• David Palmer, Steven Soule, Ravinder R Gaddam, Peter Elder, Stephen Chambers, and Matthew Doogue.
• Department of General Medicine, Canterbury District Health Board, Christchurch, New Zealand.
• Intern Med J. 2022 Jan 1; 52 (1): 89-94.

BackgroundFree concentrations of highly protein bound hormones, such as cortisol and thyroxine, are unchanged in critical illness despite substantial decreases in total concentration. Total 25-hydroxyvitamin D (25(OH)D) concentration is decreased in critical illness, but the free concentration of 25(OH)D has had less attention.AimTo compare total and calculated free 25(OH)D concentrations in critically ill patients with healthy controls.MethodsIn this case-control study, 38 patients with critical illness were compared with 68 healthy controls; 25(OH)D was measured by liquid chromatography tandem mass spectrometry (LCMS/MS) and vitamin D binding protein (VDBP) by direct sandwich enzyme-linked immunosorbent assay. Total and calculated free 25(OH)D concentrations were compared using unpaired t-tests.ResultsTotal 25(OH)D concentrations were significantly lower in critically ill patients than controls (37 (95% confidence interval 31-43) vs 57 (53-60) nmol/L). Calculated free concentrations of 25(OH)D were not lower in critically ill patients than healthy controls (26 (22-29) vs 19 (18-20) pmol/L).ConclusionsCalculated free 25(OH)D concentrations are not decreased in critical illness. Measuring total 25(OH)D concentrations in patients with critical illness potentially underestimates vitamin D and overestimates the number of patients who are deficient in vitamin D.© 2020 Royal Australasian College of Physicians.

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