The environmental chemical data are part of a large laboratory component which includes measurement of nutritional biochemistries, hematological indicators, and other clinical measures. Since NHANES 1999, a phlebotomist draws blood on examinees ages one year and older, and urine specimens are requested of all examinees ages six years and older. In the laboratory, medical technologists aliquot the blood and urine specimens for analysis. Hematological assessments (namely blood counts and pregnancy tests) are done in the mobile examination center and results are provided to examinees. The remainder of the blood and urine specimens are processed and divided into pre-labeled vials. Detailed specimen collection and storage procedures are used to minimize potential bias from contamination or loss of samples. For example, the laboratory prescreens materials for contamination and, in some cases, specially prepares collection materials to remove potential contaminants. The vials are stored under appropriate cold and frozen conditions and then shipped to various analyzed laboratories where analyses are later performed. The assay methods, details of the procedures, description of the quality control process, and calculated coefficient of variations for each environmental analyte are available in the Laboratory Manuals for each survey cycle of NHANES. You will learn where to find these documents in the Preparing an Analytic Dataset course later in this tutorial.
The majority of the environmental chemical data for NHANES 1999+ is based on the analysis of specimens from subsamples of the examinees in the Mobile Examination Center. In most cases, this is a one-third sample of persons ages 6 years or 12 years and older. Subset distributions have also changed with survey cycles (e.g., age groups have changed). Due to the issues associated with small samples, uses of the environmental chemical data may be limited. Most often, the data from two or three two-year NHANES cycles must be combined to provide an adequate sample for analytic studies. These analytic limitations and constraints are the major topics for this tutorial.