Which Tuberculosis Test Is Right For Me?
Which TB Test is Right For Me?
In the United States, there are two Interferon-Gamma Release Assay (IGRA) tests approved by the FDA which are TB Gold and T-SPOT TB Test. These tests involve a single blood draw administered by a medical professional and then sent to a laboratory for analysis. This contrasts with the traditional skin test, which requires two visits. IGRA tests can be used independently or as a secondary confirmation following a skin test. They are known for their higher accuracy and reliability compared to traditional methods, albeit at a higher cost.
IGRA tests excel in diagnosing latent tuberculosis infection (LTBI) and are particularly recommended for individuals who have received the BCG vaccine. A positive IGRA result indicates the presence of TB bacteria in the body, requiring further evaluation by a physician including a physical examination, chest x-ray, and discussion of exposure risks. A negative result suggests TB infection is unlikely, while an indeterminate result necessitates further medical investigation.
About the TB skin test
The TB skin test is used to diagnose TB infection by injecting a standardized solution of purified protein derivative (PPD), derived from tuberculin, under the skin. It triggers a T-cell mediated delayed-type hypersensitivity reaction in individuals infected with TB bacteria.
PPD, derived from proteins excreted by M. tuberculosis under specific conditions, is diluted to a standard concentration in FDA-approved solutions. It typically takes 2 to 8 weeks post-infection for the immune system to react to PPD, allowing detection via the TB skin test.
Advantages and disadvantages of the TB skin test include:
**Advantages:**
- Simple procedure (no need for lab equipment)
- Low cost
- No need for blood draw
- Well-established criteria for test interpretation
- Supported by extensive clinical and epidemiological research
**Disadvantages:**
- Requires trained personnel for administration and interpretation
- Requires proper refrigeration and inventory management of test solutions
- Involves two or more patient visits
- Potential for false-positive results due to BCG vaccination or infection with non-tuberculous mycobacteria
- Potential for false-negative results in the presence of other infections
- Rare adverse effects
- Risk of booster phenomenon
- Susceptible to biases and errors in test placement and interpretation
About The QuantiFERON TB Gold Plus
The QuantiFERON-TB Gold test assesses the immune response of T cells to potential Mycobacterium tuberculosis infection. Specifically, these T cells become sensitized to several highly specific TB antigens. The test utilizes a single mixture of these antigens, prompting the immune system to produce Interferon-γ (IFN-γ). The presence or absence of IFN-γ is indicative of a TB diagnosis. Furthermore, the test measures the magnitude of the immune response, offering clinicians valuable insights for treatment decisions.
The QuantiFERON-TB Gold test has proven beneficial in younger patients (ages 7-9) and those with HIV co-infection. It is known for its efficiency, boasting a specificity of 97%. However, some studies suggest sensitivity may decrease with age. The test typically provides results within 24 hours, though delays can occur due to laboratory or logistical reasons. One drawback is its requirement for a specialized syringe, which can occasionally affect test accuracy and necessitate retesting.
About The T-SPOT TB Test
The T-SPOT TB test is a single-visit blood test approved by the FDA, suitable even for patients as young as 2 years old. It employs the enzyme-linked immunospot (ELISPOT) technique to enumerate T cells sensitized by TB infection, similar to other tests mentioned previously. Unlike some blood tests like QuantiFERON-TB Gold, T-SPOT TB uses a larger number of cells to minimize variability and enhance sensitivity to TB infection. It utilizes distinct mixtures of highly specific TB antigens.
Studies indicate that the sensitivity of the T-SPOT TB test remains consistent across different age groups and in immunosuppressed patients. The test requires standard phlebotomy equipment and typically does not necessitate retesting. Results are usually available within 24 to 48 hours from sample acceptance.