A Synopsis of AAFP's "Update on Latent Tuberculosis Infection"

Update on Latent Tuberculosis Infection
Hartman-Adams H, Clark K, Juckett G
June 1 2014 Vol. 89 Number 11

     Tuberculosis affects about one third of the world's population. The classic symptoms include cough, hemoptysis, weight loss, fever, and night sweats. Approximately 10-15% of these people have latent infection. 5-10% of latent infections progress to active disease, which will occur within 2 years for half of these patients. Risk factors for progression include age less than 5 years, low body weight, diabetes, drug/alcohol abuse, GI bypass, immunosuppression, and others. Latent infection is noninfectious. Sputum AFB is negative.
     Screening is based on risk stratification.  Factors include recent immigration from a high risk area (especially Asia), being a health care professional, working in an "institution", and being homeless. Low-risk patients should not be tested due to possible false positive tests. The two testing options are the Tuberculin skin test (TST) and the interferon-gamma release assay (IGRA). They cannot distinguish between active and latent infection. TST is more sensitive and IGRA is more specific. High risk patients with negative TST can have a confirmatory IGRA. Low risk patients with a positive TST should have a confirmatory IGRA to avoid unnecessary treatment.  ***PEARL OF THE ARTICLE** Apparently, having the BCG vaccine within 10 years can in fact create a false positive TST test 20% of the time. Board question writers.. start your engines! In this case, IGRA is preferred over TST. It is also preferred in patients who may not return to have the test read. TST is the test of choice in patients younger than 5 years old. For patients with recent exposure, a patient may not show positive conversion with either test for 3 months. High risk patients with recent exposure should get "window prophylaxis" until a screening test can be reliably performed. Sometimes a patient may need a second TST to get a positive reaction (booster effect- ***another BOARD REVIEW PEARL**).
     There are four treatment regimens. Isoniazid can be given for 9, 6, or 4 months. The longer the treatment, the more effective, more expensive, more patient compliance, and more liver toxicity. A new regimen of 12 weekly doses of isoniazid and rifapentine are as effective as the 9 month long dose of isoniazid. It has better compliance but is more expensive.