Standardizing 25-hydroxyvitamin D values from the Canadian Health Measures Survey

Abstract

Background: The Canadian Health Measures Survey (CHMS) is an ongoing cross-sectional national survey that includes a measure of 25-hydroxyvitamin D [25(OH)D] by immunoassay. For cycles 1 and 2, the collection period occurred approximately every 2 y, with a new sample of ∼5600 individuals.

Objective: The goal was to standardize the original 25(OH)D CHMS values in cycles 1 and 2 to the internationally recognized reference measurement procedures (RMPs) developed by the US National Institute for Standards and Technology (NIST) and Ghent University, Belgium.

Design: Standardization was accomplished by using a 2-step procedure. First, serum samples corresponding to the original plasma samples were remeasured by using the currently available immunoassay method. Second, 50 serum samples with known 25(OH)D values assigned by the NIST and Ghent reference method laboratories were measured by using the currently available immunoassay method. The mathematical models for each step-i.e., 1) YCurrent = XOriginal and 2) YNIST-Ghent = XCurrent -were estimated by using Deming regression, and the 2 models were solved to obtain a single equation for converting the "original" values to NIST-Ghent RMP values.

Results: After standardization (cycles 1 and 2 combined), the percentage of Canadians with 25(OH)D values <40 nmol/L increased from 16.4% (original) to 19.4% (standardized), and values <50 nmol/L increased from 29.0% (original) to 36.8% (standardized). The 25(OH)D standardized distributions (cycles 1 and 2 analyzed separately) were similar across age and sex groups; slightly higher values were associated with cycle 2 in the young and old. This finding contrasts with the original data, which indicated that cycle 2 values were lower for all age groups.

Conclusion: The shifts in 25(OH)D distribution brought about by standardization indicate its importance in drawing correct conclusions about potential population deficiencies and insufficiencies and in permitting the comparison of distributions between national surveys.

Keywords: CHMS; adequacy; immunoassay; population survey; standardization; vitamin D.

FIGURE 1

Relation between 25(OH)D in plasma CHMS samples originally reported and values remeasured in serum via the current immunoassay for CHMS cycles 1 (A) and 2 (B). n = 106 for each cycle. The solid line represents the weighted Deming regression fit. The dashed lines represent the 95% CI. CHMS, Canadian Health Measures Survey; 25(OH)D, 25-hydroxyvitamin D.

FIGURE 2

Relation between NIST-Ghent–assigned VDSP values (VDSP_RMP) and values measured by immunoassay via the current immunoassay method for cycles 1 (A) and 2 (B). The solid line represents the weighted Deming regression fit to the data (n = 50 samples). The dashed lines represent the 95% CI. NIST, National Institute of Standards and Technology; RMP, reference measurement procedure; VDSP, Vitamin D Standardization Program.

FIGURE 3

Distribution of Canadian Health Measures Survey values (weighted for population representation) before (solid line) and after (filled gray) standardization by using Equation 1 for cycles 1 (A) and 2 (B). Vertical dashed lines show cutoff values of 30, 40, and 50 nmol/L. n = 5306 for cycle 1 and 6030 for cycle 2. VDSP, Vitamin D Standardization Program; 25(OH)D, 25-hydroxyvitamin D.

FIGURE 4

Mean ± SEM 25(OH)D values pooled by Dietary Reference Intake age categories (male + female) in cycles 1 and 2 for the original reported data (A) and the standardized data (B). See Supplemental Table 1 for the sample numbers per group and individual values. Significant differences between cycles were determined by z test and are indicated by an asterisk. 25(OH)D, 25-hydroxyvitamin D.