 Analyte-specific reagent

Analyte-specific reagents (ASRs) are a class of biological molecules which can be used to identify and measure the amount of an individual chemical substance in biological specimens.The U.S. Food and Drug Administration (FDA) defines "analyte specific reagents" (ASRs) in "21 CFR 864.4020" as “antibodies, both polyclonal and monoclonal, specific receptor proteins, ligands, nucleic acid sequences, and similar reagents which, through specific binding or chemical reaction with substances in a specimen, are intended to use in a diagnostic application for identification and quantification of an individual chemical substance or ligand in biological specimens.” 

In simple terms an analyte specific reagent is the active ingredient of an in-house test.