Quantitative detection of analytes on immunochromatographic strips

Disclosed is an improved method for determining the concentration of analyte in a test fluid by immunochromatography techniques which involves quantitatively determining the signals from captured analyte/labeled binding partner complex by instrumental means, e.g. reflectance spectrometry. In a preferred embodiment, a reflectance reading is determined for the captured complex and uncomplexed labeled binding partner which is captured in a separate zone of the immunochromatographic strip and the ratio of these reflectances is used to provide additional quantitation to the assay method.