Evaluation of Vitamin D Standardization Program protocols for standardizing serum 25-hydroxyvitamin D data: a case study of the program's potential for national nutrition and health surveys

Abstract

Background: The Vitamin D Standardization Program (VDSP) has developed protocols for standardizing procedures of 25-hydroxyvitamin D [25(OH)D] measurement in National Health/Nutrition Surveys to promote 25(OH)D measurements that are accurate and comparable over time, location, and laboratory procedure to improve public health practice.

Objective: We applied VDSP protocols to existing ELISA-derived serum 25(OH)D data from the Irish National Adult Nutrition Survey (NANS) as a case-study survey and evaluated their effectiveness by comparison of the protocol-projected estimates with those from a reanalysis of survey serums by using liquid chromatography-tandem mass spectrometry (LC-tandem MS).

Design: The VDSP reference system and protocols were applied to ELISA-based serum 25(OH)D data from the representative NANS sample (n = 1118). A reanalysis of 99 stored serums by using standardized LC-tandem MS and resulting regression equations yielded predicted standardized serum 25(OH)D values, which were then compared with LC-tandem MS reanalyzed values for all serums.

Results: Year-round prevalence rates for serum 25(OH)D concentrations <30, <40, and <50 nmol/L were 6.5%, 21.9%, and 40.0%, respectively, via original ELISA measurements and 11.4%, 25.3%, and 43.7%, respectively, when VDSP protocols were applied. Differences in estimates at <30- and <40-nmol/L thresholds, but not at the <50-nmol/L threshold, were significant (P < 0.05). A reanalysis of all serums by using LC-tandem MS confirmed prevalence estimates as 11.2%, 27.2%, and 45.0%, respectively. Prevalences of serum 25(OH)D concentrations >125 nmol/L were 1.2%, 0.3%, and 0.6% by means of ELISA, VDSP protocols, and LC-tandem MS, respectively.

Conclusion: VDSP protocols hold a major potential for national nutrition and health surveys in terms of the standardization of serum 25(OH)D data.

FIGURE 1.

Relation between serum 25(OH)D (nmol/L) in a subsample (n = 99) of National Adult Nutrition Survey samples measured by using an IDS ELISA and standardized LC-MS at University College Cork as per Vitamin D Standardization Program protocol 2. The figure shows the piecewise linear fit regression line; the shaded area represents 95% confidence bands. IDS, Immuno Diagnostic Systems Ltd; LC-MS, liquid chromatography–tandem mass spectrometry; 25(OH)D, 25-hydroxyvitamin D.

FIGURE 2.

Weighted relative frequency for serum 25-hydroxyvitamin D of National Adult Nutrition Survey samples (n = 1118) measured by using the Immuno Diagnostic Systems Ltd enzyme-linked immunosorbent assay (dotted green line) and standardized liquid chromatography–tandem mass spectrometry at University College Cork (dashed red line) and predicted as per Vitamin D Standardization Program protocol 2 [the black line with shaded area represents 95% confidence bands as estimated by using the method of Fiorio (25)].

FIGURE 3.

Bland-Altman comparison plot (y axis: standardized LC–tandem MS–measured minus LC–tandem MS–predicted differences; x axis: mean of LC–tandem MS–predicted and LC–tandem MS–measured values) of serum 25(OH)D of National Adult Nutrition Survey samples (n = 1118) measured by using standardized LC–tandem MS at University College Cork and predicted as per Vitamin D Standardization Program protocol 2. The diamonds indicate individual data points. Four black data points represent those that were deemed outliers by using an outlier limit of 4 SDs. LC–tandem MS; liquid chromatography–tandem mass spectrometry; 25(OH)D, 25-hydroxyvitamin D.