Devices, systems, and methods for measuring biomarkers in biological fluids

Platforms for enzymatic assays for biomarkers, including systems, methods, and measuring devices by which a biomarker, such as creatinine, is measured using a small amount of biological fluid, such as blood, plasma, or serum. The measuring device or biosensor can be a test strip including a layered active component assembly positioned between two outer layers which enables multi-step enzymatic reactions operating in kinetic and/or endpoint (in which the reaction is allowed to near completion), and generally includes multiple layers with primary enzyme(s), coupling enzyme(s), and reagents to produce an optical signal correlated to the concentration of a biomarker in the sample. The test strip can be read using a portable optical reader coupled to a smart phone or tablet.