ESPN 51th Annual Meeting

ESPN 2018


 
Measured Versus Calculated Free serum 25(OH)-Vitamin D Level: which one is better?
JUDITH VANSICKLE 1 TARAK SRIVASTAVA 1 UTTAM GARG 1 URI ALON 1

1- CHILDRENS MERCY HOSPITAL
 
Introduction:

Serum 25(OH)D is mainly bound to Vitamin D Binding Protein (VDBP) (~85%) and to albumin (~15%) with only a minute amount circulating as free 25(OH)D. A stronger association of free 25(OH)D than total 25(OH)D was observed with serum calcium, PTH, bone mineral density, and vascular outcomes. Free 25(OH)D can be calculated from the concentrations of total 25(OH)D, VDBP, and albumin by a mathematical method or may be directly measured by ELISA. Previous studies questioned the accuracy of calculated vs. measured free 25(OH)D. Objective: To compare the validity of the mathematically calculated versus measured free 25(OH)-Vitamin D.

Material and methods:

We collected blood samples from 74 children (ages 1-19 years old) and simultaneously measured total 25(OH)D on Tandem Mass Spectrometer, serum albumin by bromocresol green dye on COBAS chemistry analyzer, VDBP and free 25(OH)D by ELISA. Free 25(OH)D was calculated as follows: Calculated free 25(OH)D = (Total 25(OH)D)/(1+ (6*10,0000 * [albumin] )+ (7*10,0000000 *[VDBP] ) ) Pearson correlation analysis and Bland-Altman Plot were used to examine the true agreement of the two methods.

Results:

The mean age was 9.03 ± 5.07 years with 55% males and 76% Caucasians and 24% African-Americans. The mean ± SD for Total 25(OH)D was 37.12 ± 11.33 ng/mL, calculated free 25(OH)D 8.87 ± 3.66 pg/mL and measured free 25(OH)D 8.59 ± 3.41 pg/mL. Pearson correlation reflected a significant correlation between calculated and measured free 25(OH)D (r= 0.66, p< 0.0005). Bland-Altman Plot reflected a tight agreement with in a 95% limit of agreement (mean = -0.026 ± 2SD).

Conclusions:

The measured and calculated values of free 25(OH)D are in agreement and seem interchangeable. Depending on the availability of instruments and methods, free 25(OH)D can be either measured or calculated.