Plasma Protease Inhibitor Levels Can Mark Nonadherence

Netherlands researchers report that therapeutic drug monitoring can be used to measure patient compliance with HIV drug regimens. Patricia W.H. Hugen and colleagues at the University of Nijmegen Medical Center in Nijmegen and the Academic Medical Center in Amsterdam conducted a study to "determine plasma concentration ratio limits (CORALS) for HIV-protease inhibitors outside of which random plasma concentrations reflect partial compliance or noncompliance."

The researchers identified CORALS that accurately marked patients' failing to adhere to therapy for several agents. They measured plasma drug levels in patients treated with indinavir, nelfinavir, or ritonavir/saquinavir regimens. They obtained the CORALS by dividing individual drug levels by the median population levels, with CORALS defined as the 5th and 95th percentile of concentration ratios in the study cohorts, according to the report.

Plasma samples with ratios outside those percentiles had a 70-90 percent chance of coming from a patient suspected of noncompliance, data showed. After observed ingestion was used to identify falsely accused patients, extra-CORAL concentration ratios were associated with a roughly 90 percent chance of nonadherence.

The lower boundaries for concentration ratios were 0.18, 0.23, 0.28, and 0.36 for ritonavir, indinavir, saquinavir and nelfinavir, respectively, Hugen and colleagues wrote. The full report, "Therapeutic Drug Monitoring of HIV-Protease Inhibitors to Assess Noncompliance," was published in Therapeutic Drug Monitoring (2002;24(5):579-587). "By using concentration ratio limits (CORALS), plasma samples of protease inhibitors with values outside these limits are highly indicative of partial or noncompliance," the authors concluded.

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