Random Medical Fact #27: Tuberculin skin test (PPD)
11/27/07 02:26
The TST is a widely used test for detecting latent
tuberculosis infection, and requires 2-12 weeks after
primary infection for conversion to occur.
The Mantoux method (intradermal) is used in the United States, where 0.1mL (5 IU) of purified protein derivative (PPD) is injected into to volar surface of the forearm. The test should be interpreted 48 to 72 hours after administration of antigen by measuring the induration (not erythema) surrounding the injection site, in millimeters.
Interpretation
Induration greater than or equal to 5 mm: positive in highest risk population (HIV; recent contacts of TB-infected patients; patient with radiographic changes consistent with prior TB; immunosuppressed or taking >15mg/day prednisone for one month or more)
Induration greater than or equal to 10 mm: positive for individuals with an increased risk (recent immigrant (5 years) from Eastern Europe, Latin America, Asia, Africa; injection drug users; high-risk employees at hospitals, prisons, homeless shelters, nursing homes, laboratories; children exposed to adults at high risk, or any children under 4; patients with silicosis, chronic renal failure, hematologic disorders or other malignancies, weight loss of 10% of ideal body weight, gastrectomy, jejeunoileal bypass).
Induration greater than or equal to 15 mm: positive in low risk populations.
Outside the U.S., the two methods commonly used include intradermal injection of 2 IU of PPD (RT-23), or using a multiple puncture device (Heaf or tine test) on the volar or dorsal surface of the forearm. Individuals who have a history of BCG vaccination or previous tuberculous infection often test positive. Even so, tuberculin skin tests are not contraindicated in BCG-vaccinated persons and skin test reactivity should be interpreted and treated as for unvaccinated persons.
A new whole blood interferon assay (IFN-gamma, QuantiFERON-TB Gold) has been approved by the FDA and can be used instead of the TST. It is much less subjective, and only requires one patient visit.
The Mantoux method (intradermal) is used in the United States, where 0.1mL (5 IU) of purified protein derivative (PPD) is injected into to volar surface of the forearm. The test should be interpreted 48 to 72 hours after administration of antigen by measuring the induration (not erythema) surrounding the injection site, in millimeters.
Interpretation
Induration greater than or equal to 5 mm: positive in highest risk population (HIV; recent contacts of TB-infected patients; patient with radiographic changes consistent with prior TB; immunosuppressed or taking >15mg/day prednisone for one month or more)
Induration greater than or equal to 10 mm: positive for individuals with an increased risk (recent immigrant (5 years) from Eastern Europe, Latin America, Asia, Africa; injection drug users; high-risk employees at hospitals, prisons, homeless shelters, nursing homes, laboratories; children exposed to adults at high risk, or any children under 4; patients with silicosis, chronic renal failure, hematologic disorders or other malignancies, weight loss of 10% of ideal body weight, gastrectomy, jejeunoileal bypass).
Induration greater than or equal to 15 mm: positive in low risk populations.
Outside the U.S., the two methods commonly used include intradermal injection of 2 IU of PPD (RT-23), or using a multiple puncture device (Heaf or tine test) on the volar or dorsal surface of the forearm. Individuals who have a history of BCG vaccination or previous tuberculous infection often test positive. Even so, tuberculin skin tests are not contraindicated in BCG-vaccinated persons and skin test reactivity should be interpreted and treated as for unvaccinated persons.
A new whole blood interferon assay (IFN-gamma, QuantiFERON-TB Gold) has been approved by the FDA and can be used instead of the TST. It is much less subjective, and only requires one patient visit.
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