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The Mantoux test, usually referred to on the series as a PPD (for purified protein derivative) is a diagnostic test for tuberculosis. It is preferred to sputem cultures in many cases because it can also detect the disease when it is latent. However, it does have it's limitations.
The test consists of injecting 1/10th of 1 millilitre of PPD tuberculin under the patient's skin. A positive result consists of the injection site becoming hardened and palpable within 48-72 hours after the test is administered, the result of antibodies to the bacteria reacting to the proteins derived from the bacteria. Although the site will usually turn red as well in a positive test, this is not the result the physician will record because it can be the result of other factors.
The test is not perfect. It will often show a false positive if the patient has been recently vaccinated for tuberculosis or is suffering from another type of mycobacteria related to tuberculosis. It can also vary widely even for patients who are positive. In a patient also suffering from AIDS, a 5mm hardened area is generally treated as a positive result. However, if the patient has merely been in contact with a tuberculosis patient, a 10mm result is necessary for a positive result.
False negatives are also common. AIDS patients with very low T-cell counts will often test negative even when they have the disease, as do others who are immunocompromised. Patients who are using steroids, who have malnutrition or suffer from sarcoidosis as well usually have a false negative.
On the seriesEdit
In TB or Not TB, Allison Cameron tests Sebastian Charles for tuberculosis against the orders of Gregory House. Cameron was aware Charles worked with tuberculosis patients almost exclusively, and Charles himself was sure he had it. Charles was the only patient in the series who was tested and came up positive. However, House admonished Cameron for going against orders. House was also sure Charles had tuberculosis, but was also sure he had another underlying condition. However, he knew if Charles tested positive, he would stop looking for any other conditions.