1. An assay test strip, comprising: a flow path for a fluid sample; a sample receiving zone coupled to the flow path; a label that specifically binds a target analyte; a detection zone coupled to the flow path and comprising a region of interest and an immobilized test reagent that specifically binds the target analyte; and at least one position marker aligned with respect to the region of interest such that location of the at least one position marker indicates a position of the region of interest, wherein the at least one position marker is calibrated to provide at least one reference optical response to illuminating light within a specified wavelength range.