In renally impaired patients, vancomycin concentrations typically are maintained at body temperature for extended periods of time due to the drug's prolonged half-life. Both time and increased temperature potentiate production of vancomycin crystalline degradation products (CDP-1). Commercially available vancomycin assays, such as fluorescence polarization immunoassay (FPI) and radioimmunoassay, cross-react with CDP-1 isomers. Overestimation of vancomycin concentrations by 40-53% due to cross- reactivity of CDP-1 with active factor B vancomycin occurs with FPI. As FPI is the most common method of analyzing serum vancomycin, clinicians must be aware of its potential shortcomings and be prepared to alter vancomycin dosages in renally impaired patients. The possibility of adverse affects due to elevated concentrations of CDP-1 or therapeutic failures due to subtherapeutic levels of factor B vancomycin cannot be excluded.