Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 28-year-old man is evaluated after being informed his roommate at a homeless shelter was diagnosed with pulmonary tuberculosis. He reports no fever, cough, night sweats, or weight loss. Medical history is significant for injection drug use, most recently 2 weeks ago, although he reports no known infectious complications in the past. Medical history is otherwise unremarkable, and he takes no medications.

On physical examination, vital signs are normal. BMI is 22. Track marks secondary to injection drug use are present bilaterally on the antecubital fossa, without tenderness, warmth, erythema, or fluctuance. Cardiopulmonary examination and all other examination findings are normal.

A tuberculin skin test induces 7-mm induration.

Chest radiograph is normal.

Which of the following is the most appropriate next step in the management of this patient?

A: Chest CT
B: Isoniazid
C: Isoniazid, rifampin, pyrazinamide, and ethambutol
D: No further intervention

MKSAP Answer and Critique

The correct answer is B: Isoniazid.

This patient should start receiving isoniazid therapy for latent tuberculosis infection (LTBI). He is asymptomatic and has a normal chest radiograph; however, a tuberculin skin test (TST) reaction of 5-mm or larger induration is interpreted as positive in patients who have recently been in contact with a person with active tuberculosis. A TST reaction of 10-mm or larger induration is interpreted as positive in patients who use injection drugs, are recent arrivals from countries with a high prevalence of tuberculosis, or reside in homeless shelters. Treatment for LTBI with isoniazid for 9 months is recommended. Other possible treatment regimens for LTBI include a 12-week regimen of directly observed once-weekly isoniazid and rifapentine. This regimen is not recommended for patients suspected of having infection with isoniazid- or rifampin-resistant tuberculosis strains. Four months of daily rifampin therapy is acceptable for patients with LTBI that is suspected to be resistant to isoniazid or who cannot take isoniazid.

This asymptomatic patient with a normal chest radiograph has LTBI. CT may be helpful in detecting abnormalities in the lung not seen with plain radiographs. However, this patient has no indications of active disease, so CT is not indicated.

Four-drug therapy with isoniazid, rifampin, pyrazinamide, and ethambutol would be recommended as initial therapy for a patient with active tuberculosis. This patient, who has no symptoms or evidence of active infection and a normal chest radiograph, has LTBI. Therefore, four-drug therapy would not be appropriate.

Pursuing no additional evaluation or therapy would not be appropriate for this patient. Treatment of LTBI significantly reduces the risk of progression to active disease.

Key Point

• Tuberculin skin test results must be accurately interpreted in patients who are asymptomatic and were recently exposed to active tuberculosis so that treatment for latent tuberculosis infection can be initiated.

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