• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Data Processing and Editing
• Laboratory Quality Assurance and Monitoring
• Analytic Notes
• Codebook
  • SEQN - Respondent sequence number
  • WTSAF2YR - Fasting Subsample 2 Year MEC Weight
  • LBXGLU - Fasting Glucose (mg/dL)
  • LBDGLUSI - Fasting Glucose (mmol/L)
  • LBXIN - Insulin (uU/mL)
  • LBDINSI - Insulin (pmol/L)
  • PHAFSTHR - Total length of 'food fast', hours
  • PHAFSTMN - Total length of 'food fast', minutes

National Health and Nutrition Examination Survey

2011-2012 Data Documentation, Codebook, and Frequencies

Plasma Fasting Glucose & Insulin (GLU_G)

Data File: GLU_G.xpt

First Published: January 2014

Last Revised: August 2016

Component Description

Diabetes mellitus was assessed by measures of fasting plasma glucose, two-hour glucose (OGTT) and serum insulin in participants aged 12 years and over in the morning (AM) examination session only. Glycohemoglobin measures are also available for a full sample.

Diabetes is a leading cause of disease and death in the United States. Eight million Americans are known to have diabetes, and an approximately equal number 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) continue to increase. Substantial new efforts to prevent or control diabetes have begun, including the Diabetes Prevention Trial and the National Diabetes Education Program.

Eligible Sample

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

Description of Laboratory Methodology

Blood specimens were processed, stored and shipped to Fairview Medical Center Laboratory at the University of Minnesota, Minneapolis Minnesota for analysis.

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.

Insulin
Insulin is the primary hormone responsible for controlling glucose metabolism, and its secretion is determined by plasma glucose concentration. The insulin molecule is synthesized in the pancreas as pro-insulin and is later cleaved to form C-peptide and insulin. The principal function of insulin is to control the uptake and utilization of glucose in the peripheral tissues. Insulin concentrations are severely reduced in insulin-dependent diabetes mellitus (IDDM) and some other conditions, while insulin concentrations are raised in non-insulin-dependent diabetes mellitus (NIDDM), obesity, and some endocrine disorders.

The Elecsys 2010 Insulin chemiluminescent “sandwich” immunoassay employs two monoclonal antibodies which together are specific for human insulin. During the first incubation,: Insulin from a 20 μL sample, a biotinylated monoclonal insulin-specific antibody, and a monoclonal insulin-specific antibody labeled with a ruthenium complex (Tris(2,2'-bipyridyl)ruthenium(II)-complex (Ru(bpy)) form a sandwich complex. During the second incubation, after the addition of streptavidin-coated microparticles, the complex becomes bound to the solid phase via interaction of biotin and streptavidin.

The reaction mixture is aspirated into the measuring cell where the microparticles are magnetically captured onto the surface of the electrode. Unbound substances are then removed with ProCell. Application of a voltage to the electrode then induces chemiluminescent emission which is measured by a photomultiplier. The amount of light produced is directly proportional to the amount of insulin in the sample.

Oral Glucose Tolerance Test

An oral glucose tolerance test (OGTT) continued. A fasting glucose blood test was performed on all participants 12 years and older, who were examined in the morning session after a 9 hour fast. After the initial venipuncture, participants were asked to drink a calibrated dose (generally 75 grams of glucose) of TrutolTM and had a second venipuncture 2 hours (plus or minus 15 minutes) after drinking the Trutol^TM.

There are seven exclusion criteria, including hemophilia and chemotherapy safety exclusions, fasting < 9 hours, taking insulin or oral medications for diabetes, refusing phlebotomy, and not drinking all the entire Trutol solution within the allotted time.

There were no changes (from the previous 2 years of NHANES) to equipment, lab methods, or lab site.

Detailed specimen collection and processing instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM).

Data Processing and Editing

Two calculated variables were created in this data file. The formula for their creation is as follows:

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). 
 

Laboratory Quality Assurance and Monitoring

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 Laboratory/Medical Technologists Procedures Manual (LPM).

Analytic Notes

Change in methods for serum insulin

In 2009-2010, there was a change in methods for serum insulin. The insulin was performed 2005-2009 using the Mercodia sandwich ELISA assay and was switched in late 2009 to a Roche chemiluminescent immunoassay performed on the Elecsys 2010 analyzer. A crossover study of 346 specimens was performed between the two methods and the Mercodia insulin values had a higher mean and median compared to the Roche assay by approximately 7%. This was also seen in the sample participants when comparing Mercodia to Roche insulin values.

To trend insulin [LBXIN (uU/ml] from 2011-2012 to match previous NHANES cycles, the following “backward” fractional polynomial regression should be applied to 2011-2012 insulin values:

Insulin (Mercodia-equivalent) = 0.6295 + [1.0770*Insulin (Roche)] – [0.0008566*Insulin(Roche)**2].

To trend insulin [LBXIN (uU/ml] from previous NHANES cycles to match 2011-2012, the following “forward” fractional polynomial regression should be applied to previous NHANES cycles’ insulin values:

Insulin (Roche-equivalent) = 0.8868*Insulin (Mercodia) + [0.0011*Insulin (Mercodia)**2] – 0.0744.

Reporting Glucose Results

The Minnesota Laboratory Data File (GLU_G) (which contains laboratory test results for glucose - LBXGLU) was measured using the reference analytic method. However, the Iowa laboratory (BIOPRO_G), 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 Minnesota Lab (LBXGLU) should be for data analysis.

Refer to the 2011-2012 Laboratory Data Overview for general information on NHANES laboratory data.

The analysis of NHANES 2011-2012 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 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).

Sampling Weights

The analyst is strongly encouraged to use the fasting sampling weights in this file to analyze 2011-2012 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 OGTT weights and data are in a separate file (OGTT_G).

Exam sample weights should be used for analyses. Please refer to the NHANES Analytic Guidelinesand the on-line NHANES Tutorial for further details on the use of sample weights and other analytic issues.

Codebook and Frequencies