Tuberculosis (TB) remains a notable global health concern, with millions of new cases diagnosed annually. Identifying TB infection early is important for preventing its spread and ensuring timely intervention. Healthcare professionals utilize two primary types of tests to detect the presence of TB bacteria: the tuberculin skin test (TST), also known as the PPD test, and blood tests, referred to as Interferon-Gamma Release Assays (IGRAs).
The PPD Skin Test: Visual Appearance of a Positive Result
The PPD skin test involves injecting a small amount of tuberculin purified protein derivative (PPD) just beneath the skin. A positive reaction is characterized by a firm, raised, palpable swelling at the injection site. This swelling, known as induration, develops due to a localized immune response if the individual has been exposed to TB bacteria.
Redness (erythema) often surrounds the induration, but redness alone is not a positive indicator. The size and presence of the induration are the key factors for interpretation. The induration typically appears as a circular or oval-shaped bump. The reaction usually becomes visible and is read by a healthcare worker between 48 and 72 hours after the injection.
Interpreting PPD Test Results
Interpreting a PPD test involves precisely measuring the diameter of the induration. Any surrounding redness is disregarded. The size of the induration, measured in millimeters, is then assessed in conjunction with an individual’s risk factors for TB infection.
Different induration sizes are considered positive depending on an individual’s risk profile. An induration of 5 millimeters or more may be positive for individuals with weakened immune systems, close contact with active TB, or organ transplant recipients. A measurement of 10 millimeters or more is often positive for recent immigrants from high-prevalence countries, healthcare workers, or children under four years old. For individuals with no known risk factors, an induration of 15 millimeters or more indicates a positive result.
If no induration is present, or only minimal induration with fading redness, the result is typically negative. A weakened immune system might cause a false negative.
Understanding Blood Test Results (IGRA)
Interferon-Gamma Release Assays (IGRAs), such as QuantiFERON-TB Gold and T-SPOT.TB, are blood tests that detect TB infection. Unlike the PPD skin test, IGRAs do not involve a visual skin reaction. They measure the immune system’s response to specific TB bacteria proteins in a blood sample, where white blood cells from an infected person release interferon-gamma (IFN-γ) when exposed to these antigens.
IGRA results are provided in a laboratory report as “positive,” “negative,” or “indeterminate.” A positive result means TB bacteria are present. A negative result suggests TB infection is unlikely. An indeterminate result occurs when the test cannot definitively determine an outcome. Advantages of IGRAs include requiring only a single patient visit and not being affected by the BCG vaccine, which can cause false positives with the PPD test.
Next Steps After a Positive Test
A positive TB test, whether PPD or IGRA, indicates that TB bacteria are present in the body. However, it does not distinguish between latent TB infection (LTBI) and active TB disease. Latent TB means the bacteria are inactive and not causing symptoms, nor can they be spread to others. Active TB disease means the bacteria are multiplying, causing illness, and can be transmitted to other people.
Further evaluation is necessary to determine the specific diagnosis and appropriate course of action. This typically involves a review of medical history, a physical examination, and a chest X-ray to look for signs of active disease. If active TB is suspected, sputum tests may confirm the presence of TB bacteria.
If only latent TB is diagnosed, preventive treatment, often with antibiotics, may be recommended to prevent progression to active disease. Treatment for active TB involves a more intensive regimen of multiple antibiotics. Following up with a healthcare provider for proper diagnosis and management is important.