The monitoring of compliance in outpatients using serum drug levels suffers from the difficulty of relating concentrations to changes in dosage. Clearance is independent of dose for most drugs, and may be predicted using a Bayesian based forecasting program. This paper describes how the apparent clearance obtained following a single drug analysis can be compared with the Bayesian population value to derive a Compliance Index. In compliant individuals with stable hepatic and renal function this index should remain constant. We evaluated the usefulness of the Compliance Index in general practice with patients prescribed theophylline. Of 54 patients examined on up to five occasions, only 12% were deemed to be fully compliant at all times. Compliance improved during the study, but worsened again once regular consultation ceased. The results indicate that such a Compliance Index is a viable method of assessing compliance with the aid of concentration measurements.
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