AbstractBackgroundPosaconazole is an example of a highly protein‐bound drug (>98%) in which therapeutic drug monitoring (TDM) is commonplace. Total drug concentration is typically measured, and in the setting of hypoalbuminemia, total concentrations are lower despite no anticipated change in unbound concentration. Data support that unbound posaconazole concentration is responsible for antifungal activity and, in theory, is responsible for adverse effects that are dose‐related. However, the therapeutic range of posaconazole is expressed as total concentration. The objective of this study was to investigate the use of an equation to correct posaconazole concentrations for albumin concentration as a surrogate for measurement of unbound concentration.MethodsData on unbound and total posaconazole concentration were acquired retrospectively from a study of posaconazole pharmacokinetics in critically ill patients. The relationship between total and unbound concentration was explored with and without albumin as a covariate using linear regression. Correction equations were used to normalize total concentration to an albumin concentration of 4.4 g/dL.ResultsA total of 78 pairs of total and unbound concentrations were available. Total and unbound posaconazole concentrations were determined using ultra‐high‐performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS). The median fraction unbound was 0.00645 (interquartile range of 0.00331–0.00794). Albumin concentration plays a highly significant role in the interpretation of TDM results. In a patient with hypoalbuminemia, a corrected concentration (Ccorr) = Ct/(0.01 + 0.99·Alb/4.4), where Ct is the total concentration and Alb is the albumin concentration in units of g/dL, is suggested. This equation can be further simplified to Csim = Ct·4.4/Alb, where Csim is a close approximation of Ccorr.ConclusionsHypoalbuminemia is associated with lower total concentrations of posaconazole; however, the “active” unbound concentration is not expected to systematically change. As a result, total posaconazole concentrations in the therapeutic range for patients with hypoalbuminemia are more likely to be associated with toxicity, especially when doses are increased to achieve “therapeutic” concentrations.