Diagnosis and nosology in primary care.

Diagnosis in contemporary medicine is made using an underlying classification system or nosology, the basis of which was first laid down at the end of the 18th century. The International Classification of Disease (ICD) was constructed to formalise this nosology and successive revisions have attempted to capture technical developments and new discoveries across the diagnostic landscape. The ICD has proved particularly applicable in hospital practice where a selected patient population and access to comprehensive diagnostic aids enables a pathology-based diagnosis. When it came to be applied to primary care in the middle of the 20th century, however, it encountered major problems as general practice struggled to marry a classification of disease to the rawness of undifferentiated human illness and distress. Eventually a classification based on the reason the patient consulted emerged to replace that based on pathology defined disease. Analysis of the frontier zone where a dominant classification system struggles to maintain order reveals the ways in which medical nosologies, through their application in the process of diagnosis, attempt to promote and maintain a certain medical reality.