Laser nephelometric equivalence point titration for the measurements of specific proteins in turbid biological fluids.

A reliable, sensitive and specific laser nephelometric (LN) assay for the quantitative measurement of specific proteins in body fluids by equivalence point titration is described. The method is relatively free from interference encountered in conventional LN assays, e.g. unspecific background scatter, antigen excess, and the inhibition of complex formation by high ionic strength and salt concentration, low pH and elevated urea concentrations. It can be applied to turbid body fluids containing relatively small amounts of antigen. A serial dilution of the antigen preparation (human urinary albumin in the present work) is incubated with a fixed amount of highly diluted specific antiserum. The antigen concentration is determined by estimation of the equivalence point on the LN immunoprecipitation curve and comparison of this position with that on a standard curve. The data presented show good correlation with values obtained by radioimmunoassay (p less than or equal to 1 X 10(-5);log (LNET) = 0.956 log (RIA) - 0.03; n = 50).