Advances in standardization of laboratory measurement procedures: implications for measuring biomarkers of folate and vitamin B-12 status in NHANES

Abstract

Population studies such as NHANES analyze large numbers of laboratory measurements and are often performed in different laboratories using different measurement procedures and over an extended period of time. Correct clinical and epidemiologic interpretations of the results depend on the accuracy of those measurements. Unfortunately, considerable variability has been observed among assays for folate, vitamin B-12, and related biomarkers. In the past few decades, the science of metrology has advanced considerably, with the development of improved primary reference measurement procedures and high-level reference materials, which can serve as the basis for accurate measurement. A rigorous approach has been established for making field methods traceable to the highest-level reference measurement procedures and reference materials. This article reviews some basic principles of metrology and describes their recent application to measurements of folate and vitamin B-12.

FIGURE 1.

A traceability chain for serum cholesterol. By using this scheme, a routine measurement of serum cholesterol in a clinical laboratory has a calibration that is ultimately tied to a solid material of high and known purity prepared by the National Institute of Standards and Technology (NIST). Reference materials are shown on the left of the diagram and measurement procedures on the right. Each reference material is used to calibrate the next measurement procedure down the chain (right-pointing arrows), and each measurement procedure is used to assign values to the next reference material (left-pointing arrows). Each step has some associated uncertainty, which ideally can be specified, and so the overall measurement uncertainty increases as one proceeds down the chain but can be estimated for the routine clinical result. For many more complex substances, including folate and vitamin B-12, a certified solid reference material is not currently available. Folate and vitamin B-12 measurements can, however, now be made traceable to serum-based NIST reference materials analogous to SRM (standard reference material) 1951b for cholesterol. SI, International System of Units; ID-GC/MS, isotope dilution-gas chromatography/mass spectrometry; CDC, Centers for Disease Control and Prevention.

FIGURE 2.

A cholesterol-oxidase–based clinical laboratory measurement procedure compared with the Abell-Kendall (ALBK) reference measurement procedure measured cholesterol concentrations using fresh human serum (•) and lyophilized human serum (▴), which in the past has often been used as a reference material for both calibration purposes and as a “trueness control” in proficiency surveys to evaluate a clinical measurement procedure's accuracy. Reprinted with permission from reference .

FIGURE 3.

Method bias plot for folate. The scatterplot compares method bias for folate assays on 2 different materials: an artificial Proficiency Testing Material (PTM; see text) and a fresh-frozen human serum pool (FFS). Each point represents a commercial folate methodology used by ≥10 laboratories participating in a 2003 proficiency testing event of the College of American Pathologists. The method bias equals the mean result for the method minus the mean result for all methods. The method biases for the 2 materials showed no significant correlation (r = −0.224, P = 0.48), indicating that the PTM is not commutable with human serum for routine folate measurement procedures. Reprinted with permission from reference .