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 and the collected data are not nationally representative. Therefore, data collected from 2019 to March 2020 were combined with data from the NHANES 2017-2018 cycle to form a nationally representative sample of NHANES 2017-March 2020 pre-pandemic data. These data are available to the public. Please refer to the Analytic Notes section for more details on the use of the data.
Heart disease is the leading cause of death in the United States (Murphy, et. al., 2018). Blood lipid levels are fundamental measures included in NHANES that can be used for cardiovascular risk assessment. The goals the NHANES blood lipid measurements include: 1) monitoring the prevalence and trends in major cardiovascular conditions and overall risk factors in the U.S.; 2) evaluating prevention and treatment programs targeting cardiovascular disease in the U.S.; and 3) monitoring the status of hyperlipidemia.
In 2018, new Blood Cholesterol Guidelines were released, by the American College of Cardiology and American Heart Association Task Force on Clinical Practice Guidelines, which aim to reduce the risk of atherosclerotic cardiovascular disease through cholesterol management (Grundy, et. al., 2018). The blood lipids measurements in NHANES include total cholesterol, high-density lipoprotein cholesterol (HDL-C), low-density lipoproteins cholesterol (LDL-C), and triglycerides. The present file provides data on total cholesterol. Data on LDL-C and triglycerides are provided in the Cholesterol – Low – Density Lipoprotein and Triglycerides (P_TRIGLY) files, and HDL-C data are provided in Cholesterol - High - Density Lipoprotein (P_HDL).
All examined participants 6 years and older in the NHANES 2017-March 2020 pre-pandemic sample were eligible.
Total Cholesterol
The laboratory method used to measure total cholesterol is an enzymatic assay. In this enzymatic assay, esterified cholesterol is converted to cholesterol by cholesterol esterase. The resulting cholesterol is then acted upon by cholesterol oxidase to produce cholest-4-en-3-one and hydrogen peroxide. The hydrogen peroxide then reacts with 4-aminophenazone in the presence of peroxidase to produce a colored product that is measured at 505 nm (secondary wavelength = 700 nm). The final step is known as the Trinder reaction. This method is a single reagent, endpoint reaction that is specific for cholesterol.
Cholesterol, a steroid molecule with a hydroxyl group in the C3 position, is synthesized in many types of tissue, but mainly in the liver and intestinal wall. About 75 percent of cholesterol is newly synthesized, with the remainder originating from dietary intake. Cholesterol measurement is performed to screen for atherosclerotic risk and in the diagnosis and treatment of disorders involving elevated cholesterol as well as lipid and lipoprotein metabolic disorders.
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
Serum total cholesterol levels were directly measured. Please see below the Data Processing and Editing section for more details. For laboratory methods used for low-density lipoproteins cholesterol (LDL-C) and triglycerides, and HDL-C, please refer to the accompanying documentation: P_TRIGLY and P_HDL.
Total Cholesterol (Frozen) Laboratory Procedure Manual (February 2020)
Total Cholesterol (Frozen) Laboratory Procedure Manual (August 2021)
Serum specimens were processed, stored, and shipped to the University of Minnesota, Minneapolis, MN for analysis.
Detailed instructions on specimen collection and processing are discussed in the NHANES 2017-2018 and 2019-2020 Laboratory Procedures Manuals (LPM). Vials are stored under appropriate frozen (–30°C) conditions until they are shipped to University of Minnesota for testing.
The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Act 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.
One derived variable was created in this data file. The variable was created using the following formula:
LBDTCSI
The total cholesterol in mg/dL (LBXTC) was converted to mmol/L (LBDTCSI) by multiplying by 0.02586.
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. Because the collected data from 18 locations were not nationally representative, these data were combined with data from the previous cycle (2017-2018) to create a 2017-March 2020 pre-pandemic data file. A special weighting process was applied to the 2017-March 2020 pre-pandemic data file. The resulting sample weights in the demographic data file should be used to calculate estimates from the combined cycles. These sample weights are not appropriate for independent analyses of the 2019-2020 data and will not yield nationally representative results for either the 2017-2018 data alone or the 2019-March 2020 data alone. Please refer to the NHANES website for additional information for the NHANES 2017-March 2020 pre-pandemic data, and for the previous 2017-2018 public use data file with specific weights for that 2-year cycle.
Refer to the 2017-2018 and 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 2017-March 2020, approximately 76% 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.
Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for further details on the use of sample weights and other analytic issues.
Total Cholesterol Values from the NHANES 2017–March 2020 Pre-Pandemic Sample
The laboratory that performed total cholesterol assays for NHANES samples from 2017 to March 2020 participated in the CDC Lipid Standardization Program (LSP) that monitors the quality of laboratory results over time. The LSP requires comparing assay results to a CDC control pool to assess lab result quality. Acceptable performance for total cholesterol values is defined as meeting criteria for maximum allowable bias (within 3% of the CDC reference value) and maximum allowable standard deviation (within 4.0 mg/dL for values 100-149.9 mg/dL and 3% of the CDC reference value for values above 150 mg/dL). In 2019, the mean bias reported by the laboratory exceeded the criterion of 3% mean allowable bias (the maximum quarterly bias was -4.7%). The underlying cause of the downward drift in assay performance most likely resulted from reagent and calibration variation. NHANES evaluated whether this amount of drift could result in a downward shift in the population distribution for 2017-March 2020 estimates of high total cholesterol. The results indicated that there were minimal differences between prevalence estimates of high total cholesterol based on the original total cholesterol values and those based on values adjusted to compensate for the downward bias. For example, the crude prevalence of high total cholesterol among adults 20 years and older was 10.05% using original lab values versus 10.09% using adjusted values. For subgroups for sex and age, the crude prevalence estimates did not differ by more than 0.1 percentage points. Therefore, unadjusted laboratory values for total cholesterol for 2017-March 2020 were released in the dataset.
Demographic and Other Related Variables
The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2017-March 2020 Pre-Pandemic Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample design variables. The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.
The Fasting Questionnaire File includes auxiliary information, such as fasting status, length of fast, and the time of venipuncture.
The laboratory data file can be linked to other NHANES data file using the unique survey participant identifier (i.e., SEQN).
Detection Limits
The detection limits were constant for this analyte in the data set.
The lower limit of detection (LLOD, in mg/dL) for total cholesterol:
|
Variable Name |
Analyte Description |
LLOD |
|
LBXTC |
Serum Total cholesterol (Frozen) |
4 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 71 to 446 | Range of Values | 10828 | 10828 | |
| . | Missing | 1370 | 12198 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 1.84 to 11.53 | Range of Values | 10828 | 10828 | |
| . | Missing | 1370 | 12198 |