Despite rapid progress in molecular genetics, application to human hypertension still presents formidable problems and will require collection of samples from a large number of well-characterised pedigrees. A crucial difficulty in the collection of such material arises from imprecision in the measurement of BP itself and hence the adequacy of the phenotypic definition. Ascertainment of cases from a population study using ambulatory recordings has particular advantages since it will increase the power of many forms of linkage analysis including the sib-pair method. These studies were all based on a small number of clinic BP measurements. This method of estimating BP has several limitations.
Blood pressure is highly variable.
The BP measured in the clinic may be substantially higher than for the rest of the day, and it may not be the best predictor of risk. This information is available for clinic BP from the Framingham study, where subjects were followed for a prolonged period. In practice it is not always possible to wait for many months or years before deciding how to treat an individual patient. Unfortunately this study included only 1000 mild to moderate treated hypertensives and recorded only 75 fatal events during a mean follow-up period of 5 years. The analysis of the data did not relate ambulatory BP directly to cardiovascular risk.