Abstract: In an assay, an analyte in a sample is contacted with a thermostable reporter adenylate kinase coupled to a binding agent specific for the analyte, wherein a complex is formed. ADP is added, and then formation of ATP is monitored. Prior to the addition of ADP, endogenous kinase and uncomplexed thermostable reporter adenylate kinase is substantially removed by washing and residual-endogenous kinase is inactivated by heating. Prior to contacting the analyte with the thermostable reporter adenylate kinase, the sample has a background activity of at least 300,000 Relative Light Units per mg protein per ml sample when measured in the presence of luciferin/luciferase by a luminometer.