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.
Biological Specimens Collection
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 laboratories across the United States. Except for complete blood count (CBC) and pregnancy tests, which were performed in the MEC, most assays were completed in 35 laboratories across the United States. If consent was granted, some specimens were stored for future studies.
In 1999-2014, NHANES collected urine specimens for all participants aged 6 years and older. Starting 2015, the target age for urine collection was extended to participants aged 3 years and older.
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 also used: 1) to perform a urine pregnancy test on all female participants aged 12-59 years and menstruating females aged 8-11 years; 2) to aliquot, store, and transport to multiple laboratories for analysis; and 3) to aliquot and bank for future studies.
Female and male 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).
Collection procedures varied based on the specimen types. Please refer to the 2017-2018 Laboratory Procedures Manual for more details.
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 includes 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 NHANES 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 collection materials, vacuum sample vials, 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 sample vials, 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 serum 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.
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.
Low Detection Limits
For laboratory tests with a lower detection limit, results below the lower 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.
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). Each component subsample has its own designated analytic weight, which accounts for the additional probability of selection into the subsample component as well as nonresponse to the component. See the respective survey protocol and documentation for more specific information on each subsample.
Please note that when merging full sample data to the subsample data files, the analytic sample is the subsample and the subsample analytic weights must be used. Users are strongly urged to read the data file documentation to ensure proper analysis and interpretation of the data.
Please refer to the NHANES Analytic Guidelines and the Continuous NHANES Web Tutorials for further details on NHANES subsamples, the use of sample weights, and other related analytic issues.
Special Analytic Notes for the Laboratory Data
The analysis of NHANES 2017-2018 laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2017-2018 Demographics File contains data on demographic characteristics, health indicators, and other related information collected during household interviews as well as the sample weights. The Fasting Questionnaire File includes auxiliary information, such as fasting status, the time of venipuncture, and the conditions precluding venipuncture. The demographics and fasting questionnaire files may be linked to the laboratory data file using the unique survey participant identifier (i.e., SEQN).
There are over 800 laboratory tests performed. 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 2017-2018, approximately 80% of children aged 1-17 years who were examined in the MEC provided a blood specimen through phlebotomy, while 95% 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.