This is a skin test that involves the intracutaneous injection of 5 tuberculin units (TU), which is 0.1 ml of purified protein derivative (PPD), to aid in the diagnosis of mycobacterium tuberculosis (MTB) infection. Following inoculation, the injection site is then reexamined 48 to 72 hours later by a trained medical professional. The amount of induration, not erythema, is used to decide the degree of reactivity. A positive PPD in persons who are symptomatic of TB indicates tuberculosis disease; however, a positive PPD in persons who are asymptomatic indicates tuberculosis infection. Because immunocompromised persons are often anergic, the tuberculin skin test should be performed with control skin tests using two common antigens although the usefulness of simultaneous control testing is controversial. A false-positive PPD occurs for up to approximately 10 years in patients who have been vaccinated with bacille Calmette Guerin (BCG) vaccine. False-positive results may also be seen in persons who are infected with nontuberculous mycobacterium; however, the amount of induration in these patients is usually less than 10 mm. Quanti-FERON TB Gold testing of the patients blood may be performed in lieu of PPD testing or when the results prove confusing.
INTERPRETATION:
Less than 5 mm induration constitutes a negative test.
Induration of 5 mm or greater is considered a positive test in patients with recent, close contact with a known infected individual, persons with lesions on chest radiographs consistent with old tuberculosis infection (granulomas, especially in the apical regions, and signs of volume loss), and in persons infected, or at risk of infection, with the human immunodeficiency virus (HIV).
Induration of 10 mm or greater is considered a positive test if the patient has none of the above criteria but: (1) does have a medical condition that increases the risk of active disease once infection has occurred; (2) was born in Asia, Africa, or Central or South America; (3) is an African American, Native American, or Hispanic and is a member of a lower socioeconomic group; (4) abuses intravenous drugs; or (5) is a resident or employee of a long-term facility (prison, mental institution, or nursing home).
Fifteen millimeters or greater of induration is considered positive in all persons. People who are considered to be at low risk for tuberculous infection should not be subjected to testing with PPD.
The Centers for Disease Control (CDC) recommends screening the following groups:
Close contacts of persons infected with tuberculosis
Foreign-born persons from countries with a high tuberculosis prevalence
Medically underserved persons from low-income populations
Alcoholics and intravenous drug abusers
Residents or employees of long-term care facilities (prisons, mental institutions, nursing homes, etc.)
Persons with medical risk factors known to increase the risk of infection:
Silicosis
Abnormal chest radiograph consistent with prior TB infection
Gastrectomy
Jejunoileal bypass
Underweight (greater than 10% below ideal body weight)
Chronic malabsorption syndromes
Persons receiving long-term corticosteroid therapy
Hematologic disorders
Malignancies
False-negative responses to skin testing can occur for many reasons. Persons with recent viral infections, infectious mononucleosis, lymphoproliferative disorders and even persons who have recently received live viral vaccinations can have a suppressed skin reaction. Cyclophosphamide or even prednisone therapy may impair a patients response to skin testing. Certain medical conditions, such as uremia or alcohol-related disorders, may be associated with anergy. Finally, there is a small portion of the population (1-2% of persons infected with MTB) who, despite being infected with MTB, are PPD-negative without any underlying cause. This phenomenon is seen frequently in elderly persons. Therefore, a negative PPD does not exclude the diagnosis of MTB, and the diagnosis should be strongly pursued in persons who are PPD-negative but who have a high probability of infection. If this is suspected, the testing of the patients blood with Quanti-FERON TB Gold may be helpful.
In persons who were infected many years prior, the reactivity to skin testing may diminish resulting in a negative PPD. Once these patients are subjected to the PPD antigen, they may redevelop their sensitivity on subsequent testing, a phenomenon known as the booster effect. This may result in an erroneous diagnosis of MTB infection occurring in between the time of the two PPD tests. To avoid this misdiagnosis, it is important to subject persons who are to be skin tested frequently to the two-step skin test. This is done by placing a PPD in the normal fashion but reading the results in 7 not 2 or 3 days. If the first skin test is negative, a second test is placed and read in 2 or 3 days. If the second test is positive, this is indicative of the booster effect and not new infection.