Evidence suggests that interpretation of TST even by health care providers may be fraught with error. In one study of 107 health care providers including 52 practicing pediatricians, 33 pediatric house officers, and 10 pediatric academicians, 93% identified a known tuberculin converter as tuberculin negative, based on their interpretation of the degree of induration. When presented with an induration of 15 mm, the group's median reading of its size was only 10 mm (Chest 1998;113:1175–7).
Live virus vaccines—measles, mumps, rubella, and varicella—can suppress the TST response. Also be aware that in patients treated with systemic corticosteroids or inpatients who have been treated with the newer tumor necrosis factor antagonists, a false-negative test result can occur, while prior receipt of the BCG vaccine—given at birth in many TB-endemic countries—can produce a false-positive result. However, most children with a history of the BCG vaccine and a positive skin test result have latent tuberculosis. In these instances, consultation with your local infectious disease specialist will be helpful.
Perhaps most important, the identification of children with latent TB infection (LTBI) or tuberculosis disease (who rarely if ever are at risk to transmit TB when less than 10 years of age) is a sentinel event that should provoke an aggressive investigation targeting adult close contacts.
Here in Kansas City, we recently had a TB outbreak in a day care center, mostly among children born in the United States, which was related to their exposure to a foreign-born adult residing in the day care home. Epidemiologic details are being investigated; a combination of problems caused by language barrier, difficulty tracing contacts, and poor record keeping in an unlicensed facility complicate the process.
The guidelines also address treatment for latent TB infection. Daily isoniazid for 9 months is the standard treatment regimen for children and adolescents without a known source case, or those with a source case known to be infected with a susceptible strain. Intermittent regimens are acceptable if given within a directly observed therapy program. Daily rifampin for 6 months is a suitable alternative for those with isoniazid-resistant/rifampin-susceptible strains, or those who can't tolerate isoniazid.
Treatment of LTBI and tuberculosis disease generally should involve the help of your local TB expert. While the proportion of TB cases resistant to both isoniazid and rifampin remained at 1.2% from 2004 to 2005, and isoniazid remains the standard drug for LTBI treatment, we can't be complacent. In 2005, foreign-born individuals accounted for 81.5% of the 124 multidrug-resistant TB cases, and, according to the CDC, that percentage continues to grow. Treatment in such cases is more complicated, involving several drugs that are not generally used in the treatment of TB, and follow-up is important.