For each year of follow-up, the authors determined the incidence of TB per 100 person-years. Using a Cox proportional hazards model to identify index cases, household contacts, and 9 household risk factors for the disease—including sustained exposure to the index case, lower community household socioeconomic position, indoor air pollution, and “living in a household with a low number of windows per room”—they created a “continuous, integer-point risk score… [using] whole numbers rather than exact regression coefficients to create an easily calculable score for field use.” The score is calculated based on the contact’s body mass index; from that number, a certain number of points is subtracted for each risk factor present (eg, 4 points for a contact with a history of TB, 2 points for households that cook foods with crop wastes, wood, or kerosene). The authors classified those with scores >19 as low risk, those with scores between 18 and 12 as medium risk, and those with scores of ≤11 as high risk.
“Because our risk score informs actual 10-year risk of developing TB, healthcare providers are able to determine their own cut offs for a ‘high-risk’ contact, based on prescriber preferences and the availability of resources in each setting,” Drs. Saunders and Evans explained. “In Peru, we have chosen to opt for a more inclusive definition of ‘high-risk,’ where any contact judged to have a higher TB risk than the risk in the community is advised to start preventive therapy.”
In their study population, they found that the 10-year risks of TB in the low-, medium-, and high-risk groups were 2.8%, 6.2%, and 20.6%, respectively. In all, the 535 contacts classified as high risk accounted for 60% of the TB cases identified. The authors also noted that their scoring system predicted TB “independently of tuberculin skin test and index-case drug sensitivity results.” They validated the score using an urban cohort recruited in Callao, Peru.
“Our score provides a paradigm shift from the current approach where all contacts are treated the same and enables health workers to focus their resources and target TB prevention interventions to contacts who are particularly likely to benefit,” according to Drs. Saunders and Evans. “Because our score does not need any laboratory or clinical tests, in the short-term we believe our score could be immediately implemented… in other resource-constrained settings where currently very few adult contacts who are at high-risk of TB are considered for preventive therapy. However, challenging established beliefs about the use of preventive therapy and encouraging scale-up among all contact of patients with TB, not just children aged less than 5 years [of age], is more complicated and will require concerted effort and engagement from healthcare providers at all levels.”
Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.
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