MEC Operation and Schedule
NHANES collected biological specimens (biospecimens) for laboratory analysis to provide detailed information about participants' health and nutritional status. Eligibility for specific laboratory tests was based on the survey participants' gender and age (at the time of screening). The biospecimen collection took place in the mobile examination center (MEC). This included collecting, processing, storing, and shipping blood, urine, and other types of specimens. The controlled environment of the MEC allowed laboratory test to be performed under standardized conditions at each survey location.
The MEC was open 5 days per week; the non-operational days changed on a rotating basis so that appointments could be scheduled on any day of the week. Two examination sessions were conducted daily. Participants are randomly assigned to exams in the morning, or in the afternoon/evening session. Participants aged 12 years and older assigned to a morning session were asked to fast for 9 hours. Participant's fasting status was assessed by the MEC phlebotomist prior to the blood draw.
Biospecimen Collection
Collection procedures for NHANES biospecimens varied based on the specimen types. Except for complete blood count (CBC) and urine pregnancy tests, which were performed in the MEC, most NHANES biomedical tests were completed in laboratories across the United States. Some serum, plasma, urine, and DNA specimens were stored for future studies as part of the NHANES Biospecimen Program, if consent was provided by the participant. Detailed procedures in specimen collection, processing, and shipping are available at: 2019-2020 Laboratory Procedures Manual.
Blood
Blood was collected from participants aged 1 year and older by a phlebotomist. The amount of blood drawn varied by age. Blood was processed and aliquoted into vials. The vials were then refrigerated or frozen before transport to testing laboratories for analysis, or to biorepositories for future studies.
Urine
Participants aged 6 and older were asked to provide a full void urine specimen. The parents/guardians of participants aged 3-5 years were asked to help their children collect a full void urine specimen.
The date and time of the prior urine void, along with the date, time, and volume of the urine specimen collected in the MEC were used to calculate urine flow rate. The urine specimen was then aliquoted, frozen, and transported to laboratories for analysis or to biorepositories for future studies.
Vaginal/Penile swabs
All participants aged 14-59 were asked to self-collect a vaginal or penile specimen using a sterile swab. The vaginal and penile swabs were tested for the presence of multiple types of human papillomavirus (HPV).
Oral rinse
An oral rinse was collected and tested for the presence of multiple types of human papillomavirus (HPV). The examining dentist instructed participants aged 14-69 to gargle and swish with mouthwash for 30 seconds and then spit into a specimen container.
Household water sample
For each household with participants aged birth-19 years old, a tap water sample was collected by the household interviewer and sent to test for fluoride exposures. The water sample is collected from the household's primary consumption source or faucet from which the participants use the water most for drinking, preparing food, and cooking. Once identified the proper source/faucet, the interviewer would first let the water run for 10-15 seconds, then collect a 9-ml sample using the collection tube.
Household iodized salt sample
Iodized salt samples were collected from eligible households by the household interviewers and sent to test for iodine content. A household with a participant aged 3-5 years was eligible for the salt collection. In addition, a random 1/3 subsample was selected from participants aged 6 and older to be eligible. During the household interview, the interviewer asked the respondent for the salt(s) that usually used in cooking or on the table at home. If multiple salts were used in the household, the interviewer would ask the respondent to identify the 2 that used most frequently. The interviewer then reviewed the container labels to identify iodized salt for the collection. A salt sample would also be collected if no original container label available or if the label was not legible.
NHANES Laboratory Setting
Each MEC had a laboratory containing a laminar flow hood, complete blood count (CBC) with 5-part differential analyzer, two centrifuges, a portable balance (scale), refrigerators, and freezers. Each MEC laboratory team included three medical technologists and a phlebotomist. Staff were certified in accordance with guidelines set forth by the American Society for Clinical Pathology. The qualifications for these laboratory staff are described in the component training manuals. All laboratory staff were thoroughly trained to ensure the safety of the laboratory environment. This included annual training in the following laboratory safety and infection control policies and procedures:
- Occupational Safety and Health Administration (OSHA) Blood borne Pathogen Regulation;
- NHANES exposure control plan;
- Working safely with hazardous chemicals;
- Universal precautions and a set of guidelines for preventing the transmission of blood borne pathogens such as human immunodeficiency virus (HIV) and hepatitis viruses in health care settings; and
- International Air Transport Association (IATA) training for proper shipping dangerous goods.
