The NHANES program suspended field operations in March 2020 due to the coronavirus disease 2019 (COVID-19) pandemic. As a result, data collection for the NHANES 2019-2020 cycle was not completed. Data collected in 2019-March 2020 can be accessed as convenience samples through the NCHS Research Data Center (RDC). Any analyses based solely on the 2019-March 2020 data would not be generalizable to the U.S. civilian non-institutionalized population. Please refer to the Analytic Notes section for more details on the use of the data.
Normal human hemoglobin is made up of about 98% hemoglobin A (HbA) (NIH, 2020). Some people have both HbA and another type of hemoglobin, such as hemoglobin S, C, D, E or F. These less common forms of hemoglobin are called hemoglobin variants, or hemoglobinopathies and are some of the most common genetically inherited disorders.
Hemoglobin S (Sickle Cell), E, C, and D; listed in the order of prevalence where sickle cell (HbS) is the most common hemoglobin variant (Little and Roberts, 2009). Fetal hemoglobin (HgF) is the major hemoglobin during gestation, but wanes to around 1% by the end of the first year after birth. HbF can remain elevated with certain pathologic or hereditary conditions (Little and Roberts, 2009). Clinically significant interferences have been seen with some hemoglobin A1c methods for each of these variants and HbF (Little and Roberts, 2009).
All examined participants aged 12 years and older in the NHANES 2019-March 2020 convenience sample were eligible.
Detected hemoglobin variants were tested on the Sebia Cappillary2 Flex Piercing Hemoglobin Electrophoresis Assay, which uses the principle of capillary electrophoresis in free solution. This assay uses eight (8) capillaries to run the samples. With this technique, charged molecules are separated by their electrophoresis mobility in an alkaline buffer with a specific pH. Separation also occurs according to the electrolyte pH and electro osmotic flow.
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
This is a new component in the 2019-2020 survey cycle.
Hemoglobin Variants (March 2024)
Whole blood specimens were processed, stored, and shipped to the University of Missouri, Columbia, MO for analysis.
Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials were stored under appropriate frozen (–30°C) conditions until they were shipped to the University of Minnesota for testing.
The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Amendments mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.
Mobile Examination Centers (MECs)
Laboratory team performance is monitored using several techniques. NCHS and contract consultants use a structured competency assessment evaluation during visits to evaluate both the quality of the laboratory work and the QC procedures. Each laboratory staff member is observed for equipment operation, specimen collection and preparation; testing procedures and constructive feedback are given to each staff member. Formal retraining sessions are conducted annually to ensure that required skill levels were maintained.
Analytical Laboratories
NHANES uses several methods to monitor the quality of the analyses performed by the contract laboratories. In the MEC, these methods include performing blind split specimens collected on “dry run” sessions. In addition, contract laboratories randomly perform repeat testing on 2% of all specimens.
NCHS developed and distributed a QC protocol for all CDC and contract laboratories, which outlined the use of Westgard rules (Westgard et al., 1981) when testing NHANES specimens. Progress reports containing any problems encountered during shipping or receipt of specimens, summary statistics for each control pool, QC graphs, instrument calibration, reagents, and any special considerations are submitted to NCHS quarterly. The reports are reviewed for trends or shifts in the data. The laboratories are required to explain any identified areas of concern.
The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.
The COVID-19 pandemic required suspension of NHANES 2019-2020 field operations in March 2020 after data were collected in 18 of the 30 survey locations in the 2019-2020 sample. Data collection was cancelled for the remaining 12 locations. Calculation of survey weights for this partial cycle is not possible due to incomplete data collection. Therefore, data from survey components that were only collected in 2019-March 2020 are made available as convenience samples through NCHS's Research Data Center (RDC) because unbiased estimates for the NHANES target population cannot be produced with these samples.
For survey components conducted in both 2017-2018 and 2019-2020 cycles, data collected from 2019 to March 2020 were combined with data from 2017 to 2018 to form a nationally representative sample of NHANES 2017-March 2020 pre-pandemic data. Please see the NHANES 2017-March 2020 pre-pandemic data page for detailed information on this combined sample.
Refer to the 2019-2020 Laboratory Data Overview documents for general information on NHANES laboratory data.
There are over 800 laboratory tests performed on NHANES participants. 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 2019-2020, 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 be aware of this and 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 as needed.
Demographic and Other Related Variables
The analysis of NHANES laboratory data may require additional demographic variables. The NHANES 2019-March 2020 Demographics File contains demographic data, health indicators, and other related information collected during household interviews.
The laboratory data file can be linked to the Demographics file and other NHANES data files in the 2019-March 2020 convenience sample using the unique survey participant identifier (i.e., SEQN).
Detection Limits
Since the data is semi-quantitative and reported in percentages, the use of lower limits of detection isn’t applicable. For the purposes of this study, presumptive hemoglobin variants S, C, D, and E (abnormal variant observed), and elevated HbF (over 2%), initially screened by the G8 HPLC method and reflexed to the Capillary 2 Flex Piercing will be reported as percentages.
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 28 to 92.9 | Range of Values | 36 | 36 | |
| . | Missing | 3972 | 4008 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| . | Missing | 4008 | 4008 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 20.2 to 25.7 | Range of Values | 7 | 7 | |
| . | Missing | 4001 | 4008 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0.3 to 27.9 | Range of Values | 39 | 39 | |
| . | Missing | 3969 | 4008 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 26.3 to 51.1 | Range of Values | 100 | 100 | |
| . | Missing | 3908 | 4008 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 1 to 57.7 | Range of Values | 12 | 12 | |
| . | Missing | 3996 | 4008 |