Tuberculosis (TB) is a serious infection caused by the bacterium Mycobacterium tuberculosis that primarily attacks the lungs. Although it is a common worldwide health concern, most people exposed to the bacteria do not develop active disease. Initial screening relies on two main types of tests to determine if a person has been infected: a skin test and a blood test.
Visual Interpretation of the Skin Test
A positive result from the Tuberculin Skin Test (TST), often called the Mantoux test, is not a simple red mark. A small amount of tuberculin protein is injected just beneath the skin, and the reaction is assessed 48 to 72 hours later.
The positive sign is an area of induration, which is a hardened, raised, and palpable swelling, not merely redness or bruising. This induration is a delayed-type hypersensitivity reaction, indicating the immune system has previously encountered the TB bacteria. The healthcare provider measures the diameter of this induration in millimeters (mm) across the forearm.
The size criteria for a positive result depend on the individual’s risk factors:
- For people at the highest risk (e.g., those with HIV or recent close contact with an active TB patient), 5 mm or more is considered positive.
- For individuals with increased probability of infection (e.g., recent immigrants from high-prevalence areas), 10 mm is the threshold.
- In people with no known risk factors, a reaction of 15 mm or more is required to be considered positive.
Understanding Positive Blood Test Results
An alternative to the skin test is a TB blood test, known as an Interferon-Gamma Release Assay (IGRA), which includes tests like QuantiFERON-TB Gold. This test measures the immune response by detecting the release of interferon-gamma from white blood cells after they are mixed with TB-specific antigens.
The laboratory reports the result qualitatively as “positive,” “negative,” or sometimes “indeterminate.” A positive result means the individual’s blood cells recognized the TB antigens and released the immune chemical, suggesting Mycobacterium tuberculosis infection. This reaction is not affected by the BCG vaccine, which can sometimes cause a false positive result on the TST.
A positive IGRA result strongly suggests the presence of TB bacteria in the body. The test cannot, however, differentiate between a latent infection and active TB disease. The advantage of the IGRA is that it requires only one patient visit and provides a more specific result for TB infection.
Distinguishing Latent Infection from Active Disease
A positive TB test result, whether from a skin test or a blood test, only confirms that the person has been infected with the bacteria. It does not indicate whether the infection is latent or active, which is a critical distinction for determining the next steps.
Latent TB Infection (LTBI)
LTBI occurs when the bacteria are present but inactive, meaning the immune system has contained them. Individuals with LTBI do not feel sick, display no symptoms, and cannot spread the bacteria to others. The bacteria remain dormant, but there is a risk they may reactivate into disease later, particularly if the immune system weakens. Without treatment, approximately 5% to 10% of infected persons will develop active TB disease at some point in their lives.
Active TB Disease
Active TB Disease occurs when the bacteria multiply and overcome the body’s defenses, causing illness. People with active TB often experience symptoms like a prolonged cough, unexplained weight loss, fever, and night sweats. If the disease is in the lungs or voice box, the person is contagious and can transmit the bacteria through the air by coughing or sneezing.
Determining the patient’s state requires a thorough medical evaluation. Active TB is a serious condition requiring immediate and aggressive treatment, so a positive screening test must always be followed up to rule out the active, contagious form of the disease.
Confirmatory Diagnosis and Treatment Overview
Following a positive screening test, the healthcare provider determines if the patient has active TB disease. This evaluation starts with a physical examination and a detailed medical history to check for symptoms. A chest X-ray is used to look for abnormalities in the lungs that are characteristic of active TB.
If the chest X-ray is normal and the patient has no symptoms, the diagnosis is confirmed as Latent TB Infection. If the X-ray is abnormal or symptoms are present, a definitive diagnosis requires collecting sputum samples. These samples are tested in a lab using acid-fast staining, nucleic acid amplification tests, and cultures to confirm the presence of multiplying TB bacteria.
The treatment approach varies significantly depending on the diagnosis. For Latent TB, treatment usually involves a shorter course of one or two antibiotics, typically lasting three to nine months, to prevent future activation. Active TB disease requires a more intensive regimen, typically a combination of three or four different drugs taken for a longer period, often six months or more.