Wednesday, September 28, 2016

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.


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