Abstract: A system and method of determining absence or presence of an illicit substance in a flowing mixture of requisite reagents by agglutination or lack thereof. A plurality of photodetectors are positioned over an illuminated read area oblique to a path of the flowing mixture. Signal samples of each of the detectors are processed individually. Successive samples are processed over a period time to detect differences or edges between relatively clear liquid and relatively opaque clusters in the flowing mixture. The illicit substance is determined in accordance with the number of variations in signal samples for a predetermined number of detectors.