What are the other Names for this Test? (Equivalent Terms)
- Interferon-gamma Release Assays (IGRA)
- Latent Tuberculosis Infection Test
- QuantiFERON®-TB Gold (QFT-G)
What is TB Screening QuantiFERON Test? (Background Information)
- QuantiFERON Test for tuberculosis is a type of Interferon-gamma Release Assay (IGRA’s). This test is used to check for a latent tuberculosis infection
- Latent tuberculosis (TB) infection is a non-communicable form of tubercular infection in which, the patient is not symptomatic. In latent tuberculosis, the TB infection persists for a long period of time (may last for years). When an individual’s immune system decreases due to a variety of reasons, the infection may become active and cause an active tuberculosis disease
- The QuantiFERON testing helps in the diagnosis of a latent tuberculosis infection. This could help in prompt intervention resulting in the preventionof an active TB disease.
The QuantiFERON Test is an in-vitro diagnostic test that measures an immune response of an individual. The immune response that is measured is a cell-mediated immune reactivity to mycobacterium tuberculosis. The principle behind the test is the release of gamma interferon chemical from a lymphocyte that has been sensitized to mycobacterium tuberculosis.
Individuals who have latent tuberculosis infection have circulating lymphocytes in the blood that are sensitized to mycobacterial antigens and hence during the test, these lymphocytes will release gamma interferon chemical.
- It is important to follow a positive QuantiFERON blood test with further testing of the individual to rule out an active tuberculosis infection. Further diagnostic intervention includes (but is not limited to); acid fast bacilli smear of sputum, culture for acid fast bacilli in the microbiology lab, or a routine chest x-ray
What are the Clinical Indications for performing the TB Screening QuantiFERON Test?
The QuantiFERON testing is part of the screening process for an individual, for any exposure to tuberculosis infection. Individuals who are at high risk of getting tuberculosis are screened. These include:
- Healthcare workers who come in contact with individuals having active tuberculosis infection
- Individuals diagnosed with HIV/AIDS
- Those who have a weakened immune system, due to chronic steroid therapy, use of immunosuppressive medications
- Individuals inhabiting confined living conditions, such as in refugee camps, correctional facilities, homeless shelters
- Those living/working in nursing homes, working at schools
- Individuals who are on illicit drugs, and such other substances
- Individuals from endemic areas for tuberculosis
- As part of a routine screening check of a new employee (job orientation process)
How is the Specimen Collected for TB Screening QuantiFERON Test?
Sample required: Blood
Process: Insertion of a needle into a vein (arm)
Preparation required: None
What is the Significance of the TB Screening QuantiFERON Test?
QuantiFERON Test results may be positive, negative, or indeterminate. The following are the interpretations of the results:
A positive test result means:
- Prior exposure to tuberculosis infection, either as a latent infection, or as an active TB infection
- Rarely, the QuantiFERON Test may be false positive. False positive test results can occur with individuals who have infection from other types of mycobacterium, such as mycobacterium szulgai, mycobacterium kansasii, and mycobacterium marinum
A negative test result means:
- Non-exposure to tuberculosis infection in the past, or rarely,the result may be false negative
- A negative QuantiFERON Test also does not rule out an active tuberculosis infection, because certain patients, who have a very severe, active tuberculosis infection, may have a negative test result
- False negative result can result from a specimen that is older than twelve hours; causing the white cells to become less active or dysfunctional. These inactive white cells may end-up giving a false negative report
An intermediate test result means:
- Test needs to be repeated after an appropriate time, as determined by your healthcare provider
The QuantiFERON Test has certain advantages and disadvantages. Advantages of the test include:
- The test requires a single visit. During the visit of the individual, a blood sample is drawn via phlebotomy
- The test results are usually available within 24-hours, unlike a tuberculin skin test which may require up to 72-hours to be performed
- Unlike the tuberculin skin test, where the immune system response is bolstered by subsequent tests, a QuantiFERON Test does not have a bolstered immune response, during subsequent testing
- Tuberculin skin test requires an individual to measure the site of tuberculin skin test for positivity. A QuantiFERON Test does not require one to measure individually, whether the patient is positive or not. The positivity is determined by a standardized assay and hence, there is no reader bias involved
- Unlike the tuberculin skin test, which is affected in cases, where individuals have received prior BCG vaccination, a QuantiFERON test is not affected by a prior BCG vaccination
The disadvantages and limitations of QuantiFERON Test include:
- The specimen has to be processed within 12-hours after collection, because QuantiFERON Test requires well-functioning white blood cells to determine the functionality of the assay. A specimen that is older than 12-hours, results in the white cells becoming less active, or dysfunctional, which may lead to a false negative report
- QuantiFERON testing in children, younger than 17 years age, is being debated. There is limited data available and hence, current guidelines indicate that a QuantiFERON Test should not be used in individuals, less than 17 years old
- Use of the QuantiFERON Test is limited in individuals, who have been recently exposed to mycobacterium tuberculosis bacteria. It may give rise to false negative results
- It also has a limited diagnostic value in immune-compromised individuals, such as those who have genetic immune-compromised disorders, individuals who have active HIV infection, and those who have acquired immunodeficiency syndrome. The test may result in false negative results
- There are other immune-compromised individuals, where the test could be of a limited diagnostic value. These include leukemias, lymphomas, and patients on chemotherapy, individuals with severe diabetes, those who have had recent transplants, and also individuals suffering from chronic renal failure. In such cases, the results may be false negative
The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.
Additional and Relevant Useful Information:
- It is important to note that a QuantiFERON Test does not help one distinguish, between a latent infection and an active tuberculosis infection
- To diagnose an active tuberculosis infection, one must have other tests performedin the clinical laboratory, which indicate the presence of acid fast organisms in sputum, or cultures that are positive for mycobacterium tuberculosis
- It is important to note that a positive QuantiFERON Test does not mean that the individual is having an active TB infection
- A negative QuantiFERON Test also does not rule out an active tuberculosis infection, because certain patients with a very severe, active TB infection may have a negative QuantiFERON Test
- A tuberculin skin test was used earlier to the QuantiFERON Test to diagnose latent tuberculosis infection. It had a certain drawback, because individuals vaccinated previously with BCG,may test positive with the tuberculin skin test, but test negative on the QuantiFERON Test. Hence, QuantiFERON Test is superior to the tuberculin skin test for detecting latent tuberculosis infections in individuals, who have received BCG vaccination
Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.