Comparison Between Activated Partial Thromboplastin Time and Anti-Xa Heparin Assays on Patients on Unfractionated Heparin Therapy

The clinical laboratory at Veterans Affairs Medical Center–Memphis recently implemented anti-Xa heparin assays to monitor therapy for unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH). The pharmacy became concerned by possible discrepant results between the new assay and the activated partial thromboplastin time (aPTT), and we reviewed early data generated by the anti-Xa assay. Consecutive anti-Xa heparin results (n = 105 from 30 patients) were collected that had sufficient specimen for a simultaneous aPTT. Both assays used reagent and equipment from Diagnostica Stago (Parsippany, NJ) and were performed according to manufacturer’s directions. A single hybrid calibration curve was used for both UFH and LMWH. Of 105 specimens, 85 were from patients receiving UFH and the latter were grouped into three categories according to the results of the anti-Xa assay: subtherapeutic (group 1 [anti-Xa result < 0.3 IU/mL]) (n = 45), therapeutic (group 2 [0.3-0.7 IU/mL]) (n = 30) and supertherapeutic (group 3 [> 0.7 IU/mL]) (n = 10). Additionally, 10 specimens came from a patient receiving rivaroxaban and UFH, and 10 specimens were from 6 patients receiving LMWH. Statistical analysis used the Kruskal-Wallis nonparametric one-way analysis of variance. When required for statistical analysis, values of aPTT > 235 and > 276 seconds were assigned random number between 235 and 275 seconds, and 277 and 299 seconds respectively, using the randbetween function on Excel software (Microsoft, Richmond, WA). The median aPTTs were 47.7 seconds (25.8, 123.6), 136 seconds (76.2, 276) and > 235 seconds (39, > 276) in groups 1-3 respectively (P < .001). In group 1, 43/45 (95.5%) were < 100 seconds, while in group 2, 23/30 (76.6%) were > 100 seconds, and 6/30 specimens (20%) were > 200 seconds. In group 3, 8/10 (80%) were > 200 seconds. Although there is a correlation between heparin anti-Xa activity and aPTT, results of the latter are frequently increased beyond 100 seconds in patients whose anti-Xa UFH activity is within the therapeutic range. It remains an open question as to which parameter is to be preferred for monitoring patients receiving UFH.