The timing of the reaction of moisture-sensitive reagents for detecting the presence of analytes, e.g. leukocytes in urine samples, is used to detect when the reagents have been compromised by excess humidity. The ratio of light reflectance at wavelengths characteristic of the products of reaction between the reagents and the analyte and an infra-red reference dye is measured at two preset times after a urine sample has been applied to a test strip and used to determine whether the reagents have been compromised by excessive humidity. The presence of unusually dark samples is determined from the reflected light at 470 and 625 nm in order to confirm that the test strips are humidity-compromised.