A red mark on your forearm is common after receiving a tuberculosis (TB) test, a procedure that screens for exposure to the bacteria causing this disease. The test, formally known as the Tuberculin Skin Test (TST) or Mantoux test, involves injecting a small amount of purified protein derivative (PPD) just beneath the skin’s surface. This injection creates a temporary pale bump, or wheal, indicating the fluid was placed correctly. A healthcare professional checks the body’s immune response to the PPD between 48 and 72 hours later to determine the result.
The Difference Between Redness and Swelling
The redness you notice at the injection site is called erythema, representing a normal, localized inflammatory reaction to the injection itself. Erythema is almost always irrelevant to the actual result of the TB test. The body often sends increased blood flow to the area as a basic defense mechanism against the foreign material.
The medical professional focuses on the swelling, or induration, to determine the result. Induration is a raised, firm, and palpable area of skin that feels hard to the touch. This firm swelling indicates a delayed-type hypersensitivity reaction, suggesting the immune system has previously encountered Mycobacterium tuberculosis proteins. When reading the test, the provider ignores the size of the redness and focuses only on measuring the diameter of the induration.
How The Clinical Measurement Is Taken
The clinical measurement must occur within the 48-to-72-hour window after the injection; otherwise, the results are unreliable. The health professional uses a flexible ruler to measure the diameter of the firm, raised area of induration, not the surrounding redness. The measurement is taken across the forearm, perpendicular to the long axis of the limb.
The result is recorded in millimeters (mm). A positive outcome is not determined by a single universal size, but the threshold depends entirely on the patient’s individual risk factors for TB infection and progression to disease. This variable interpretation ensures that higher-risk individuals receive necessary follow-up even with a smaller reaction.
Positive Thresholds Based on Risk
An induration of 5 millimeters or greater is considered positive for individuals in the highest-risk categories, such as:
- People with HIV.
- Those who have had recent close contact with a person with active TB.
- Patients who are significantly immunocompromised due to organ transplants or high-dose corticosteroid use.
A measurement of 10 millimeters or greater is the positive cut-off for moderate-risk groups, including:
- Recent immigrants from countries with high TB prevalence.
- Intravenous drug users.
- Residents or employees of high-risk congregate settings like hospitals, prisons, or nursing homes.
A reaction of 15 millimeters or more is required for the test to be considered positive for persons with no known risk factors for TB infection. This tiered system ensures that the interpretation is weighted by the individual patient’s potential exposure and immune status.
Understanding Latent Versus Active Infection
A positive TST result simply indicates that the body has been exposed to the Mycobacterium tuberculosis bacteria at some point. It does not distinguish between a latent infection and active disease, which necessitates further evaluation by a healthcare provider.
Latent TB Infection (LTBI) means the bacteria are present in the body but are inactive and not causing symptoms. People with LTBI do not feel sick and cannot spread the bacteria to others. However, they risk the infection progressing to active disease later, so treatment for LTBI is often recommended to prevent this progression.
Active TB Disease occurs when the bacteria multiply and overwhelm the immune defenses, causing symptoms like a prolonged cough, unexplained weight loss, fever, or night sweats. A person with active TB, particularly of the lungs or larynx, is typically infectious and can spread the disease. A positive TST in a person with symptoms requires immediate follow-up testing, such as a chest X-ray and sputum sample analysis, to diagnose active disease.
In cases of a positive skin test, especially for people who received the Bacille Calmette-Guérin (BCG) vaccine, a follow-up blood test is often used to confirm the result. The Interferon Gamma Release Assay (IGRA) blood test, such as QuantiFERON-TB Gold, offers higher specificity because it is less likely to produce a false-positive reaction from the BCG vaccine. The IGRA measures the immune system’s response to specific TB proteins and is now frequently used as an alternative or confirmatory test to the TST.