Heparinised plasma samples have falsely high blank values in the automated immunoprecipitation system when blanks are measured by omitting antiserum from the assay. This is probably due to an interaction of heparin and serum proteins, together with either fibrinogen or polyethylene glycol. A more accurate estimate of the blank of heparinised plasma samples is obtained by measuring it with a solution of nonimmune serum substituted for the antiserum.
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