, renal function, and malignancy context
- Always identify medication causes: diuretics, ACEi/ARB, spironolactone, NSAIDs, SSRIs, PPIs, lithium, supplements, laxatives, and antacids
Approach to the Presentation
Electrolyte abnormalities are presentations, not diagnoses. The immediate question is whether the patient is symptomatic or at risk of arrhythmia, seizure, coma, respiratory failure, or sudden deterioration. Interpret the electrolyte in context of renal function, acid-base status, medications, volume status, glucose/osmolality, and ECG. The MCC has specific objectives for hyponatraemia, hypokalaemia, hyperkalaemia, and calcium disorders; magnesium is clinically tied to potassium and calcium physiology.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Severe hyperkalaemia | must-not-miss | Weakness, paralysis, arrhythmia, AKI/CKD, ACEi/ARB, spironolactone, missed dialysis; peaked T waves/QRS widening | Repeat potassium if pseudo possible, ECG, renal function, acid-base |
| Severe symptomatic hyponatraemia | must-not-miss | Headache, confusion, seizures, coma; acute water excess, SIADH, thiazides, adrenal insufficiency, postoperative state | Serum osmolality, urine osmolality, urine sodium, glucose, cortisol/TSH when indicated |
| Hypercalcaemia of malignancy | must-not-miss | Dehydration, confusion, constipation, bone pain, renal impairment, cancer symptoms | Corrected/ionized calcium, PTH suppressed, PTHrP/vitamin D work-up, malignancy evaluation |
| Hypocalcaemia with tetany/seizure/QT prolongation | must-not-miss | Perioral tingling, cramps, tetany, seizures, Chvostek/Trousseau, post-thyroidectomy, CKD, pancreatitis | Ionized calcium, magnesium, phosphate, PTH, ECG QT interval |
| Hypokalaemia | common | Weakness, cramps, ileus, arrhythmia; diuretics, vomiting, diarrhoea, hyperaldosteronism | ECG, serum magnesium, urine potassium, acid-base status, BP |
| SIADH | common | Euvolaemic hyponatraemia, malignancy, CNS disease, pulmonary disease, SSRIs, carbamazepine | Low serum osmolality, inappropriately concentrated urine, urine sodium usually elevated |
| Hypovolaemic hyponatraemia | common | Vomiting, diarrhoea, diuretics, orthostasis, tachycardia, dry mucosa | Clinical volume status; urine sodium low unless renal salt loss/diuretics |
| Hypomagnesaemia | common | Tremor, tetany, refractory hypokalaemia/hypocalcaemia; alcohol, diarrhoea, PPIs, diuretics | Serum magnesium; check calcium and potassium |
| Pseudohyperkalaemia / pseudo-electrolyte abnormality | less common | Haemolysed sample, high platelets/WBC, delayed processing, no ECG changes | Repeat plasma potassium, ECG correlation |
Red Flags & Key History
Symptoms
Seizure, coma, severe confusion — sodium or calcium emergency
Palpitations, syncope, muscle paralysis — potassium/magnesium emergency
ECG changes in hyperkalaemia, hypokalaemia, hypocalcaemia, or hypercalcaemia
AKI/CKD or missed dialysis with abnormal potassium
Cancer symptoms, bone pain, dehydration with high calcium
Thiazides, SSRIs, carbamazepine, diuretics, ACEi/ARB, spironolactone, NSAIDs, PPIs, lithium
Signs
Altered mental status, seizures, respiratory compromise
Tetany, carpopedal spasm, Chvostek/Trousseau signs
Volume depletion or overload signs
Hypertension with hypokalaemia — hyperaldosteronism clue
Approach to Investigation
First-line
Repeat critical electrolyte if unexpectedExclude haemolysis/pseudohyperkalaemia while treating if ECG changes or symptoms are present
ECGMandatory for significant potassium, calcium, or magnesium disturbance
Creatinine/eGFR, bicarbonate, glucose, serum osmolalityRenal function, acid-base and osmotic context drive diagnosis and treatment
Medication and fluid historyDiuretics, ACEi/ARB, spironolactone, SSRIs, PPIs, lithium, supplements, vomiting/diarrhoea, water intake
Second-line
Urine osmolality and urine sodiumCore tests for hypotonic hyponatraemia classification
Urine potassium, renin/aldosterone if indicatedHypokalaemia evaluation when renal wasting or hyperaldosteronism suspected
PTH, phosphate, magnesium, vitamin DCalcium disorder evaluation; magnesium is required to correct some calcium/potassium disorders
Specialist
Nephrology/endocrinology/ICUSevere or refractory electrolyte disorder, dialysis requirement, adrenal/pituitary disease, severe calcium disorder, or complex sodium correction
Dialysis planningRefractory hyperkalaemia, severe renal failure, toxin-related disturbances, or severe volume/electrolyte complications
Management Principles
MCC electrolyte objectives + Canadian nephrology/endocrinology practice + KDIGO renal guidance1
Hyperkalaemia
- If ECG changes or severe elevation: IV calcium first
- Shift potassium intracellularly with insulin/dextrose and beta-agonist; bicarbonate only selected acidotic cases
- Remove potassium: diuretics if producing urine, potassium binders in selected non-emergency contexts, or dialysis if refractory/severe renal failure
2
Hyponatraemia
- If severe neurologic symptoms: hypertonic saline with close monitoring
- Determine hypotonic vs non-hypotonic and volume status; treat cause
- Avoid overly rapid correction, especially chronic hyponatraemia, malnutrition, alcohol use, liver disease, or hypokalaemia
3
Calcium and magnesium
- Symptomatic hypocalcaemia: IV calcium and correct magnesium
- Severe hypercalcaemia: isotonic fluids, calcitonin/bisphosphonate or denosumab depending on renal function and cause
- Hypomagnesaemia: replace magnesium to allow potassium/calcium correction
Complications & Pitfalls
- Hyperkalaemia order: Calcium stabilizes the heart; insulin only shifts potassium and does not protect the myocardium first.
- Hyponatraemia overcorrection: Rapid correction can cause osmotic demyelination.
- Glucose correction: Hyperglycaemia lowers measured sodium; correct sodium interpretation before labelling true hyponatraemia.
- Magnesium link: Refractory hypokalaemia or hypocalcaemia often needs magnesium replacement.
- ECG normality: A normal ECG does not guarantee safety in significant hyperkalaemia.
MCCQE1 Exam Tips
- 1Hyperkalaemia with ECG changes: IV calcium is first
- 2Hyponatraemia work-up: serum osmolality → urine osmolality → urine sodium → volume status
- 3Chronic hyponatraemia correction must be slow
- 4Hypokalaemia plus hypertension suggests hyperaldosteronism/renal potassium wasting
- 5Low magnesium causes refractory low potassium and low calcium
- 6Hypercalcaemia: check PTH first to separate PTH-mediated from malignancy/vitamin D causes
- 7Always review medication causes in electrolyte questions
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