Data files are organized into four categories, as listed below. All environmental chemical variables are included in the Laboratory section and organized by sample specimens and chemical groups. For each two-year data release included in an analysis, it is important to review the laboratory section because you will find the general information for overall laboratory measurements and specific information related to each environmental chemical may change.
It is advisable to read the Documentation to fully understand this important information and identify the correct variables for your analysis.
For each two-year cycle of NHANES, review the general information and the Variable List to decide which environmental chemical variables are needed in your analysis. Use the search feature to speed up your search for variables relevant to your analysis. Carefully read the Documentation and related Laboratory Procedure Manuals for each of your search results.
For many enviromental chemical measures, a variable is present in the data file to indicate if the measured value of the environmental chemical is beyond the limit of detection of the laboratory assay. It is helpful to include this variable when creating an analytic dataset. See Module 11 for further information on the LOD variables.
In reviewing the documentation you will notice that some of the blood measurements are lipid-adjusted and some are unadjusted. In data files for serum dioxins, furans, polychlorinated biphenyls, polybrominated diphenyl ethers, and organochlorine pesticides, each analyte is expressed as two forms, lipid-adjusted and unadjusted, because these compounds are lipophilic and concentrate in the body’s lipid stores, including the lipids in serum. Serum levels reported per gram of total lipid reflect the amount of these compounds that are stored in body fat.
Similarly, for chemicals measured in urine, levels can be presented two ways: per volume of urine and per gram of creatinine. Urinary levels are expressed both ways in the literature and used for different purposes. Levels per gram of creatinine (i.e. "creatinine corrected" values) adjust for urine dilution. For example, if one person has consumed more fluids than another person, his or her urine output is likely higher and the urine more dilute (has less urine creatinine) than that of the other person. Interpretation of creatinine corrected results should also recognize that creatinine correction can also partially adjust for differences in lean body mass or renal function among persons. Unlike the lipid-adjusted measurements, which are included on the data files, "creatinine corrected" values need to be created by the data user with the information available on the data file. Further details about lipid and creatinine adjustment is available in Module 13.