The Mantoux test (also called the Mendel–Mantoux test, tuberculin sensitivity test, or PPD test) is a method used to screen for tuberculosis (TB) infection. It has largely replaced older skin testing techniques such as the tine and Heaf tests. The test involves injecting a small amount of purified protein derivative (PPD) tuberculin just under the skin of the forearm. If performed correctly, the injection creates a small, pale bump called a wheal. The test site is examined a few days later for swelling or hardening of the skin, an immune response that would be expected if the person had been exposed to tuberculosis. However, additional tests are usually required to confirm active infection.
History
Tuberculin is a glycerol extract of the tubercle bacillus. Purified protein derivative (PPD) tuberculin is a precipitate of species-nonspecific molecules obtained from filtrates of sterilized, concentrated cultures. The tuberculin reaction was first described by Robert Koch in 1890. The test was first developed and described by the German physician Felix Mendel in 1908. It is named after Charles Mantoux, a French physician who built on the work of Koch and Clemens von Pirquet to create his test in 1907. However, the test was unreliable due to impurities in tuberculin which tended to cause false results. Esmond R. Long and Florence B. Seibert identified the active agent in tuberculin as a protein. Seibert then spent a number of years developing methods for separating and purifying the protein from Mycobacterium tuberculosis, obtaining purified protein derivative (PPD) and enabling the creation of a reliable test for tuberculosis. Her first publication on the purification of tuberculin appeared in 1934. By the 1940s, Seibert's PPD was the international standard for tuberculin tests. In 1939, Russian M.A. Linnikova created a modified version of PPD. In 1954, the Soviet Union started mass production of PPD-L, named after Linnikova.
Procedure In the Mantoux test, a standard dose of tuberculin is injected intradermally (into the dermis, just beneath the outer layer of skin) on the forearm using a small syringe. The standard dose specified by the U.S. Centers for Disease Control and Prevention is 5 tuberculin units (TU) in 0.1 ml of solution, while the U.K. National Health Service specifies 2 TU in the same volume. The injection is given with the needle bevel facing upward, producing a small, pale, raised bump (called a wheal) about 6 to 10 mm in diameter if placed correctly. The test site is then examined 48–96 hours later—ideally at 72 hours—and the result is recorded based on the size of the reaction. The reaction occurs because a person previously exposed to Mycobacterium tuberculosis is expected to mount an immune response to the injected proteins. This is a classic example of a delayed-type hypersensitivity (type IV) reaction, in which T cells and other immune cells gather at the site over 24 to 72 hours, producing localized inflammation. The response is measured by the diameter of induration (a firm, raised area of skin), recorded in millimeters across the forearm. Redness (erythema) is not measured. A result with no induration is recorded as "0 mm." Earlier versions of the test, such as the Pirquet test, applied tuberculin to the skin through a shallow scratch (scarification).
Classification of tuberculin reaction The results of this test must be interpreted carefully. The person's medical risk factors determine at which increment (5 mm, 10 mm, or 15 mm) of induration the result is considered positive. A positive result indicates TB exposure.
5 mm or more is positive in An HIV-positive person Persons with recent contacts with a TB patient Persons with nodular or fibrotic changes on chest X-ray consistent with old healed TB Patients with organ transplants, and other immunosuppressed patients 10 mm or more is positive in Recent arrivals (less than five years) from high-prevalence countries Injection drug users Residents and employees of high-risk congregate settings (e.g., prisons, nursing homes, hospitals, homeless shelters, etc.) Mycobacteriology lab personnel Persons with clinical conditions that place them at high risk (e.g., diabetes, prolonged corticosteroid therapy, leukemia, end-stage renal disease, chronic malabsorption syndromes, low body weight, etc.) Children less than four years of age, or children and adolescents exposed to adults in high-risk categories 15 mm or more is positive in Persons with no known risk factors for TB A tuberculin test conversion is defined as an increase of 10 mm or more within a two-year period, regardless of age. Alternative criteria include increases of 6, 12, 15 or 18 mm.
Interpretation process According to the guidelines published by Centers for Disease Control and Prevention in 2005, the results are re-categorized into 3 parts based on their previous or baseline outcomes:
Baseline test: ≥10 mm is positive (either first or second step); 0 to 9 mm is negative Serial testing without known exposure: Increase of ≥10 mm is positive Known exposure: ≥5 mm is positive in patients with baseline of 0 mm ≥10 mm is positive in patients with negative baseline or previous screening result of >0 mm
False positive result TST (tuberculin skin test) positive is measured by size of induration. The size of the induration considered to be a positive result depends on risk factors. For example, a low-risk patient must have a larger induration for a positive result than a high-risk patient. High-risk groups include recent contacts, those with HIV, those with chest radiograph with fibrotic changes, organ transplant recipients, and those with immunosuppression. Due to the test's low specificity, most positive reactions in low-risk individuals are false positives. A false positive result may be caused by nontuberculous mycobacteria or previous administration of BCG vaccine. Vaccination with BCG may result in a false-positive result for many years after vaccination. False positives can also occur when the injected area is touched, causing swelling and itching. If the swelling is less than 5 mm, it is possibly due to error by the healthcare personnel causing inflammation to the area. Another source of false positive results can be allergic reaction or hypersensitivity. Although rare (about 0.08 reported reactions per million doses of tuberculin), these reactions can be dangerous and precautions should be taken by having epinephrin available.
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