Electrolyte Disorders: Calcium, Magnesium and Phosphate

The answer first

Calcium, magnesium and phosphate questions look like biochemistry and behave like physiology. Five rules carry you through almost every DWE vignette and every ward call ^[3]:

1. Trust the right calcium. A total calcium without an albumin is half a result. Correct for albumin as a screen, but reach for ionised calcium whenever the correction assumptions break — critical illness, acid-base disturbance, paraproteinaemia, or when the story and the number disagree ^[1].
2. PTH is the first test in hypercalcaemia. One number splits the differential in two: high or inappropriately normal PTH means parathyroid disease; suppressed PTH means malignancy, vitamin D or one of the rare mimics ^{[2] [3]}.
3. Severe hypercalcaemia is saline first. Volume repletion before any antiresorptive; calcitonin for speed within hours; a bisphosphonate — or denosumab in CKD — for durability over days ^{[3] [7]}.
4. Magnesium before potassium before calcium. Refractory hypokalaemia and refractory hypocalcaemia are both magnesium stories until proven otherwise — the kidney cannot hold potassium and the parathyroid cannot work without it ^{[18] [19]}.
5. Thiamine before calories. In the malnourished patient, feeding without thiamine and slow refeeding is how refeeding syndrome happens — phosphate, potassium and magnesium crash together ^[26].

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References28ShowHide

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