The basis for monitoring strategies in clinical guidelines: a case study of prostate-specific antigen for monitoring in prostate cancer

Abstract

M onitoring involves the scheduled, repeated use of a test or tests in an individual over time to make decisions about the management of a disease or condition. It is a central activity in the management of care, taking up a considerable part of the clinical workload and associated cost.1 In contrast, the volume of published literature on the evaluation and use of tests for monitoring purposes is relatively sparse.Mant and others have provided a framework for developing and evaluating a monitoring strategy with four main steps:1-3 deciding whether to monitor, choosing a test, specifying and assessing the monitoring strategy to be used, and implementing the strategy. Underlying this framework is the key concept that the "signal" from the test, reflecting the status of the underlying condition, should be greater than the surrounding "noise," or measurement variability, that may affect the interpretation of the results. 2,3If the noise is too high in relation to the signal, one's certainty in a given test result will be considerably reduced.The repeated measurement of prostate -specific antigen among men who have received primary treatment for prostate cancer is an ap parently successful example of a rule-based monitoring strategy. The levels of prostate -specific antigen following radical treatment vary. Recurrence of disease following radical prostatectomy is associated with the presence of prostate-specific antigen; following radical radiotherapy, it is associated with a rise in the level of prostate-specific antigen.4 When a predefined level is reached, biochemical failure is said to have occurred. The usefulness of testing for prostate-specific antigen as a form of monitoring is based on the assumption that biochemical failure predates clinical failure within some clinically meaningful time frame. The decision to initi-

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