The Clinical Importance Of Heparin Assays In Plasma. Critical Aspects In Antixa Techniques

Different clotting assays for heparin measurement in plasma, based on AntiXa potentiating effect were studied, to determine the causes of variability in results in the currently used techniques. A modified 2 steps technique was also used (step I:0.1ml test plasma (PPP)+0.3ml buffer+0.1 mlXa.2' incubation at 37°C:step II: 0.1ml of step I mixtu- re+0.2ml of substrate plasma+CaCl 0.1ml. (Xa and VII-X defficitary bo vine plasma+cefalin: Thame, Oxon. Triz-Mal buffer,ionic strength 0.15, PH7.5 and CaCl 0.025M). Three modifications of the technique were used, varying test plasma treatment: a) using oxalated PPP, adsorbed with BaS04 b) like in a) plus heating it at 56°C 15'; c)using untreated plasma.

Using a) assay (K dependant factors absent), straight regression lines and low variability of results was obtained, relating heparin concentrations to clotting times;usinga b) it was seen that heat diminishes AntiXa reactivity to hepa- riny using c)high variability in results was obtained. To show the difficulties in monitoring heparin in cases where Iox AntiXa concentrations, variable amounts of clotting factors and differents amounts of heparin might be present, a pool of normal plasmas, a normal plasma, and a cirrhotic one were compared, using the modified AntiXa assay in its 3 variations. Wide differences in results may be obtained while studying the cirrhotic plasma, using techniques that can vary AntiXa reactivity, or mask the real heparin in plasma, because of its variable content in K dependant factors.

We did not find a way of clearing test plasma of K dependant factors when heparin is in it without altering heparin concentration or AntiXa reactivity.

Thus, it is suggested, for clinical monitoring of the drug to use heated or untreated plasma, and to compare results with the same individuals plasma, studied under equal conditions, and not with a pool of normal plasmas, as common ly suggested.