Development of a RIA for clonidine and its comparison with the reference methods.

A radioimmunoassay for clonidine was optimized by introducing, as a tracer ligand, an iodinized clonidine derivative specifically labeled with more than 500 Ci/mMol. The detection limit of clonidine was 10 pg/ml. The high assay specificity was demonstrated by a double isotope technique. The intraassay coefficient of variance (VK) was less than 4% for any concentration, the interassay VK did not exceed 8.3%. The assay reliability was increased when the plasma samples were diluted 1:5, prior to analysis, in normal human blood plasma, which was also used for the preparation of the calibration samples. In a comparative manner, this assay system was subjected to double-blind accuracy control tests, including all other reference methods so far described for clonidine analysis in blood plasma samples. The RIA technique presented here turned out to be the most sensitive and most reliable method, requiring, moreover, the smallest volumes of blood plasma (0.05 ml). This procedure was appropriate for routine tests, since one technician could perform 500 sample analyses a day. The advantages and disadvantages of each method are discussed.