What's in a word? Case definitions in sexual health medicine

In clinical medicine, establishing case definitions for diseases, including sexually transmitted infections, is often an inexact and arbitrary exercise. The definition is affected by the intended purpose, be it for disease surveillance, clinical care, service provision, research, or funding of services. A case must be defined precisely in order to monitor disease trends, detect outbreaks, and evaluate the effectiveness of interventions. The definition ensures consistent measurement of disease, specificity of reporting, and improves data standardization through time and across regions. The definition itself is one of the criteria used to evaluate the quality of a surveillance system. Designing definitions is fraught with problems relating to appropriate diagnostic criteria, sensitivity and specificity, context and relevance to current knowledge, and/or clinical practice. There is also the problem of reaching a consensus, especially for complex syndromes such as pelvic inflammatory disease.