Key Concepts About the Environmental Chemical Data Files

NHANES laboratory data files primarily include findings from analyses of blood and urine. In selected years, assessment of environmental exposures also included other specimens, such as: hair, air, household dust, and home tap water. Specimens are collected at the MEC or in the home. Each specific laboratory tests target populations are based on the survey participant's gender and age at the time of the Household Interview.  

The NHANES laboratory component tasks include the collection, processing, storage, and shipment of blood, urine, and other environmental specimens to analytic laboratories.

This tutorial focuses its discussion on the environmental chemical data measured in blood and urine specimens. The blood collection procedure consists of administering a questionnaire to screen for conditions that exclude participants from the blood draw. Fasting status is recorded. The urine collection procedure consists of urine specimen collection and processing, and pregnancy testing.

Venipuncture Exclusion Criteria

The following exclusion criteria apply to all tests that require blood specimens:

• Participants with hemophilia;
• Participants who received chemotherapy within the last four weeks;
• Participants with the presence of rashes, gauze dressings, casts, edema, paralysis, tubes, open sores or wounds, withered arms or limbs missing, damaged, sclerosed or occluded veins, allergies to cleansing reagents, burned or scarred tissue, or shunt or intravenous lines on both arms.

Data Collection

Automated data collection procedures are used in NHANES. In the MEC and in NHANES analytic laboratories, data for the laboratory component are recorded directly into a computerized database. Survey forms are also automated. The data collection and reporting systems are integrated with the main NHANES survey database. While the complete blood count and pregnancy analyses are performed in the MEC laboratory, most of the laboratory analyses are conducted off-site.

Data Processing and Preparation

NHANES data processing guidelines provide NCHS and contractor staff with standards for naming variables, filling missing values, and handling missing records. NCHS staff, assisted by contract staff, developed data editing specifications that check data sets for valid codes, ranges, and skip pattern consistencies and examine the consistency of values between interrelated variables. Comments are reviewed and recoded. NCHS staff verifies extremely high and low values. Numerous consistency checks are performed during data preparation. Nevertheless, data users should examine variable ranges and frequencies and other descriptive statistics before analyzing the data.

Data Editing

The NCHS data editing specifications for laboratory data include the following:

• Check age and gender ranges for each assay;
• Enumerate number of observations complete for each field;
• Verify field overlap, truncated values, or unusual results;
• Direct data entry (DDE) errors;
• Confirm out-of-range test results with lab;
• Verify data algorithm procedures are working as intended;
• Check comment codes to resolve missing results and missing records;
• Check for all missing results and missing MEC examination records;
• Verify and delete duplicate records;
• Apply the SI conversion of units when appropriate; and
• Apply below detection limit formula.