[Influence of heparin on the radioimmunological assay of ACTH].

Heparin traps plasma ACTH, promoting the formation of aggregates with apparent high molecular weight as shown by chromatography on Sephadex G 50 fine columns. The percentage of 125I-ACTH which appeared in the void volume of the column, increased linearly with the log. dose of heparin. Heparin at concentrations of up to 100 IU/ml does not impair ACTH adsorption on either silicic acid or Quso G 32 as well as further elution by acetic acid/acetone/water (I : 40 : 59; V/V) or HCl O.I N. Silicic acid traps selectively ACTH but not heparin. Heparin interferes with direct RIA-ACTH in the plasma by decreasing 125I-ACTH binding to the antibodies and modifying the slope of the standard curve. Unsuitable artefacts induced by heparin, as overestimation or underestimation of plasma ACTH levels by RIA, can be avoid by previous hormone extraction from heparinized plasmas. Such results emphasized the importance of the sample preparation in order to obtain consistent results.