Analyte assay using superaggregated complex as labelled reagent

In a method for the qualitative or quantitative determination of an analyte in a test sample comprising:

• immobilizing a labelled reagent comprising a gold sol bound to a protein or fragment thereof specifically binding to said analyte, or a gold sol bound to a protein or fragment thereof which is a specific binding partner for a substance which specifically binds to said analyte, in bound form on a solid phase;
• and
• detecting the presence or absence of said labelled reagent thereby providing an indication of the presence or quantity of the analyte in the sample;
• wherein the improvement comprises as the labelled reagent a superaggregated complex of a plurality of molecules of said protein or fragment and a plurality of particles of said gold sol wherein the superaggregated complex has a mean diameter of 50-5000 nm and wherein at least 75% by weight of the gold particles of the gold sol have a mean diameter of less than 20 nanometers,
• wherein the superaggregated complex is obtained by the process of:
• (i) mixing said protein or fragment thereof and gold sol at a pH of 1-5, or at neutral pH provided that the gold sol is used in molar excess relative to the protein and the protein molecule exhibits at least two positively charged groups at the pH used, so as to form macroscopic aggregates;
• (ii) collecting the macroscopic aggregates so formed;
• (iii) resuspending the macroscopic aggregates in a pH-neutral medium, optionally with ultrasonic treatment, to form a suspension of stable superaggregated complexes.