Field of the Invention
The present invention relates to diagnostic assays for analytes in a liquid sample.  In some embodiments, methods and devices for the detection of an analyte in a body fluid using, impedance are provided. 
Description of the Related Art
Many types of ligand-receptor assays have been used to detect the presence of analytes in body fluids such as saliva, urine or blood.  These assays typically involve antigen-antibody reactions, synthetic conjugates comprising enzymatic, fluorescent, or visually observable tags, and specially designed reactor chambers.  In most of these assays, there is a receptor (e.g., an antibody) specific for the selected analyte (e.g., antigen), and a means for detecting the presence and/or amount of the antigen-antibody reaction product.  Although some commercially available tests are designed to a quantitative determination, in many circumstances all that is required is a qualitative indication (e.g., positive/negative).  Examples of such qualitative assays include blood typing, pregnancy testing, ovulation prediction and many types of urinalysis. 
Diagnostic assays should generally be very sensitive because of the often low concentrations of analytes of interest, present in a test fluid.  False positives can be problematic particularly with agglutination and other rapid detection methods such as dipstick and color change tests.  Because of these problems, sandwich assays which use metal sols or other types of colored particles have been developed that rely on the interaction between avidin and biotin-tagged antibodies.  For example, in some commercially available pregnancy tests, an antibody-antigen sandwich complex comprising a colloidal gold-labeled anti-hCG antibody and an anti-hCG biotin-labeled antibody is used.  Test strips of this nature are known in the art, and are described in more detail in, for example, FIGS.  2-6 and the accompanying description of U.S. Pat.  No.  6,319,676, the content of which is hereby incorporated by reference in its entirety. 
Other proposals have sought to exploit the changes in impedance that occur due to the presence of metal-labeled antibodies.  For example, International Application Publication No.  WO2013/083686, the content of which is herein incorporated by reference in its entirety, discloses electrodes located on test and reference regions measuring changes in capacitance.  A difference in capacitance between the test and reference regions is indicative of the presence and quantity of a bound metal and thus of the target analyte.  However, in such systems the changes in the capacitance are small, and the layout thus requires precision.  This requires more precise tools and other improvements to the designing, building and assembling of such devices.  Thus, burdensome constraints are imposed on production, driving up costs for end users.  Therefore, it is desirable to have further improvements in the design, reliability, and ease of manufacture for impedance-based assays and to provide the market with affordable test kits.