Clinical Lab Handbook

The LIAISON® QuantiFERON®-TB Gold Plus assay is an in vitro diagnostic test for the detection of interferon- (IFN-) in human lithium heparin plasma by chemiluminescence immunoassay (CLIA) using the LIAISON® XL Analyzer. QIAGEN QuantiFERON®-TB Gold Plus Blood Collection Tubes, containing a peptide cocktail simulating ESAT-6 and CFP-10 proteins, are used in conjunction with the LIAISON® QuantiFERON®-TB Gold Plus assay to stimulate cells in heparinized whole blood. Detection of IFN- is used to identify in vitro responses to these peptide antigens that are associated with Mycobacterium tuberculosis infection.

The assay is a qualitative indirect test for M. tuberculosis infection (including disease) and is intended for use in conjunction with risk assessment, radiography, and other medical and diagnostic evaluations to assist the clinician in making individual patient management decisions. The LIAISON® QuantiFERON®-TB Gold Plus assay must be performed using the LIAISON® XL Analyzer.

Tuberculosis (TB) is a communicable disease, transmitted almost exclusively by cough aerosols carrying pathogens of the M. tuberculosis complex. TB continues to be a major public health threat, causing an estimated 10.4 million new cases and 3 million deaths from TB in 2016. Pathogenesis is characterized by a period of asymptomatic subclinical infection, defined broadly as latent tuberculosis infection (LTBI), which might last for weeks or decades. However, there is no diagnostic gold standard for LTBI. Two tests are available for the identification of LTBI: the tuberculin skin test (TST) and the interferon gamma release assay (IGRA). They represent indirect markers of M. tuberculosis exposure and indicate a cellular immune response to M. tuberculosis.

Test Result Comments:

Negative: No interferon-gamma response to Mycobacterium tuberculosis antigens was detected in either of the TB1 or TB2 tubes. Latent infection with M. tuberculosis is unlikely; however, a single negative result does not exclude infection with M. tuberculosis. In patients at high risk for M. tuberculosis infection, a second test should be considered in accordance with References 1and 2 (see below). When interpreting QuantiFERON-TB Gold plus results for the diagnosis or exclusion of tuberculosis disease and the assessment of the probability of latent infection, a combination of epidemiological, historical, medical, as well as diagnostic findings are required. Reference 1: Official ATS/IDSA/CDC Clinical Practice Guidelines for Diagnosis of Tuberculosis in Adults and Children [Lewinsohn DM et. al. Clin. Infect. Dis. 2017;64(2):111−115] Reference 2: www.cdc.gov/tb/

Positive: Interferon-gamma (IFN-gamma) response to Mycobacterium tuberculosis antigens was detected in either TB1 and/or TB2 tubes, suggesting infection with Mycobacterium tuberculosis.A positive result suggests infection with M. tuberculosis but does not distinguish latent infection from active disease. Positive results in patients at low-risk for TB should be interpreted with caution and repeat testing on a new sample should be considered. When interpreting QuantiFERON-TB Gold plus results for the diagnosis or exclusion of tuberculosis disease and the assessment of the probability of latent infection, a combination of epidemiological, historical, medical, as well as diagnostic findings are required (See References 1&2 below). Reference 1: Official ATS/IDSA/CDC Clinical Practice Guidelines for Diagnosis of Tuberculosis in Adults and Children [Lewinsohn DM et. al. Clin. Infect. Dis. 2017;64(2):111−115] Reference 2: www.cdc.gov/tb/

Indeterminate: Results are indeterminate for interferon-gamma responsiveness to M. tuberculosis antigens. It is recommended to repeat testing with new specimen. If TB is suspected, additional evaluations should be performed before or simultaneously with the repeat of AuantiFERON-TB Gold Plus Testing. Indeterminate results may be related to the status of the immune system of the patient. Reference 1: Official ATS/IDSA/CDC Clinical Practice Guidelines for Diagnosis of Tuberculosis in Adults and Children [Lewinsohn DM et. al. Clin. Infect. Dis. 2017;64(2):111−115] Reference 2: www.cdc.gov/tb/

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Processing Instructions

Tubes may be stored (up to 16 hours post collection) at room temperature; do not refrigerate or freeze the blood samples.

Incubation

1. Each of the four tubes must be transferred to a 37° C +/- 1° C incubator as soon as possible and within 16 hours of collection for 16-24 hours. If the blood is not incubated immediately after collection, re-mixing of the tubes by inverting 10 times must be performed immediately prior to incubation. The specimen rack should sit off the floor of the incubator and the tubes should be upright.

Post Incubation

1. Following Incubation: Centrifuge each of the 4 incubated collection tubes for 5-15 minutes at 2000- 3000 RCF(G). The gel plug should separate the cells from the plasma. If this does not occur, the tubes should be re-centriguged.
2. Samples will be stable for 3 days at 4-27C prior to centrifugation
3. After centrifugation, the plasma must be stored in original collection tubes. Plasma should be stored at 2–8°C no longer than 28 days.
4. Transport in original collection tubes at a refrigerated temperature.

Note: After centrifugation, avoid pipetting up or down or mixing prior to harvesting.

Collection Site Instructions

1. Order of Draw:
2. QFT-Plus tubes should be considered as Heparin for "Order of Draw"
3. Order of draw for the collection kit is Grey->Green->Yellow->Purple
4. The use of butterfly needles for collection of QFT-Plus collection should be avoided whenever possible. If a "butterfly needle" is needed, than a "purge" tube should be used to ensure that the tubing is filled with blood prior to the QFT-PLUS tubes being used.
5. For each patient, collect 1 mL of blood by venipuncture directly into each of the 4 QFT-PLUS blood collection tubes (tubes need to be at room temperature at time of draw). As 1mL tubes draw relatively slowly, keep the tube on the needle for 2-3 seconds once the tube has completed filling. Fill volume should fall within the black markings on the outside of the tube. If the level of blood is outside of the indicator mark, a new sample should be obtained.
6. Immediately after filling the tubes, shake them 10 times just firmly enough to ensure the entire surface of the tube is coated with blood, to solubilize antigens on tube walls. Motion should be similar to a "firm hand shake" or a "karate chop".

NOTE: Under or over filling the tubes may lead to erroneous results. Vigorous mixing may cause gel to disrupt and could lead to abnormal results.

Additional Resource: QuantiFERON-TB Gold Plus Poster Blood Draw Critical Points WW PROM-21864-001.pdf