The tuberculin test is used to know if a person has had contact with the tuberculosis bacillus or Koch’s bacillus (Mycobacterium tuberculosis).
It is performed injecting a small amount (0.1 mL) of tuberculin (an extract of tuberculosis bacilli) intradermally, that is, into the skin. A small bubble is formed in the skin and disappears in a few minutes. The test is read 48-72 hours after the injection.
The test has no relation to allergy or asthma. It is sometimes performed in asthmatic patients as part of a differential diagnosis. The symptoms of tuberculosis may resemble those of asthma, and the test is performed to rule out tuberculosis.
The test is considered positive it a reaction of induration appears at the point of the injection. A hard raised area appears which can be palpated and is measured in mm. If no induration appears, the test is considered negative, even if some redness can be seen. The test is usually considered positive if the diameter of the induration is at least 5 mm. Sometimes, a larger diameter (10 or even 15 mm) is required to be considered positive, depending on specific conditions of the patient. A positive test means that the patient has had contact with the tuberculosis bacillus, but it does not discriminate whether this has been a recent or a past contact, or whether the bacillus has been cleared or is still active at the body. The doctor will take into account other data or tests to interpret the test. Once the test is positive, it will usually remain positive lifelong. False positive results (the test is positive but the patient has not really had contact with the tuberculosis bacillus) may appear due to contact with other bacteria similar to that of tuberculosis. False negative results (the test is negative even though the patient has really had contact with the bacillus) may appear due to some other diseases or to medical therapies, specially those that cause immunosuppression in the patient, or if the contact with the bacillus has been very recent
The tuberculin test is also called Mantoux test, or Pirquet test, or PPD test.
The injection of tuberculin causes pain, usually tolerable and transient. No specific preparation is needed if you are scheduled for a tuberculin test.
Photo: "Mantoux tuberculin skin test"by Greg Knobloch (PHIL, #6806)
Angel Mazon MD, PhD