All staff also completed privacy protection and confidentiality training, and cardiopulmonary resuscitation training. In addition, all laboratory staff completed component-specific training to learn the standardized NHANES laboratory protocols.
Automated Data Collection
In the MECs and analytical laboratories, data for the laboratory component were recorded directly into a computerized database. Related questionnaire forms (e.g., fasting status) were also automated. The laboratory data collection and reporting systems were integrated with the main survey database.
Quality Control Monitoring
The NHANES MEC laboratory has been a Clinical Laboratory Improvement Act (CLIA)-certified laboratory of moderate complexity. Quality assurance and quality control (QA/QC) involved both internal and external surveillance. QA/QC procedures were performed in the MEC as well as in contract and CDC laboratories. As part of the overall QA process, all blood and urine collection materials, vacuum sealed blood tubes, pipettes, and storage containers used were initially prescreened for background contamination (e.g., lead, mercury, etc.). EDTA (ethylenediaminetetraacetic acid) tubes were used after prescreening for contamination. The lot number and expiration dates for all vacuum sealed blood tubes, pipettes, needles, and reagents were recorded.
Specific QC procedures were followed in the laboratory. For example, the freezers, refrigerators, and centrifuges were cleaned before the MEC opened, and a temperature reading on these items was conducted daily. On-site calibrations were performed twice each year. The NCHS biomedical engineer certified the revolutions per minute (rpm) of the centrifuges periodically and replaced the high-efficiency particulate air filters as necessary. All instrument maintenance was recorded. NHANES laboratories participated in the College of American Pathologists (CAP) proficiency-testing program. CAP samples were sent three times a year for the CBC and qualitative urine human chorionic gonadotropin. In addition, blind split samples were used for QC determinations.
Contract laboratories followed QA/QC guidelines when working with NHANES specimens and were required to be CLIA-certified. In addition, to ensure the data quality, NHANES staff conducted annual laboratory inspections and reviewed the QC data from each laboratory.
Data Preparation
Routine data preparation procedures included a review of frequency data, outliers, and technician notes. Analysts should review the data reported for each component or laboratory assay prior to beginning data analyses.
Detection Limits
For laboratory tests with a lower or a higher detection limit, results below the lower detection limit or higher than the higher detection limit were replaced with a value equal to the detection limit, divided by the square root of two. This value was created to help users distinguish a nondetectable laboratory test result from a measured laboratory test result.
Laboratory Subsamples
Some NHANES components were collected or processed for a subsample of participants. Subsampling was done to reduce participant burden and facilitate the scheduling and completion of examinations. Each subsample was selected to be nationally representative. For example, some participants were selected to give a fasting blood sample on the morning of their examination. The subsamples selected for these components were chosen at random with a specified sampling fraction (e.g., one-half of the total examined group). See the respective survey protocol and documentation for more specific information on each subsample.
Special Analytic Notes for the Laboratory Data
There are over 800 laboratory tests performed using NHANES biospecimens. However, not all participants provided biospecimens or enough volume for all the tests to be performed. The specimen availability can also vary by age or other population characteristics. For example, in current cycle, approximately 71% of children aged 1-17 years who were examined in the MEC provided a blood specimen through phlebotomy, while 94% of examined adults age 18 and older provided a blood specimen. Analysts should evaluate the extent of missing data in the dataset related to the outcome of interest as well as any predictor variables used in the analyses to determine whether additional re-weighting for item non-response is necessary.
We strongly encourage data users to read all relevant documentation on the survey overall and for the specific data files to be used in their analysis. Specific data file documentation can be found via the link next to the respective data file on the NHANES website. Data users should also review the NHANES Analytic Guidelines prior to beginning any analyses.