National Health and Nutrition Examination Survey

Diabetes is a leading cause of disease and death in the United States. Approximately eight million Americans are known to have diabetes, and it is estimated that an equal amount have undiagnosed diabetes. In 1993, nearly 18 percent of all deaths for persons over the age of 25 were among people with diabetes. The prevalence of diabetes and overweight (one of the major risk factors for diabetes) continues to increase. Substantial new efforts to prevent or control diabetes have begun, including the Diabetes Prevention Trial and the National Diabetes Education Program.

Diabetes testing provides population data to: 1) determine a national estimate of diabetes disease prevalence (diagnosed and undiagnosed); 2) identify the risk factors of diabetes disease; 3) permit a national cohort to be established for follow-up studies of this condition; and 4) provide critical information to clinicians and public health officials for the development of preventive care and community-based interventions.

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

Glucose
In this enzymatic method glucose is converted to glucose-6-phosphate (G-6-P) by hexokinase in the presence of ATP, a phosphate donor. Glucose-6-phosphate dehydrogenase then converts the G-6-P to gluconate-6-P in the presence of NADP+. As the NADP+ is reduced to NADPH during this reaction, the resulting increase in absorbance at 340 nm (secondary wavelength = 700 nm) is measured. This is an endpoint reaction that is specific for glucose.

There are seven exclusion criteria, including hemophilia and chemotherapy safety exclusions, fasting less than nine hours, taking insulin or oral medications for diabetes, refusing phlebotomy, or not drinking the entire Trutol^TM solution within the allotted time.

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

There were changes to the lab method, lab equipment, and lab site for this component in the NHANES 2013-2014 cycle. In the 2011-2012 cycle, the University of Minnesota performed this testing. The University of Missouri-Columbia began testing plasma glucose in the 2013-2014 cycle.

Fasting Glucose (January 2016)

Plasma specimens were processed, stored, and shipped to the University of Missouri-Columbia, Columbia, MO for analysis.

Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–70°C) conditions until they are shipped to University of Missouri-Columbia 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 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 Missouri’s quality control and quality assurance performance criteria for accuracy and precision, similar to the Westgard rules.

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

Two variables were created in this data file. The variables were created using the following formulas:

LBXGLU and LBDGLUSI:
The fasting glucose value in mg/dL (LBXGLU) was converted to mmol/L (LBDGLUSI) by multiplying by 0.05551 (rounded to 3 decimals).

LBXIN and LBDINSI:
The insulin value in µU/mL (LBXIN) was converted to pmol/L (LBDINSI) by multiplying by 6.0 (rounded to 2 decimals).

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

Sampling Weights

Glucose and insulin were measured in a fasting subsample of persons 12 years and older.

The analyst is strongly encouraged to use the fasting sampling weights in this file to analyze 2013-2014 glucose and insulin levels.

There will be two weight files associated with the subsample for the diabetes data. Use the fasting sample weights (WTSFA2YR) when analyzing the fasting glucose and insulin levels only. Use the OGTT sample weights (WTSOG2YR) when analyzing the insulin, fasting AND OGTT glucose levels or when analyzing ONLY OGTT glucose levels. NOTE: the Insulin and OGTT weights and data are in separate files (INS_H and OGTT_H, respectively).

Reporting Glucose Results

The University of Missouri’s Data File (GLU_H) (which contains laboratory test results for glucose - LBXGLU) was measured using the reference analytic method. However, the Iowa laboratory (BIOPRO_H), that measures biochemistry profiles, also included measurements of serum glucose. The serum glucose values (LBXSGL) reported in the Iowa lab should not be used to determine undiagnosed diabetes or prediabetes. Instead, plasma glucose values from the University of Missouri’s lab (LBXGLU) should be used for data analysis.

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2013-2014 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, 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).

This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier SEQN.

Detection Limits

The detection limits were constant for the analyte in the data set. The variable prefixed LBX (ex., LBXGLU) provides the analytic result for that analyte.

The lower limit of detection (LLOD, in mg/dL) for fasting glucose:

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.