Skin testing is still the best method of testing for latent infection by prior exposure to M. tuberculosis. Intradermal testing with 5 tuberculin units (0.1 mL) of PPD is more accurate than multiprong tine testing. Interpretation depends on the patient's risk of disease. Patients with a history of direct exposure to active cases of TB, or with impaired immunity such as HIV, should be considered to have a positive test if the area of induration is greater than 5 mm at 48 to 72 hours. Most other patients should be considered positive with induration greater than 10 mm. Very-low-risk patients (age >5 years, no history of exposure, normal immune system, low rates of TB in population) may be considered positive only with induration greater than 15 mm. These criteria are summarized in Box 18-5 (CDC, 2000). Persons vaccinated with bacille Calmette-Guerin (BCG) vaccine may still be accurately tested with PPD skin testing. For high-risk populations, a percentage tuberculin response higher than 15 on the QuantiF-ERON-TB test (QFT) performed on venous whole blood is moderately correlated with a positive skin test. Neither PPD nor QFT is recommended as routine screening in low-risk populations (CDC, 2003a).

Clinical diagnosis in endemic areas is often based on history of exposure, clinical signs, AFB smears, and chest x-ray findings. Sputum cultures can confirm the diagnosis and also

Box 18-5 Criteria for Tuberculin Positivity by Risk Group

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