Review Question - QID 217138

Patients that fit into different risk categories for tuberculosis (TB) exposure have different standards when it comes to positivity for a PPD test: a positive PPD is 15 cm or greater in healthy, low-risk individuals, 10 cm or greater in intermediate risk patients (immigrants, history of intravenous drug use, employees in high risk settings, etc.) and 5 cm or greater in high risk patients (HIV, immunosuppressed, recent TB exposure).

The purified protein derivative (PPD) test, sometimes known as the tuberculosis skin test (TST), is a method of screening for tuberculosis. To perform the test, a small amount of tuberculin is injected into the skin of the arm. The person given the skin test must then return in 48 to 72 hours, at which point the area of reaction (usually seen as hardness or swelling at the arm) is measured. A positive test means that the person was infected with TB and further testing is needed to determine whether this is latent TB infection or TB disease. A negative test means that the person did not react to the test, indicating that latent TB or TB disease is unlikely. Patients with different risk factors for TB have differing standards for positivity of a PPD test: a positive PPD is 15 cm or greater in healthy, low-risk individuals, 10 cm or greater in intermediate risk patients (immigrants, history of intravenous drug use, employees in high risk settings, etc.) and 5 cm or greater in high risk patients (HIV, immunosuppressed, recent TB exposure).

Pahal et al. discuss active versus latent TB as well as the two types of screening tests. They found that both modalities are effective screening tests. They recommend that TB screening should only be performed in high-risk populations such that active or latent TB infections may be detected and treated.

Incorrect Answers:
Answer 1: These three patients fit into different risk categories for TB PPD screening and thus have different cutoffs for positivity.

Answer 2: The cutoff for a positive PPD for an employee of high risk settings (e.g. healthcare facilities, prison) would be 10cm or greater. The cutoff for a positive PPD for a generally healthy, low-risk person would be 15cm or greater.

Answer 3: A generally healthy, low-risk individual would have a positive PPD at 15cm or greater.

Answer 5: A patient with history of renal transplant on immunosuppression is considered high risk and would have a positive PPD at 5cm or greater.

Bullet Summary:
Patients that fit into different risk categories for tuberculosis (TB) exposure have different standards when it comes to positivity for a PPD test.