A test strip or electrochemical sensor for measuring the amount of an analyte in a hiological fluid, e.g., the glucose content of whole blood, includes a size self-limiting reagent formulation employing an enzyme system for reaction with the analyte, the reactive system mixed into a water-soluble swellable polymer matrix containing small water-insoluble particles having a nominal size of about 0.05 to 20 .mu.m, preferably about 1 to 10 .mu.m. The weight ratio of the water-insoluble particles to the water-soluble swellable polymer matrix is about 1/2 to 2/1. The reagent formulation is deposited onto a non-porous substrate to form a thin layer about 6-16 .mu.m thick, providing a rapid and stable response to application of a sample, while being insensitive to the amount of the sample.