Analytical elements and methods for the selective determination of an analyte in aqueous fluids containing the analyte. These elements and methods feature means for continuously releasing chromogenic indicator reagent from a reagent zone to a reaction zone. The continuous release means is responsive to the application of a sample of the fluid to continuously release reagent into the reaction zone at a rate producing color response corresponding to interaction of the indicator with the analyte and reduced interaction of the indicator with interferents. In preferred embodiments, albumin is determined in the presence of interfering proteins such as globulins using buffered chromogenic indicator reagent. In such embodiments, when protein interferents are present, their interference can be substantially eliminated for up to three minutes, during which time color response is substantially only from the interaction of albumin and reagent. The determination of albumin follows from such color response.
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