The Mantoux test, officially known as the Tuberculin Skin Test (TST), is a widely utilized diagnostic procedure. It serves as a standard screening method to determine if a person has been infected with Mycobacterium tuberculosis, the bacterium responsible for tuberculosis (TB). This simple skin test helps healthcare providers identify individuals who have developed an immune response to the bacteria. The TST remains a valuable tool in public health efforts to control the spread of this infectious disease.
Purpose and History of the Test
The primary function of the Mantoux test is to screen for latent tuberculosis infection (LTBI), which is a non-symptomatic state where the bacteria live in the body without causing illness. Unlike active TB disease, individuals with LTBI cannot spread the infection to others, but they risk the infection progressing to active disease later on. Identifying LTBI allows for preventive treatment to keep the bacteria from becoming active.
The test is named after the French physician Charles Mantoux, who developed the intradermal injection technique in the early 20th century. The material injected is a standardized solution called Purified Protein Derivative (PPD), a precipitate of non-specific molecules derived from the bacteria’s culture filtrate. The PPD solution used for the test is standardized at a dose of five Tuberculin Units (TU) contained within 0.1 milliliters of fluid.
How the Test is Performed
The Mantoux test is administered through a precise procedure involving an intradermal injection, meaning the substance is placed directly into the top layer of the skin. A healthcare professional uses a small tuberculin syringe and a fine needle to inject 0.1 milliliters of the PPD solution into the inner surface of the forearm. The needle is inserted bevel-up at a shallow angle, just beneath the skin’s surface.
If the injection is performed correctly, it produces a distinct, pale, and raised blister, known as a wheal. The wheal confirms that the PPD has been delivered into the dermal layer where the immune reaction occurs. The patient is then advised to protect the injection site and must return to have the test result read.
The result cannot be determined immediately because the immune system’s reaction is a delayed-type hypersensitivity response. The patient must return to the healthcare setting for a professional reading between 48 and 72 hours after the injection. Failure to return within this specific window of time usually necessitates repeating the entire procedure.
Understanding the Test Results
A positive Mantoux test result is determined by measuring the diameter of the induration, which is the firm, raised, and palpable area that develops at the injection site. This induration represents the accumulation of immune cells reacting to the PPD proteins. Its size is measured in millimeters across the forearm, perpendicular to the long axis. An induration of zero millimeters is recorded if no raised area can be felt.
The interpretation of a positive result depends heavily on the patient’s individual risk factors for TB infection.
High-Risk Individuals
For the highest-risk individuals, a reaction of five millimeters or greater is considered positive. This small cutoff applies to people with HIV, those who have had recent close contact with an active TB patient, organ transplant recipients, and other significantly immunosuppressed patients.
Moderate-Risk Individuals
A larger induration of ten millimeters or greater is considered positive for moderate-risk individuals. This category includes people who have recently immigrated from countries with a high prevalence of TB, residents and employees of high-risk congregate settings like correctional facilities or nursing homes, and those with certain underlying medical conditions such as diabetes or chronic kidney failure.
Low-Risk Individuals
The largest cutoff, fifteen millimeters or greater, is reserved for individuals with no known risk factors for tuberculosis. This tiered approach ensures that the test’s sensitivity remains high for those most likely to have the infection.
Context and Follow-up Testing
Several factors can influence the accuracy of the Mantoux test, potentially leading to false-positive or false-negative results.
False Positives
Prior vaccination with the Bacillus Calmette-Guérin (BCG) vaccine, used in many countries outside the United States, can cause a false-positive reaction. This occurs because the vaccine introduces similar mycobacterial proteins, triggering a cross-reactive immune response.
False Negatives
A false-negative result can occur in people with a weakened immune system, a state known as anergy. Immunosuppression due to conditions like HIV or certain medications may prevent the body from mounting a sufficient immune response to the PPD, even if the person is infected. A recent TB infection, within the preceding eight to ten weeks, can also yield a negative result if the immune system has not yet developed a full response.
A positive Mantoux test indicates only that the person has been infected with M. tuberculosis bacteria at some point, not necessarily that they have active disease. Therefore, a positive result requires immediate follow-up diagnostic testing to determine the clinical status. This typically involves a chest X-ray to check for signs of active disease in the lungs. Follow-up may also include a blood test, such as an Interferon-Gamma Release Assay (IGRA), which offers an alternative way to confirm the immune response to TB-specific antigens.