Prevalence and annual risk of tuberculosis infection in Laos.

Setting: Laos, where the implementation of a national tuberculosis program started in 1995.

Objective: To estimate the prevalence (P) and annual risk of tuberculosis infection (ARTI) and relate this to case rates in the tuberculosis program.

Methods: Tuberculin survey in schools in Vientiane municipality and three provinces. Sampling was not representative, but included schools in accessible districts in provinces with different characteristics. A group of 150 tuberculosis patients were tested for comparison.

Results: An intra-dermal tuberculin test (PPD RT23/ Tween80) was applied to 11237 children; 9968 reactions were read, 4035 among first and second grade children with no BCG scar: 818 in Vientiane (mean age 8.4), 803 in Savannakhet (8.9), 1563 in Xiengkhuang (8.4), and 851 in Luangprabang (7.9). If reactions of > or =10 mm are counted, ARTI is estimated as 1.1% for the areas included in the survey with a strong variation by province (Vientiane 1.3%; Savannakhet 1.4%; Luangprabang 0.6%; Xiengkhuang 0.1%). The difference in ARTI is in accordance with differences in reported case rates in the provinces. The ARTI varied from 0.5% to 1.2%, depending on the method used for calculating P (Vientiane 0.6%-1.4%; Savannakhet 0.5-1.5%; Xiengkhuang 0.1%-0.2%; Luangprabang 0.4%-0.9%).

Conclusions: The ARTI is unequally distributed geographically and/or by ethnic group. It is generally higher in south and lower in central and north Laos. Improved methods are needed for estimating the prevalence of infection.