Variability in free 25(OH) vitamin D levels in clinical populations

Our goal was to determine total and directly measured free 25-hydroxy vitamin D (25(OH)D) serum levels in humans with a range of 25(OH)D levels and clinical conditions associated with low and high vitamin D binding protein levels. Serum samples and clinical data were collected from 106 subjects: 62 without cirrhosis or pregnancy, 24 cirrhotic patients with albumin <2.9g/dL, and 20 pregnant women. Total 25(OH)D (LC/MS/MS) and "free" 25(OH)D (immunoassay) were measured. Total 25(OH)D was significantly lower in liver disease patients but free 25(OH)D concentrations were significantly higher in this group (p<.001). Neither total nor free 25(OH)D concentrations were significantly different in pregnant women vs. the comparator group. There were significant direct positive relationships between free 25(OH)D and total 25(OH)D concentrations for the entire dataset and for each group (p<.0001), however slopes of relationships differed in the cirrhotic group compared to pregnant women or the comparator group. In cirrhotics: y (free 25(OH)D)=2.52+0.29×X(total 25 (OH)D), r(2)=.51, p<.001; y=1.45+0.09×X; r(2)=.77, p<.0001 for pregnant women; and y=1.11+0.12×X; r(2)=.72, p<.0001 for the comparator group).

Conclusions: directly measured free 25(OH)D serum concentrations and relationships between total and free 25(OH)D vary with clinical conditions, and may differ from those predicted by indirect estimation methods. This article is part of a Special Issue entitled 'Vitamin D Workshop'.