Methodological issues in setting up a surveillance system for birth defects in India.

India is undergoing an epidemiological transition--communicable diseases are on the decline due to better living conditions and healthcare delivery. On the other hand, the relative increase in the prevalence of non-communicable, chronic and genetic diseases threatens to be a public health problem in India. One such group of disorders is congenital malformations. Though several studies have been done on congenital malformations in India since the early 1960s, coinciding with the thalidomide tragedy in the West, no uniform methods are available for the surveillance of birth defects. Each study has come out with varying results, not only because of the geographic variation in birth defects but also due to the varying standards adopted by each study in data collection, case definition and other methodological issues. Setting up a mechanism to understand the extent and nature of birth defects would involve the creation of a birth defects registry. The goals and objectives of such a registry should be formulated before it is set up. There are three types of registries-descriptive, analytical and preventive. These can also be classified as population- or hospital-based. Whether a registry is population- or hospital-based depends largely on the movement of mothers for delivery, registration of vital events in an area defined by the programme, as well as the resources available to the registry. Data can be collected in a passive or active manner, which also depends on the resources available to the registry. Every registry should have its own working definition of eligible cases to be reported, depending on the diagnostic services available in that area, and multiple sources of information should be used to improve the ascertainment rate. All the diagnostic terms should be coded and the information collected should be stored in a well-constructed database, preferably a relational type. Registries must evaluate their methods of data collection periodically to estimate the number of false-positive and false-negative reports. Ethical issues, cost and funding for the employment of various specialized professionals should be considered before setting up a registry.