Anti Xa monitoring during treatment with low molecular weight heparin or danaparoid: Inter-assay variability

If laboratory monitoring of low molecular weight heparin (LMWH) therapy is required the test of choice is the anti Xa activity assay. The relationship between anti Xa results obtained using different techniques is unknown. Th e aim of the present study was to compare anti Xa results obtained with eig ht different commercially available anti Xa activity assays (five chromogen ic and three clotting based assays) in samples from patients receiving eith er therapeutic or prophylactic LMWH (enoxaparin or dalteparin) or danaparoi d. We have demonstrated that highly significant differences exist between resu lts obtained using different techniques. The mean anti Xa activity in patie nts receiving treatment or prophylaxis with enoxaparin ranged from 0.28 to 0.64 iu/ml. A similar relationship was present in samples from patients tre ated with dalteparin, mean anti Xa results ranging from 0.43 to 0.69 iu/ml. The Heptest clotting assay as used here in combination with the Automated Coagulation Laboratory instrument, was associated with lower results than o ther clotting or chromogenic techniques. In patients receiving danaparoid f or heparin induced thrombocytopaenia (HIT) mean results with three clotting based assays were 0.30 to 0.36 u/ml, compared to mean results of 0.47 to 0 .65 u/ml for chromogenic assays. Our data clearly indicate that the selection of anti Xa assay method could influence patient management since the dose required to achieve the therape utic range would differ according to the assay employed. This is particular ly important since the frequency of hamorrhagic side effects has been shown by others to be dose dependant, irrespective of the concomitant anti Xa ac tivity results. In danaparoid therapy the clotting assays studied here shou ld not be employed for monitoring without a modified target range, unless i t can be demonstrated that the higher doses required to achieve the therape utic range are safe.