Persistently Positive TB Skin Test
A 35-year-old man presents to his primary care provider after a positive tuberculin skin test result (12 mm) during a pre-employment physical. He has no significant medical history. He was born in Mexico and immigrated to the United States 20 years ago, where he now lives with his wife and two children. He denies fever, cough, night sweats, loss of appetite, or weight loss. A physical exam is unremarkable, and his chest x-ray is normal.
He states that his tuberculin skin test has been positive for 15 years. In the past, he was told that he did not need treatment for tuberculosis (TB) because the skin test result was most likely a false-positive reaction from a previous bacilli Calmette-Guérin (BCG) vaccine (Figure 1).
Because the patient's chest x-ray is normal, the clinician decides to monitor him for signs and symptoms of TB over the next few months.
Why is this a diagnostic error?
The difficulties in selecting whom to test for TB infection have been addressed in recent guidelines. Patients with a positive tuberculin skin test who have had a previous BCG vaccine should be evaluated with an IGRA. Other indications for IGRA include adults who are likely to be infected with Mycobacterium tuberculosis but who may not reliably return for a tuberculin skin test reading after 48 hours, or those who are at low to intermediate risk for disease progression. This patient comes from a country with a high prevalence of TB and could be at risk for reactivation. The positive skin test and IGRA result are confirmatory of latent TB infection, even without symptoms or radiographic findings.
Among the available therapeutic options for latent TB infection, the most commonly used is isoniazid daily with vitamin B6 supplementation. The World Health Organization recommends the following treatments for latent tuberculosis (Figure 2).
An interferon gamma release assay (IGRA) was positive. After discussing the risks and benefits, the patient was started on isoniazid and vitamin B6 supplementation for the treatment of latent TB.
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