A healthy nurse in a suburban community hospital presents reporting she thinks she has a positive PPD from a skin test that had been placed 48 hours ago. She has no known specific TB exposures. What finding is indicative of a PPD test in this patient?


(a) Any amount of induration
(b) Induration > 5 mm
(c) Induration > 7.5 mm
(d) Induration > 10 mm
(e) Induration > 15 mm


The correct answer is E. Because the nurse works in a suburban area, with presumably low incidence of TB, and she has no history of immunocompromise, she is considered a lower risk category (despite working in health care facility)

The TST is performed by injecting 0.1 ml of tuberculin purified protein derivative (PPD) with needle bevel facing upwards intradermally such that there is a wheal of 6-10 mm diameter. The test must be read between 48 – 72 h of administration.

Classification of the Tuberculin Skin Test Reaction

Induration > 5 mm is positive:

-HIV patients
-Having a recent TB contact
-Fibrotic change on CXR consistent w prior TB
-Patients with organ transplants
-Persons who are otherwise immunosuppressed (prednisone / TNF-a antagonists)

Induration > 10 mm is positive:

-Recent immigrants (< 5 years) from high-prevalence countries
-Injection drug users
-Residents and employees of high-risk settings (nursing home; areas with high prevalence)
-Mycobacteriology lab personnel
-Persons with clinical conditions that place them at high risk
-Children < 4 years
– Infants, children, and adolescents exposed to adults in high-risk categories

Induration >  15 mm  is positive:

-No known risk factors for TB.