Validity of case-finding tools in a national tuberculosis programme

Abstract

For case-finding under the District Tuberculosis Programme (DTP) in India, the recommended method is sputum smear microscopy of chest symptomatics attending the health institutions on their own. It is universally followed, with a slight variation in the procedure of initial screening of outpatient attendance respectively at the District Tuberculosis Centre (DTC) and at peripheral health institutions (PHI). However it may be argued that smear microscopy of all patients with chest symptoms results in a high workload in terms of positivity rate for examinations done as well as for its comparatively lower predictive value of negativity. It would be interesting in this context to estimate validity of screening and diagnosis obtained under operational conditions. The data available from some of the field surveys and from the DTP situation have been analysed for the above purpose. Examined in the context of statistical reliability of tests, the methodology of prior X-ray screening adopted by the DTC for case-finding for tuberculosis appears to be well founded. In contrast to the DTC, the need for X-ray screening at PHIs does not arise, as the procedure of patients being subjected to screening for the presence of chest symptoms has in itself a very high specificity of about 97%. The results show that X-ray and smear microscopy should not be used indiscriminately as case-finding tools in mass case-finding programmes, as their predictive values of positivity are likely to be very low at the current case prevalence rates in the community, being 2-8 per thousand.