• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Data Processing and Editing
• Laboratory Quality Assurance and Monitoring
• Analytic Notes
• Codebook
  • SEQN - Respondent sequence number
  • LBXTBA - TB Antigen result
  • LBXTBN - TB Nil control result
  • LBXTBIN - TB coded result
  • LBXTBM - TB Mitogen control result

National Health and Nutrition Examination Survey

2011-2012 Data Documentation, Codebook, and Frequencies

Tuberculosis - Quantiferon_In_Gold (TB_G)

Data File: TB_G.xpt

First Published: November 2013

Last Revised: NA

Component Description

TB skin testing was added to the NHANES 2011–2012 to provide comprehensive data about the extent of tuberculosis infection in the United States. To determine the prevalence of TB infection, NHANES participants 6 years of age and older who consented to this component were skin tested with a tuberculin-purified protein derivative (PPD) product, tubersol, a commercially available antigen. Additionally, NHANES participants were secondarily screened with an FDA-approved blood test, QuantiFERON^®-TB Gold In Tube test (QFT-GIT), for TB infection.

To aid in the interpretation of skin test results and provide for risk factor analysis, participants were asked questions about TB skin testing, disease, exposure to and treatment for Mycobacterium tuberculosis. These questions were asked in the household interview prior to laboratory screening.

Eligible Sample

Participants aged 6 year and older, who do not meet any of the exclusion criteria, weare eligible.

Description of Laboratory Methodology

The QuantiFERON^®-TB Gold IT test is a test for Cell Mediated Immune (CMI) responses to petide antigens that simulate mycobacterial proteins. These proteins, ESAT-6, CFP-10, and TB7.7 are absent from all BCG strains and from most non-tuberculosis mycobacteria with the exception of M. kansasii, M. szulgai and M. marinum. Individuals infected with M. tuberculosis complex organisms (M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti) usually have lymphocytes in their blood that recognize these and other mycobacterial antigens. This recognition process involves the generation and secretion of the cytokine, IFN-γ. The detection and subsequent quantification of IFN-γ forms the basis of this test.

The QuantiFERON^®-TB Gold IT system uses specialized blood collection tubes, which are used to collect whole blood via venipuncture, which include a Nil control tube, TB Antigen tube and a Mitogen tube (positive control). The tubes are shaken to mix antigen with the whole blood and incubated at 37° C + 1° C for 16 to 24 hours. Following the incubation period, plasma is harvested and the amount of IFN-γ produced in response to the peptide antigens is measured by ELISA. Results for the test samples are reported in International Units (IU) relative to a standard curve prepared by testing dilutions of a recombinant human IFN-γ standard.

Heterophile antibodies in plasma of certain individuals are known to cause interference with immunoassays. The effect of heterophile antibodies in the QuantiFERON^®-TB Gold IT ELISA is minimized by the addition of normal mouse serum to the green diluent and the use of F(ab’)2 monoclonal antibody fragments as the IFN-γ capture antibody coated the microplate wells.

A test is considered positive for an IFN-γ response to the TB Antigen tube that is significantly above the Nil IFN-γ IU/mL value. The Nil samples adjust for background, heterophile antibody effects, or non-specific IFN-γ in blood samples. The mitogen stimulated plasma sample serves as an IFN-γ positive control for each specimen tested. A low response to mitogen (<0.5 IU/mL) indicates an Indeterminate result when a blood sample also has a negative response to the TB antigens. This pattern may occur with insufficient lymphocytes, reduced lymphocyte activity due to prolonged specimen transport or improper specimen handling, including filling/mixing of the blood tubes, or inability of the patient’s lymphocytes to generate IFN-γ. Elevated levels of IFN-γ in the Nil sample may occur with the presence of heterophile antibodies, or to intrinsic IFN-γ secretion.

In the NHANES Mobile Examination Center (MEC) on the same day each eligible participant was TB skin tested he/she was given a blood test to screen for tuberculosis infection. For each subject, 1 mL of blood was collected by venipuncture directly into each of three QFT GIT blood collection tubes in the order Nil (grey cap), TB Antigen (red cap), Mitogen (purple cap). In order to mix the tubes appropriately, the tubes were shaken 10 times firmly enough to ensure the entire inner surface of tube was coated in blood. Each tube (Nil, TB Antigen, Mitogen) was identifiable by its label once the cap was removed.

The tubes were transferred to a 37°C ± 1°C incubator as soon as possible, and within 16 hours of collection. Prior to incubation, tubes were maintained at room temperature (22°C ± 5°C). If the tubes were not incubated immediately after collection, they were mixed again immediately prior to incubation. The tubes were incubated upright.

After incubation of the blood with antigens for 16 to 24 hours and in conjunction with a regularly scheduled shipment, specimens were sent to a contracted laboratory on frozen refrigerant packs or dry ice where the QFT-GIT test for tuberculosis infection was conducted.

Data Processing and Editing

Whole blood samples were sent to the University of Washington, Seattle, Washington for TB_QFT analysis.

Detailed specimen collection and processing instructions are discussed in the NHANES LPM. Vials are stored under appropriate frozen (–20°C) conditions until they are shipped to National Center for Environmental Health for testing.

Laboratory Quality Assurance and Monitoring

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 Laboratory/Medical Technologists Procedures Manual (LPM). Read the LABDOC file for detailed QA/QC protocols.

Analytic Notes

The following criteria must be met for the QuantiFERON-TB Gold assay to be interpreted as reactive:

LBXTBN (Nil) value must be =< 8.0 IU gamma interferon (IF)/ml
LBXTBA (TB antigen value) minus LBXTBN (Nil) value must be => 0.35 IU gamma interferon (IF)/ml AND
LBXTBA (TB antigen value) minus LBXTBN (Nil value) must be => 25% of the LBXTBN (Nil) value.

 

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).

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.

Codebook and Frequencies