Tuberculosis testing: Which patients, which test?
The most appropriate test to identify latent TB depends on the patient’s risk for developing active TB and other factors. This review provides practical guidance on who to test, how, and when.
Since 2011, the World Health Organization has endorsed the use of a new molecular diagnostic test called Xpert MTB/RIF in settings with high prevalence of HIV infection or multidrug-resistant TB (MDR-TB).35 This test is able to detect M. tuberculosis as well as rifampin resistance, a surrogate for MDR-TB, within 2 hours, with sensitivity and specificity approaching that of culture.36
“Culture-negative” TB? A small but not insignificant proportion of patients will present with risk factors for, and clinical signs and symptoms of, active TB; their cultures, however, will be negative. In such cases, consultation with an infectious disease or pulmonary specialist may be warranted. If no alternative diagnosis is found, such patients are said to have “culture-negative active TB” and should be continued on anti-TB drug therapy, although the course may be shortened.37 This highlights the fact that while cultures are key to diagnosing and treating active TB, the condition is—practically speaking—a clinical diagnosis; treatment should not be withheld or stopped simply because of a negative culture or rapid diagnostic test.
CASE 1 › Based on her risk factors (being a health care worker, born in a country with a high prevalence of TB), Ms. C’s cutoff for a positive test is >10 mm, so her TST result is negative and she is not considered to have LTBI. The increase to 8 mm seen on the second TST probably represents either childhood BCG vaccination or previous infection with nontuberculous Mycobacterium.
CASE 2 › Strictly speaking, 3-year-old Patrick does not need testing, because he was exposed only to LTBI, which is not infectious. However, because children under age 5 are at particularly high risk for progressing to active TB and poor outcomes, it would be best to confirm the mother’s story with the day care center and/or health department. If it turns out that Patrick had, in fact, been exposed to active TB, much more aggressive management would be required.
CORRESPONDENCE
Jeff Hall, MD, Family Medicine Center, 3209 Colonial Drive Columbia, SC 29203; jeff.hall@uscmed.sc.edu
