Sepsis and multisystem organ failure are common diagnoses affecting nearly three-quarters of a million Americans annually. Infection is the leading cause of death in acute kidney injury, and the majority of critically ill patients who receive continuous dialysis also receive antibiotics. Dialysis equipment and prescriptions have gradually changed over time, raising concern that current drug dosing recommendations in the literature may result in underdosing of antibiotics. Our research group directed its attention toward antibiotic dosing strategies in patients with acute renal failure (ARF), and we sought data confirming that patients receiving continuous dialysis and antibiotics actually were achieving therapeutic plasma drug levels during treatment. In the course of those investigations, we explored "fast-track" strategies to estimate plasma drug concentrations. As most antimicrobial antibiotics are small molecules and should pass freely through modern high-flux hemodialyzer filters, we hypothesized that continuous renal replacement therapy (CRRT) effluent could be used as the medium for drug concentration measurement by reverse-phase high-pressure liquid chromatography (HPLC). Here we present the first data demonstrating this approach for piperacillin-tazobactam. Paired blood and dialysate trough-peak-trough samples were drawn from 19 patients receiving piperacillin-tazobactam and continuous venovenous hemodialysis (CVVHD). Total, free, and dialysate drug concentrations were measured by HPLC. Dialysate drug levels predicted plasma free drug levels well (r(2) = 0.91 and 0.92 for piperacillin and tazobactam, respectively) in all patients. These data suggest a strategy for therapeutic drug monitoring that minimizes blood loss from phlebotomy and simplifies analytic procedures.