Effect of N-acetylcysteine on the accuracy of the prothrombin time assay of plasma coagulation factor II+VII+X activity in subjects infused with the drug. Influence of time and temperature.

OBJECTIVES

The prothrombin time (PT) assay of factor II+VII+X activity is an important predictor of liver damage in paracetamol poisoned patients. It complicates interpretation of results that the antidote, acetylcysteine (NAC) depresses this activity. The aim was to investigate if NAC influences the accuracy of the plasma PT assay.

MATERIALS AND METHODS

The accuracy of Nycotest PT was studied using plasma added NAC in vitro and plasma from subjects infused with NAC. The latter results were compared with those obtained by analysis of PT by CoaguChek S.

RESULTS

Therapeutic NAC concentrations added to plasma in vitro decreased factor II+VII+X activity at 37 degrees C in a time-dependent manner. This effect was quenched at temperatures <24 degrees C. Activity lost at 37 degrees C could partly be recovered by subsequent incubation at 5 or 20 degrees C. Incubation at 37 degrees C prior to assay led to a significant additional depression of factor II+VII+X activity in plasma from subjects infused with NAC during the first 3h of infusion indicating that it contained reactive NAC. The risk that this NAC interfered with the accuracy of the PT assay was considered minimal with samples stored below 24 degrees C. This was supported by similarity of results obtained by analysis of appropriately stored plasma and simultaneously drawn blood by CoaguChek S.

CONCLUSIONS

Residual reactive NAC does not interfere with the accuracy of the PT assay of plasma stored below 24 degrees C, but NAC-induced loss in activity at 37 degrees C may be partly recovered during subsequent storage below 24 degrees C.