The invention features a method of adjusting the concentration of at least one but not all of a plurality of analytes in a fluid sample to match a known working range of detection of an analyte assay system, where each of the plurality of analytes may or may not be present within an expected initial concentration range having a high end and a low end, and at least one analyte has a high end expected concentration range that exceeds the high end of the working range of the assay system. The expected concentration of the high concentration analyte is adjusted by a proportional scaling constant, .alpha., so that the high end of the adjusted expected concentration range is less than or equal to the high end of the working range, without adjusting the expected concentration range of at least one other of the plurality of analytes. Adjustment is preferably accomplished by adding to the solution phase of the assay one or more scaling agents, each scaling agent binding with specificity to an analyte and thereby preventing it from being detected by the assay system, e.g., by competing with binding to immobilized capture agent. This scaling method contrasts with prior methods, in which a concentration of available analyte is offset by a fixed amount to adjust the detectable threshold of the assay. Here, the amount of scaling agent is proportional to a scaling coefficient, and the scaling agent is present in the solution phase of the assay at high concentrations relative to analyte. Due to the equilibrium conditions established by the laws of mass transfer, the amount of free analyte remaining in solution in the presence of scaling agent is predictable and finite, and can be measured as a quantitative indicator of the initial concentration of the analyte in the sample.