Reaction 15 mm of Induration

Persons with no risk factors for TB

*Risk of TB in patients treated with corticosteroids increases with higher dose and longer duration.

tFor persons who are otherwise at low risk and are tested at the start of employment, a reaction of >15 mm induration is considered positive.

are important for identifying patterns of drug resistance. In infants and young children, sputum AFB smears and cultures plus gastric aspirates each morning for 3 days yields the diagnosis only 50% of the time. Other cases may need to be treated presumptively, based on exposure, symptoms, and chest radiograph. In culture-negative cases of TB, it is essential to find the index case and to obtain sputum cultures and drug sensitivities from that patient to guide therapy for patients with negative cultures but active disease.

Laboratory diagnosis ofTB historically has relied on the use of sputum smears for AFB and culturing of the M. tuberculosis organism. Culture results can require 2 to 8 weeks, but more rapid methods can detect early growth within 5 to 14 days (Katoch, 2004; Schluger, 2003). Gene amplification using PCR techniques can be performed on sputum samples for rapid results, as well as on cerebrospinal fluid (CSF), gastric or pleural aspirates, and urine. PCR is highly sensitive (95%-98%) for diagnosing TB from sputum in smear-positive and culture-positive cases, but it has lower sensitivity (57%-78%) for smear-negative and culture-positive cases (Rattan, 2000). PCR may also be used on organisms obtained from early growth on positive cultures to detect drug resistance more rapidly, taking advantage of the genetic polymorphisms in the M. tuberculosis organism, which are almost always associated with drug resistance. Although positive results are highly specific, failure to detect mutations does not entirely rule out drug resistance (Hazbon, 2004; Nachamkin et al., 1997).

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