BAYESIAN AND THRESHOLD PROBABILITIES IN THERAPEUTIC DRAG MONITORING -WHEN CAN SERUM DRUG CONCENTRATIONS ALTER CLINICAL DECISIONS

The use of Bayesian and threshold probabilities is examined with respe ct to the range of probabilities of toxicity for which obtaining a pat ient's serum drug concentration leads to information that is potential ly useful in altering a clinical decision. For the situation of potent ial drug-induced toxicity, three threshold probabilities are needed to characterize the decision process: the decision threshold for decidin g between continuing and discontinuing the drug regimen, the testing t hreshold that separates the decision to continue the regimen without t esting the serum drug concentration from the decision to test before d eciding, and the companion testing threshold that separates the decisi on to discontinue the regimen without testing from the decision to tes t before deciding. For digoxin, theophylline, aminoglycosides, vancomy cin, and phenytoin, three prototypical decision threshold probabilitie s, 0.33, 0.2, and 0.1, are used, along with published true-positive an d false-positive rates, to calculate serum concentration testing thres holds for each drug. Practitioners can be more effective in their use of serum drug concentration data when a Bayesian approach to probabili ty assessment is used in conjunction with testing thresholds.