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. 2022 Jan;52(1):89-94.
doi: 10.1111/imj.15096.

Unbound vitamin D concentrations are not decreased in critically ill patients

David Palmer  1 , Steven Soule  1   2   3 , Ravinder R Gaddam  4 , Peter Elder  4   5 , Stephen Chambers  4 , Matthew Doogue  1   2   6
Affiliations

Unbound vitamin D concentrations are not decreased in critically ill patients

David Palmer et al. Intern Med J. 2022 Jan.

Abstract

Background: Free 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.

Aim: To compare total and calculated free 25(OH)D concentrations in critically ill patients with healthy controls.

Methods: In 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.

Results: Total 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).

Conclusions: Calculated 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.

Keywords: carrier protein; critical illness; protein binding; steroid; vitamin D; vitamin D-binding protein.

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