Interpretation Of PPD Skin Testing (Tuberculin Skin Test)

An 80 Year old female living in nursing home comes to her doctor for the interpretation of her tuberculosis (TB) skin test. 5-tuberculin unit injection of purified protein derivative (PPD) was interpreted as 0 mm diameter of induration after 48 hours.

What should be the next step?

A) Repeat the PPD now.
B) Repeat the PPD in 2 weeks.
C) Repeat the PPD in 1 year.
D) Declare the patient free of TB.
E) Obtain a chest x-ray.

Answer And Discussion: Discussion:
The correct answer is “B.”(Repeat the PPD in 2 weeks.)

 Because the prevalence of a positive PPD(Mantoux test) doubles between the first and second tests in initial non responders, the U.S. Public Health Task Force recommends a two-step PPD by the Mantoux method for screening high-risk populations (e.g., persons living in nursing homes). If
the first PPD is negative, a second test should be performed approximately 2 weeks later in order to detect the “booster phenomenon.”
Individuals admitted to a nursing home with a positive PPD and no symptoms of active TB may be presumed to have a distant history of infection.

The PPD is repeated 2 weeks later and is interpreted again as 0 mm diameter of induration after 48 hours. The patient is reassured that her tuberculin test is negative.
What would most accurately represent a positive tuberculin skin test in THIS patient?

A) Erythema 5 mm in diameter.
B) Induration 5–10 mm in diameter.
C) Erythema of 10 mm in diameter.
D) Induration 10 mm in diameter.
E) Erythematous induration of any size.

Answer And Discussion:

The correct answer is “D.” (Induration 10 mm in diameter. )

Routine screening is not recommended for low-risk patients (e.g., community dwelling individuals from a low-risk country).
Screen only the following:

• contacts of those with TB, 
• HIV infected patients, 
• IV drug users, 
• those with predisposing factors to TB infection (diabetes, immunosuppressive drugs, lung cancer, etc.), 
• foreign-born individuals arriving in the United States within the last 5 years, 
• health-care workers, 
• nursing home and other institutionalized individuals and the homeless. 

Measure the induration and not the erythema. The definition of a positive PPD changes with the population tested.