NHANES monitoring of serum 25-hydroxyvitamin D: a roundtable summary

Abstract

A roundtable to discuss monitoring of serum 25-hydroxyvitamin D [25(OH)D] in the NHANES was held in late July 2009. Topics included the following: 1) options for dealing with assay fluctuations in serum 25(OH)D in the NHANES conducted between 1988 and 2006; 2) approaches for transitioning between the RIA used in the NHANES between 1988 and 2006 to the liquid chromatography tandem MS (LC-MS/MS) measurement procedure to be used in NHANES 2007 and later; 3) approaches for integrating the recently available standard reference material for vitamin D in human serum (SRM 972) from the National Institute of Standards and Technology (NIST) into the NHANES; 4) questions regarding whether the C-3 epimer of 25-hydroxyvitamin D3 [3-epi-25(OH)D3] should be measured in NHANES 2007 and later; and 5) identification of research and educational needs. The roundtable experts agreed that the NHANES data needed to be adjusted to control for assay fluctuations and offered several options for addressing this issue. The experts suggested that the LC-MS/MS measurement procedure developed by NIST could serve as a higher order reference measurement procedure. They noted the need for a commutability study for the recently released NIST SRM 972 across a range of measurement procedures. They suggested that federal agencies and professional organizations work with manufacturers to improve the quality and comparability of measurement procedures across all laboratories. The experts noted the preliminary nature of the evidence of the 3-epi-25(OH)D3 but felt that it should be measured in 2007 NHANES and later.

FIGURE 1

The NHANES 2003–2004 QC performance data for 25(OH)D (20) (A). The NHANES 2005–2006 QC performance data for 25(OH)D (21) (B). Each inflection point represents the mean of 2 separately processed samples measured in duplicate in each assay. At any given point in time, there are 3 QC performance pools–high, medium, and low. The vertical lines indicate the time when pools were changed. Different line patterns also indicate different pools. The name of each pool is given above its respective data line. Shifts of concern occur within a QC pool (same line pattern, same pool number, within the same vertical lines). Differences between QC pools are not meaningful. Unit conversion: 2.5 nmol/L = 1 ng/mL.

FIGURE 2

Scatter plot with Deming regression fit of 25(OH)D concentrations as measured by the reformulated DiaSorin RIA (adjusted for the 2004 upward assay drift) compared with 25(OH)D concentrations as measured by the original DiaSorin RIA (2) (A). Bland-Altman difference plot of 25(OH)D concentrations between the reformulated DiaSorin RIA (adjusted for the 2004 upward assay drift) and the original DiaSorin RIA (2) (B). For Figure 2A, each data point represents the mean of duplicate measurements in each assay. The Deming equation was: NHANES 1988–1994 25(OH)D_{corrected 2004 RIA} = [0.8429 × NHANES 1988–1994 25(OH)D_{1988–1994 RIA}] + 2.5762 nmol/L. The SE was 0.0342 for the slope and 2.3909 for the intercept. For Figure 2B, each data point represents the relative difference between the 2 assays as a function of the mean between the 2 assays. Unit conversion: 2.5 nmol/L = 1 ng/mL.