[Study investigating infection in contacts of tuberculosis patients in a semi-urban area]

The most important factors for establishing priorities in studies of contacts are the bacteriological status of the patient and the degree of exposure of contact persons to the index case. The aim of the present study was to analyze the results of tuberculosis contact tracing and the relationship between these results and the bacteriological situation of the index case and the proximity of exposure.

We studied the contacts of tuberculosis cases examined in the Center for Tuberculosis Control and Prevention of Lleida (a city in the autonomous region of Catalonia, Spain) over the period 1991-1997. The Mantoux test with 2 TU of PPD-RT23 was used to identify infected persons. Diagnosis of tuberculosis disease was made on evidence from the clinical situation, chest radiography and bacteriological status. Associations between infection or disease in the contact, and the bacteriological status of the index case and proximity of exposure, were analyzed. The chi-square test was used for statistical analyses; significance was set at a p value of, 0.05. The odds ratio was calculated as a measure of association.

The prevalence of tuberculosis infection among the contacts was 44% (763/1733). Tuberculin conversion was observed in 7.8% of contacts, and 31 new cases of tuberculosis were detected (1.8%). The percentages of tuberculosis infection, tuberculin conversion and case detection were higher among persons exposed to sputum smear-positive patients and among those in close contact with the index case. Contact infection was more highly associated with the bacteriological status of the index case and degree of proximity of exposure when the analysis was restricted to contacts less than 15 years old. Case detection was 4% among close contacts living with a sputum smear-positive patient.

Investigation into tuberculosis contacts offered a high yield of detection of infected contact persons and new tuberculosis cases, even among contacts of culture-negative pulmonary tuberculosis patients and contacts of extrapulmonary tuberculosis patients.