For several years, most coagulation specialists have supported using the chromogenic anti-Xa assay in place of the PTT for monitoring unfractionated heparin therapy. We contend the anti-Xa is more accurate and reproducible as it is less prone to interference. Many medical center laboratories have converted to the anti-Xa as their primary means for monitoring UFH , and can also use it to measure low molecular weight heparin and synthetic pentasaccharide therapy. These institutions still offer the PTT as a coagulopathy screen and many surgeons and physicians insist on using both the anti-Xa and the PTT for monitoring UFH.
A pathologist colleague recounts that surgeons at his institution have encountered a few patients whose anti-Xa values were below the therapeutic range limit, 0.3–0.7 units/mL while the PTTs from the same specimen were prolonged beyond the upper limit of the therapeutic target range in seconds. Increasing the drip rate to achieve the anti-Xa therapeutic target resulted in serious hemorrhaging.
In a single instance, the patient's PTT was prolonged to within the therapeutic target range but the anti-Xa was below the limit, and the antithrombin level was 30%. When the patient was given antithrombin concentrate, the PTT remained nearly the same but the anti-Xa value rose to within the therapeutic target range.
I don't have a good answer for my friend, however I know there are many who consider the anti-Xa , a primary measure, to be inferior to the PTT , a secondary measure, because the PTT is a surrogate for the outcome of UFH therapy and not merely UFH concentration. They say the PTT more closely reflects the patient's clinical condition. I'd like to get some comments from colleagues who have similar experiences and who can both explain anti-Xa –PTT discrepancies and who can advise which of the measures to rely upon in the clinic or surgery.