Antiretroviral Concentrations in Untimed Plasma Samples Predict Therapy Outcome in a Population with Advanced Disease

This study was designed to examine the relationship between untimed antiretroviral concentrations measured in plasma samples collected for virus-load testing and response to highly active antiretroviral therapy. Plasma nonnucleoside reverse-transcriptase-inhibitor and protease-inhibitor concentrations were retrospectively measured in all virus-load plasma samples collected during the first year of therapy, for 122 patients in British Columbia, Canada, who initiated therapy between August 1996 and September 1999 and who had CD4 counts <50 cells/micro L. Drug levels were designated a priori as "low" if the concentrations were below the published Ctrough-SD. A single low drug level measured shortly after initiation of therapy (median, 6 weeks) is common (30%) and is predictive of both more-rapid immunological failure (P=.06) and failure to achieve virologic success during the first year of therapy (P=.01). These results may reflect incomplete adherence, since a strong association (P<.001) was found between low drug levels and an imperfect prescription-refill record (<95%).