National Health and Nutrition Examination Survey

The goals of this component are: 1) to monitor the prevalence and trends in major cardiovascular conditions, and overall risk factors in the U.S.; and 2) to evaluate prevention and treatment programs targeting cardiovascular disease in the U.S.

Blood lipid levels are the main elements of the cardiovascular disease laboratory in NHANES. Cardiovascular disease is the leading cause of death in the United States. The data will be used to monitor the status of hyperlipidemia, and the success of the National Cholesterol Education Program.

Participants aged 12 years and older who were examined in the morning session were eligible.

This method is based on the work by Wahlefeld using a lipoprotein lipase from microorganisms for the rapid and complete hydrolysis of triglycerides to glycerol followed by oxidation to dihydroxyacetone phosphate and hydrogen peroxide. The hydrogen peroxide produced then reacts with 4-aminophenazone and 4-chlorophenol under the catalytic action of peroxidase to form a red dyestuff (Trinder endpoint reaction). The color intensity of the red dyestuff formed is directly proportional to the triglyceride concentration and can be measured photometrically.

Triglycerides are fatty acid esters of glycerol that have three hydroxyl groups. Because they are insoluble in water, the triglycerides are transported with other more polar lipids. Elevated triglyceride measurements are associated with diabetes mellitus, pancreatitis, alcoholism, glycogen storage disease, hypothyroidism, nephrosis, pregnancy, use of oral contraceptives and gout. Triglyceride levels are decreased in hyperthyroidism, use of certain lipid-lowering drugs and malabsorption syndrome.

Refer to the Laboratory Method Files section for detailed laboratory procedure manual(s) of the methods used.

There were no changes to the lab method or lab site for this component in the NHANES 2013-2014 cycle. However, there was a change to the total cholesterol, HDL-cholesterol, and triglycerides lab equipment. In the 2011-2012 cycle, the three analytes were measured on the Roche modular P chemistry analyzer. In the 2013-2014 cycle, the three analytes were measured on the Roche modular P and Roche Cobas 6000 chemistry analyzers.

Triglycerides - MODP (March 2016)

Triglycerides - COBAS (March 2016)

Blood specimens were processed, stored, and shipped to University of Minnesota, Minneapolis, MN for analysis.

Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–30°C) conditions until they are shipped to University of Minnesota for testing.

The NHANES quality control and quality assurance protocols (QA/QC) 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 quality assurance evaluation during unscheduled visits to evaluate both the quality of the laboratory work and the quality-control 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 samples collected on “dry run” sessions. In addition, contract laboratories randomly perform repeat testing on 2% of all specimens.

NCHS developed and distributed a quality control protocol for all the contract laboratories which outlined the use of Westgard rules (Westgard et al, 1981) when running 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.

All QC procedures recommended by the manufacturers were followed. Reported results for all assays meet the University of Minnesota’s quality control and quality assurance performance criteria for accuracy and precision.

The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.

Three derived variables were created in this data file. The variables were created using the following formula:

LBDTRSI

The triglycerides value in mg/dL (LBXTR) was converted to mmol/L (LBDTRSI) by multiplying by 0.01129.

LBDLDL

Serum LDL-cholesterol levels were derived on examinees that were examined in the morning session only. The distribution of serum LDL-cholesterol should be estimated only on examinees aged 12 and above who fasted at least 8.5 hours or more but less than 24 hours in the morning session. LDL-cholesterol is calculated from measured values of total cholesterol, triglycerides, and HDL-cholesterol according to the Friedewald calculation:

[LDL-cholesterol] = [total cholesterol] – [HDL-cholesterol] – [triglycerides/5]

where all values are expressed in mg/dL. The calculation is valid for triglycerides less than or equal to 400 mg/dL.

LBDLDLSI

The LDL-cholesterol in mg/dL (LBDLDL) was converted to mmol/L (LBDLDLSI) by multiplying by 0.02586.

Refer to the 2013-2014 Laboratory Data Overview for general information on NHANES laboratory data.

Sampling Weights

Triglycerides were measured in a fasting subsample of persons 12 years and older. Special sample weights are required to analyze these data properly. Specific sample weights for this subsample are included in this data file and should be used when analyzing these data.

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2011-2012 Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample weight 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, the time of venipuncture, and the conditions precluding venipuncture.

The laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier (i.e., SEQN).

LBXTR and LBXLDL

Serum levels were measured for examinees that were examined in the morning session only. The distribution of serum triglycerides should be estimated only on examinees aged 12 and above who fasted at least 8.5 hours or more but less than 24 hours.
The Laboratory TRIGLY_H data file contains laboratory test results for triglycerides (LBXTR), which uses the reference analytic method. The NHANES Biochemistry Profile also includes measurements of triglycerides. The Biochemistry Profile variable name is LBXSTR. The appropriate variable to use from the TRIGLY_H data file is LBXTR.

Detection Limits

The detection limits were constant for this analyte in the data set. The variable prefixed LBX (ex., LBXTR) provides the analytic result for that analyte.
The lower limit of detection (LLOD, in mg/dL) for triglycerides:

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.