The Centers for Disease Control and Prevention has established three criteria that must be met to establish that a patient with tuberculosis is noncontagious: adequate treatment for tuberculosis for at least 2 weeks, improvement of symptoms, and three consecutive negative sputum smears (collected at 8- to 24-hour intervals, including one early morning collection). This patient has met two of these criteria but must complete at least 2 weeks of treatment with rifampin, isoniazid, pyrazinamide, and ethambutol before he is considered noncontagious.

Factors associated with the contagiousness of a patient infected with tuberculosis include presence of a lung cavity, cough, or acid-fast bacilli on sputum smear; receiving inadequate treatment; pulmonary or laryngeal tuberculosis; coughing without covering the mouth; and sputum culture positivity. The absence of a cavity does decrease the contagiousness of a patient but is not a criterion for establishing that a patient is noncontagious.

A positive result on a tuberculin skin test (TST) indicates that a patient has been infected with Mycobacterium tuberculosis. A negative TST result makes it unlikely that the patient has been infected. However, false-negative results do occur. Physiologic causes of false-negative TST results include active tuberculosis infection, HIV infection, use of immunosuppressive agents (such as glucocorticoids), kidney injury, lymphoproliferative disorders, and recent vaccination with live virus. Therefore, the lack of a positive result does not correlate with contagiousness.

The leukocyte count may be normal, decreased, or increased in patients with active tuberculosis. It is not used as a criterion for determining whether a patient is contagious.