Tacrolimus Monitoring — SCE Rheumatology MCQ

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ModerateRheumatology PharmacologyTacrolimus MonitoringSCE Rheumatology

A 34-year-old woman with refractory lupus nephritis has been taking oral tacrolimus for 4 months. Blood pressure, serum creatinine and serum potassium (4.9 mmol/L) are stable, and trough tacrolimus levels are within target. She reports intermittent calf cramps and a fine tremor. Which serum analyte should be checked periodically in addition to renal function, because tacrolimus causes renal tubular wasting of it?

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Correct answer: A — Magnesium

Magnesium is correct. Calcineurin inhibitors downregulate distal tubular magnesium transport, producing renal magnesium wasting and hypomagnesaemia; cramps, tremor, arrhythmia and rarely seizures follow, so serum magnesium is checked periodically alongside creatinine, potassium, blood pressure and glucose, with oral replacement when low. Sodium is the main distractor because it appears in every electrolyte profile, but tacrolimus characteristically causes potassium retention and a type-4 renal tubular acidosis pattern rather than renal sodium loss, and this patient's potassium is at the upper normal range. Iron, zinc and copper deficiency are not features of calcineurin-inhibitor tubulopathy and have no place in routine tacrolimus safety surveillance. Concurrent proton pump inhibitor therapy compounds hypomagnesaemia risk and should be reviewed.

Reference: Nijenhuis T et al. / Kabel J et al. Tacrolimus-induced hypomagnesemia and hypercalciuria requires FKBP12, suggesting a role for calcineurin (mechanism of renal magnesium wasting), 2020: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6944708/ — with clinical corroboration in connective tissue disease: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8173076/ (2021), in the UK context of the 2025 BSR csDMARD monitoring guideline: https://pubmed.ncbi.nlm.nih.gov/41235543/