Problems connected with the estimation of the incidence of tuberculosis infection
Narain, Raj; Nair, S.S.; Chandrasekhar, P. and Ramnatha Rao, G. (1965) Problems connected with the estimation of the incidence of tuberculosis infection. Indian Journal of Tuberculosis, 13 (1). pp. 5-23.
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It is necessary to obtain a measure of the incidence of new infection for organising tuberculosis control measures and for a better planning of mass BCG campaigns. Remarkably difficult problems are encountered in the estimation of this apparently simple but [much neglected index. Conventional methods for calculation of incidence of infection were applied to material from fifty villages in Bangalore District in South India, where repeated Tuberculosis Prevalance Surveys are being conducted on a random sample of villages. Incidence rates based on these methods are wholly unacceptable. The commonly used method of “conversion rates” is subject to serious errors. A new approach to the estimation of the number newly infected, based on the distribution of the difference of tuberculin reactions to 1 TU from one round to another is presentep. The newly infected, as obtained by this method, form a homogeneous group with a normal distribution (mean increase being about 24 mm and standard deviation 4 mm). 98% of this group show an increase of 16 mm or more from first to second round indurations. However there are others, who in the absence of new infection, as a result of boosting, nonspecific allergy and other factors show such large increases of allergy from first to second round. The number of such persons, who constitute the main source of error in estimating newly infected by the criterion of definite increase in allergy increases with age and is likely to be more in areas with high prevalence of non-specific allergy. Problems concerning the estiestimation of those, who may be considered to be at risk of new infection are also examined. The tentative incidence rates obtained are very low even for a country like India inspite of the vast tuberculosis problem, congestion and malnutrition, and with little or no precautions possible against the spread of infection. That the number of infected among those below 1 year of age was practically nil in the two rounds of the survey supports the finding of low incidence rates of infection.
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