SPARCL? (Spatial Proximity Analyte Reagent Capture Luminescence) assays use two specific antibodies; one conjugated to HRP, the other to acridan, a chemiluminescent substrate. When HRP and acridan conjugated antibodies bind to their target biomarker they are brought into close proximity. Upon addition of hydrogen peroxide (trigger solution), HRP catalyzes oxidation of proximal acridan molecules causing a flash of chemiluminescence that is proportional to biomarker concentration. Acridan molecules distant from HRP are not oxidized and therefore produce no luminescence. This principle allows development of a range of rapid and sensitive homogeneous immunoassays.