A quantitative dot immunobinding assay for coagulation factor XII in plasma.

A dot immunobinding assay on nitrocellulose (NC) membranes has been developed for the quantification of human coagulation factor XII (F XII). Plasma samples were dotted on to NC filters and F XII was detected using a polyclonal antiserum followed by a radiolabelled antigen overlay. Dilutions of either pooled normal human plasma (NHP) or purified F XII in F XII deficient plasma were used as standards. Quantification was performed by measuring the radioactivity of bound 125I-F XII. Precise measurements of F XII antigen (F XII: Ag) were possible with a sensitivity down to 0.12 ng. Thus, dotting samples containing 0.5 microliter of plasma permitted detection of a F XII concentration corresponding to 1% of the level in NHP. The intra-assay coefficient of variation (CV) was less than 5% and the interassay CV was less than 16%. F XII:Ag in plasma samples of 50 healthy adults ranged from 12 micrograms/ml to 47 micrograms/ml. A good correlation (r = 0.93) existed between F XII:Ag and F XII clot promoting activity (F XII:C) in these samples. NHP contained 24.1 micrograms/ml F XII:Ag confirming earlier results obtained by other methods. In 16 pregnant women levels of F XII:Ag as well as of F XII:C were elevated, but F XII:Ag was disproportionately higher compared with F XII:C. The immunobinding assay has the following advantages: (1) rapid quantification of large numbers of samples is possible, (2) the sensitivity down to 1% of NHP is better than that of several other methods, (3) only very small amounts of both test material and reagents are needed.