Type of test Skin
Normal findings
PPD: Negative; reaction < 5 mm
Test explanation and related physiology
Tuberculin testing is performed for persons who are
• Suspected of having active TB
• At increased risk for progression to active TB
• At increased risk for latent TB infection (LTBI) (e.g., health-care workers, recent immigrants, or IV drug abusers)
• At low risk for LTBI, but are tested for other reasons (e.g., entrance to college)
For this test, a PPD of the tubercle bacillus is injected intra dermally. If the patient is infected with TB (whether active or dormant), lymphocytes will recognize the PPD antigen and cause a local reaction; if the patient is not infected, no reaction will occur. If the test result is negative and the physician still strongly suspects TB, a second-strength PPD can be used. If this test result is negative, the patient does not have TB. A positive reaction usually occurs 6 weeks after infection. If positive, a localized skin reaction will recur with any subsequent testing throughout the person’s life. This test is used to detect TB infection but is unable to indicate whether the infection is active or latent.
When a patient known to have active TB receives a PPD test, the local reaction may be so severe that it causes complete skin slough and requires surgical care. When these patients are eliminated from PPD testing, the test has no complications. The PPD test will not cause active TB because no live organisms exist in the test solution.
The PPD test also can be used as part of a series of skin tests to assess the immune system. If the immune system is nonfunctioning because of poor nutrition or chronic illness (e.g., neoplasia, infection), the PPD test will be negative despite the patient having had an active or dormant TB infection.
There are several alternative tests for diagnosing Mycobacterium tuberculosis infection.
Contraindications
• Patients with known active
• Patients who have received bacille Calmette-Guérin (BCG) immunization against PPD, because these patients will demonstrate a positive reaction to the PPD vaccination even though they have never had TB infection
Procedure and patient care
Before
- Explain the procedure to the patient.
* Assure the patient that TB cannot result from testing.
• Assess the patient for a previous history of TB. Report a positive history to the physician.
• Evaluate the patient’s history for previous PPD results and BCG immunization.
During
• Prepare the patient’s forearm with alcohol and allow it to dry.
• Intradermally inject the PPD. A skin wheal will occur.
• Circle the area with indelible ink.
• Record the time at which the PPD was injected.
After
• Read the results in 48 to 72 hours.
• Examine the test site for induration (hardening). Measure the area of induration (not redness) in millimeters.
• If the test is positive, ensure that the physician is notified and the patient is treated appropriately.
• If the test is positive, check the patient’s arm 4 to 5 days after the test to be certain that a severe skin reaction has not occurred.
Abnormal findings
Positive results
TB, infection
Nontuberculous mycobacteria infections
Negative results
Possible immunoincompetence
