Lactulose interferes in the alkaline picrate assay for creatinine

Errors of more than 10 mg/L in measured serum creatinine concentrations were encountered for an azotemic patient who was given lactulose orally. The magnitude of the apparent error fluctuated with the dose of lactulose. Additions of lactulose to serum produced a linear increase in the creatinine measured by each of two automated methods that involve use of the alkaline picrate (Jaffé) reaction. A lactulose concentration of 100 g/L produced positive interferences of 30 and 65 mg/L in kinetic (Beckman Astra) and continuous-flow (Technicon SMAC) assays, respectively, but caused no problem in an enzymatic assay for creatinine. The results of creatinine assays must be interpreted with caution in patients treated with lactulose.