Colorimetric determination of potassium in plasma and serum by reflectance photometry with a dry-chemistry reagent.

We evaluated a colorimetric assay of potassium in plasma and serum with the Boehringer Mannheim Reflotron reflectance photometric analyzer, which is designed for near-patient testing in hospitals and physicians' offices. This potassium method does not require calibration or instrument maintenance by the operator. Analysis of 30 microL of plasma or serum takes approximately 140 s. Within-day imprecision (CV) was 1.0-1.2%. Total CVs over a 1-month period were 1.0-1.4%. Patients' results from the Reflotron correlated well with those from the IL 643 flame photometer and the Beckman Synchron CX3 ion-selective electrode methods. The accuracy of Reflotron values was also verified with Standard Reference Material 956 from the National Institute of Standards and Technology.