Tuberculosis, commonly referred to as TB, is a bacterial infection caused by Mycobacterium tuberculosis that primarily affects the lungs. The diagnosis of TB involves a combination of clinical evaluation, radiographic imaging, and laboratory tests. Among these, the tuberculin skin test (TST) and interferon-gamma release assays (IGRAs) are pivotal in determining if an individual has been infected with M. tuberculosis. Understanding the implications of positive and negative test results is crucial for appropriate management and treatment.
The TST, also known as the Mantoux test, involves injecting a small amount of purified protein derivative (PPD) into the skin. The reaction, measured after 48 to 72 hours, is indicative of previous exposure to M. tuberculosis. IGRAs, on the other hand, are blood tests that measure the release of interferon-gamma by T cells in response to TB-specific antigens. Both tests are used to diagnose latent TB infection (LTBI) rather than active TB disease.
Interpreting Tuberculin Skin Test (TST) Results
The interpretation of TST results depends on the size of the induration (not the redness) and is influenced by the individual's risk factors for TB. The Centers for Disease Control and Prevention (CDC) guidelines categorize TST results as follows:
- For individuals with no risk factors, a result of ≥15 mm is considered positive.
- For those with some risk factors (e.g., recent exposure to TB, fibrotic changes on chest radiograph, patients with organ transplants, and other immunosuppressed patients), a result of ≥10 mm is considered positive.
- For individuals with a high risk of TB (e.g., HIV infection, recent contact with a person known to have TB, fibrotic changes on chest radiograph consistent with prior TB, patients with organ transplants, and other immunosuppressed patients), a result of ≥5 mm is considered positive.
Understanding Positive TST Results
A positive TST result indicates that an individual has been infected with M. tuberculosis at some point in their life. It does not differentiate between latent infection and active disease. A positive result necessitates further evaluation to rule out active TB disease, including a chest radiograph and possibly sputum smear microscopy and culture.
Understanding Negative TST ResultsA negative TST result, on the other hand, suggests that the individual may not have been infected with M. tuberculosis or that the immune system is not responding adequately to the test. False-negative results can occur in individuals with compromised immune systems, such as those with HIV/AIDS, severe malnutrition, or those taking immunosuppressive drugs. In cases where there is a high suspicion of TB despite a negative TST, further testing with IGRAs or repeat TST may be considered.
Interferon-Gamma Release Assays (IGRAs)
IGRAs offer an alternative to the TST for diagnosing LTBI. These blood tests measure the production of interferon-gamma by T cells in response to specific TB antigens. The results are not influenced by prior BCG vaccination, which can affect TST results.
Interpretation of IGRA Results
IGRA results are typically reported as positive, negative, or indeterminate. A positive IGRA result indicates exposure to M. tuberculosis and suggests latent infection. Like the TST, IGRA does not distinguish between latent infection and active disease. An indeterminate result may occur due to various factors, including laboratory issues or the patient's immune status, and may require retesting.
| Test Type | Positive Result Implication | Negative Result Implication |
|---|---|---|
| TST | Latent TB infection or past exposure | No infection or false-negative |
| IGRA | Latent TB infection or past exposure | No infection or false-negative |
Key Points
- Tuberculosis (TB) diagnosis involves clinical evaluation, radiographic imaging, and laboratory tests like TST and IGRAs.
- TST results are interpreted based on induration size and individual risk factors.
- A positive TST or IGRA result indicates latent TB infection but does not confirm active disease.
- Negative TST results may be false-negative in immunocompromised individuals.
- IGRAs are not affected by BCG vaccination and offer an alternative to TST.
Implications for Patient Care
The accurate interpretation of TB test results has significant implications for patient care. Individuals with positive test results require further evaluation to rule out active TB disease and may be considered for treatment of latent TB infection to prevent progression to active disease.
Preventive Therapy
Preventive therapy, typically with isoniazid or rifapentine, is recommended for individuals with latent TB infection, especially those at high risk of progression to active disease. The decision to initiate preventive therapy is based on a combination of factors, including the test results, clinical evaluation, and individual risk assessment.
What does a positive TB test result mean?
+A positive TB test result, whether from a TST or IGRA, indicates that an individual has been infected with M. tuberculosis at some point in their life. It signifies latent TB infection but does not confirm active TB disease.
Can a negative TB test result rule out TB?
+A negative TB test result does not entirely rule out TB, especially in immunocompromised individuals who may exhibit false-negative results. In cases of high clinical suspicion, further testing or evaluation may be necessary.
How are IGRA results interpreted?
+IGRA results are interpreted as positive, negative, or indeterminate. A positive result suggests latent TB infection, while a negative result suggests no infection or a false-negative. Indeterminate results may require retesting.