Abstract: A test means, test device and method for use for determining the presence of an ion in a test sample are disclosed. The test means comprises a substantially nonpolar, nonporous carrier matrix incorporated with an ionophore capable of forming a complex with the specific ion and reporter substance capable of interacting with the complex of ionophore and ion to produce a detectable response. The test means disclosed is particularly useful for serum potassium determinations. The test means forms the reagent layer in a multilayer format particularly suited to whole blood electrolyte determinations.