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Comparative Study
. 2014 May;99(5):1631-7.
doi: 10.1210/jc.2013-3874. Epub 2014 Jan 31.

A comparison of measured and calculated free 25(OH) vitamin D levels in clinical populations

J B Schwartz  1 , J Lai, B Lizaola, L Kane, S Markova, P Weyland, N A Terrault, N Stotland, D Bikle
Affiliations
Comparative Study

A comparison of measured and calculated free 25(OH) vitamin D levels in clinical populations

J B Schwartz et al. J Clin Endocrinol Metab. 2014 May.

Abstract

Objective: Our goal was to compare direct quantitation of circulating free 25-hydroxyvitamin D (25(OH)D)levels to calculated free 25(OH)D levels and their relationships to intact PTH (iPTH), a biomarker of 25(OH)D effect, in humans with a range of clinical conditions.

Patients and methods: Serum samples and clinical data were collected from 155 people: 111 without cirrhosis or pregnancy (comparison group), 24 cirrhotic patients with albumin <2.9 g/dL, and 20 pregnant women (second and third trimester). Total 25(OH)D (LC/MS/MS), free 25(OH)D (immunoassay), vitamin D binding protein (DBP) (immunoassay), albumin, and iPTH (immunoassay) were measured.

Results: Total 25(OH)D, DBP, and albumin were lowest in patients with cirrhosis, but measured free 25(OH)D was highest in this group (P < .001). DBP was highest in pregnant women (P < .001), but measured free 25(OH)D did not differ from the comparison group. Calculated free 25(OH)D was positively correlated with measured free 25(OH)D (P < .0001) but explained only 13% of the variability with calculated values higher than measured. African Americans had lower DBP than other ethnic populations within all clinical groups (P < .03), and differences between measured and calculated free 25(OH)D were greatest in African Americans (P < .001). Measured free 25(OH)D was correlated with total 25(OH)D (P < .0001; r(2) = 0.51), but calculated free 25(OH)D was not. Similarly, both measured free 25(OH)D (P < .02) and total 25(OH)D (P < .05) were correlated with iPTH, but calculated free 25(OH)D was not.

Conclusions: Calculated free 25(OH)D levels varied considerably from direct measurements of free 25(OH)D with discrepancies greatest in the data for African Americans. Differences in DBP binding affinity likely contributed to estimation errors between the races. Directly measured free 25(OH)D concentrations were related to iPTH, but calculated estimates were not. Current algorithms to calculate free 25(OH)D may not be accurate. Further evaluation of directly measured free 25(OH)D levels to determine its role in research and clinical management of patients is needed.

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Figures

Figure 1.
Figure 1.
Directly measured free 25(OH)D concentrations are plotted on the x-axis, and calculated free 25(OH)D estimates (based on albumin, DBP levels, and published affinity constants) are plotted on the y-axis. Triangles represent data from pregnant women, closed circles represent data from liver failure patients, and open circles represent data from the comparison group. The dotted line represents the line of identity for 1:1 correlation. Although the measures were significantly related (P < .0001, r2 = 0.13), calculated free 25(OH)D concentrations were higher than directly measured free 25(OH)D concentrations.
Figure 2.
Figure 2.
Total 25(OH)D concentrations are plotted on the x-axis, and directly measured free 25(OH)D levels are plotted on the y-axis. Triangles represent data from pregnant women, closed circles represent data from liver failure patients, and open circles represent data from the comparison group. Directly measured free concentrations were related to total 25(OH)D concentrations (P < .0001), but the relationship varied slightly for each clinical group with free 25(OH)D concentrations highest in liver failure patients despite lower total 25(OH)D concentrations. Relationships between total 25(OH)D and pregnant women did not differ from the comparator group.
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