Hughes, Michael David; (1990) Blood pressure variability over time: statistical implications for risk assessment and screening policies. Doctoral thesis (Ph.D), University of London, Royal Free Hospital School of Medicine.

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Abstract

Screening programmes based on quantitative factors are designed to identify individuals at high risk of disease. When these factors vary within individuals over time, observations on repeat occasions should aid risk classification. However, there have been few statistical developments for assessing accumulated evidence during screening to determine whether intervention is indicated. This study addresses this issue in the context of screening diastolic blood pressure for cardiovascular risk. Data on a cohort of 11299 middle-aged men is used to develop models describing variability during a period of four years. From annual diastolic pressure measurements, a model based on normal variation about an underlying subject mean level, with standard deviation dependent on level, fitted well. No evidence for a risk relationship with trends or variability about the mean level was found: increased risk appears to be established only through a raised underlying mean level. As this cannot be measured directly, a survival model based on observed level is fitted and adjustment for the effect of "regression dilution" made to determine the magnitude of this association. Two alternative statistical models for screening strategies are proposed. The first emphasizes precision in determining an individual's underlying level and hence also their risk. Substantial numbers of measurements over several months may be required particularly when the level is borderline for intervention. The second approach takes a public health perspective and aims to identify that proportion of the population (of a given size) in which expected risk is maximized subject also to a screening cost constraint (the average number of visits per person). Using this rule, most gains in identified risk can be achieved by averaging only a little more than one visit per person. These rules should enhance the value of screening providing an informed assessment of risk before commencing intervention.

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