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Useful For
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The purpose of this assay is to perform an in-vitro test on heparinized whole blood using a peptide cocktail that simulates ESAT-6 and CFP-10 proteins to stimulate cells. The detection of interferon-γ (IFN-γ) via enzyme-linked immunosorbent assay (ELISA) is used to identify in vitro responses to these peptide antigens that are linked to Mycobacterium tuberculosis infection.
However, it is important to note that the results obtained from QFT-Plus testing should be considered alongside an individual's epidemiological history, current medical status, and other diagnostic evaluations.
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Method name and description
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Method name: Enzyme-Linked Immunosorbent Assay (ELISA).
The QTF-Plus system involves using specialized blood collection tubes to collect whole blood, which is then incubated in the tubes for 16 to 24 hours. After incubation, the plasma is harvested and tested for the presence of IFN-γ produced in response to peptide antigens. The QTF-Plus test is conducted in two stages, beginning with the collection of whole blood into each of the QTF-Plus blood collection tubes, including the Nil Control tube, TB1 tube, TB2 tube, and Mitogen tube. The Mitogen tube serves as a positive control, especially in cases where an individual's immune status is uncertain, as well as a control for proper blood handling and incubation. The tubes should be incubated at 37ºC as soon as possible and within 16 hours of collection. Following the incubation period, the tubes are centrifuged, and the plasma is separated and measured for the amount of IFN-γ in IU/mL using ELISA. The QTF-Plus microplate strips, which are coated with anti-human IFN-γ monoclonal antibody, bind to the anti-human IFN-γ conjugate complex. After incubation, the non-bound conjugate is washed away, and the bound complex is incubated with the enzyme substrate solution. The reaction is stopped, and the optical density (OD) in the mixture is measured. To interpret the test results, the response for patient samples is directly compared to the calibration curve.
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Clinical information
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Tuberculosis (TB) is an infectious disease caused by the Mycobacterium tuberculosis complex (M. tuberculosis, M. bovis, and M. africanum), which is transmitted through airborne droplet nuclei from individuals with respiratory TB disease. While some individuals can develop active TB disease within weeks to months of being infected, most individuals remain asymptomatic. However, some individuals with latent TB infection can develop TB disease months or years later. The primary objective of diagnosing latent TB infection is to initiate medical intervention to prevent TB disease.
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Aliases
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Quantiferon TB
Quantiferon TB Gold Plus
Latent TB
Latent Tuberculosis
M. tuberculosis
IGRA
Interferon-gamma release assay
QuantiFERON
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Specimen type / Specimen volume / Specimen container
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Whole blood samples (1 ml per tube) collected in four tubes supplied with the QF kit as follow:
Four special tubes are:
Nil tube (Grey cap)
TB1tube (Green cap)
TB2 tube (Yellow cap)
Mitogen tube (Purple cap)
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Collection instructions / Special Precautions / Timing of collection
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The laboratory provides four special tubes for collecting samples for TB testing (Quantiferon). It is important to follow the specific guidelines for collecting the samples, including collecting exactly 1 ml of blood as indicated by the black mark on the side of the tubes.
The tubes must be transferred to the laboratory as soon as possible and within 12 hours of collection, and should not be refrigerated or frozen. If the blood cannot be incubated immediately after collection, the tubes should be mixed again just prior to incubation.
The tubes should be incubated upright at 37ºC for 16 to 24 hours. Any deviation from these instructions may affect the accuracy of the result.
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Storage and transport instructions
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The blood collection tubes should be transported to the laboratory promptly, ideally within 12 hours of collection. Refrigerating or freezing the blood samples is not recommended.
If incubation is not possible immediately after collection, it is necessary to repeat the mixing of the tubes immediately before incubation.
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Specimen Rejection Criteria
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Overload volume
QNS
Wrong Collection
Refrigerate or freeze the blood samples
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Biological reference intervals and clinical decision values
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| Nil (IU/ml) |
TB1 minus Nil (IU/ml) |
TB2 minus Nil (IU/ml) |
Mitogen minus Nil (IU/ml) |
QFT-Plus Result |
Interpretation |
| ≤ 8.0 |
≥0.35 and ≥25% Of Nil value |
Any |
Any |
Positive |
M. tuberculosis infection likely |
| Any |
≥0.35 and ≥25% Of Nil value |
| <0.35 or ≥0.35 and <25% Of Nil value |
<0.35 or ≥0.35 and <25% Of Nil value |
≥0.5 |
Negative |
M. tuberculosis infection NOT likely |
| <0.35 or ≥0.35 and <25% Of Nil value |
<0.35 or ≥0.35 and <25% Of Nil value |
< 0.5 |
Indeterminate |
Results are indeterminate for TB Antigen responsiveness |
| > 8.0 |
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Any |
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Indeterminate |
Results are indeterminate for TB Antigen responsiveness |
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Factors affecting test performance and result interpretation
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- This test is not recommended for use for diagnosis of active tuberculosis infection.
- Deviations from the procedure (blood collection, handling of the blood samples, volume and incubation).
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Turnaround time / Days and times test performed / Specimen retention time
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Turnaround Time: 48 hours.
Days and Times test performed: sample collected from Sunday through Wednesday from 7:00 to 13:00.
Specimen retention time: Approximately 1 day.
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