When a patient is exposed to tuberculosis, the body will mount a cell-mediated immune response. This response will create memory cells capable of generating a delayed (Type 4) hypersensitivity reaction when subsequently exposed to PPD.
The Mantoux test is performed by injecting purified protein derivative (PPD) intradermally at a 15° angle on the dorsal side of the patient’s forearm; the fluid containing the PPD will form a wheal under the skin. Patients should be instructed not to itch or cover the test site with a bandaid.
The results of a tuberculosis skin test must be read by a nurse or other healthcare provider within 48 to 72 hours of receiving the PPD injection. Results cannot be interpreted before or after the allotted time frame. A positive PPD result occurs when the body has been sensitized to tuberculosis and has created antibodies that react with the protein in the Mantoux test injection. A positive result will create an induration or hardened mass, whose diameter is then measured to determine if the patient has tested positive or negative for the disease. The area of redness at the injection site is NOT measured as part of the induration.
In an immunocompromised patient, an induration or hardened mass measuring greater than or equal to 5 millimeters in size is considered a positive result.
An induration measuring greater than or equal to 5 millimeters is considered a positive result in immunosuppressed patients. This group of patients includes any of the following: HIV-positive, radiographic signs of TB on chest x-ray, taking chronic immunosuppressants (e.g. after organ transplant or for another disease), or recent contact with another person who is infected with TB.
In high risk patients, an induration measuring greater than or equal to 10 millimeters in size is considered a positive result.
Patients with high risk of reactivation include any of the following: intravenous drug use, homelessness, immigration from a country with a high prevalence of TB, chronic illness (e.g. diabetes, kidney disease, lung disease, or malignancy), occupational or residential setting in healthcare, prison, or microbiology lab.
An induration measuring greater than or equal to 15 millimeters is always considered positive regardless of risk factors. There may be evidence of blistering at the test site as well.
A patient who presents with a positive tuberculosis skin test should obtain a chest x-ray to confirm the diagnosis or to rule out a false positive test result. It is important to remember that a patient with TB may have a negative chest x-ray. Confirmative diagnosis of TB is by sputum culture.
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