PTT Vs Anti Xa
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The PTT continues to be the principle method by which most laboratories monitor IV and fractionated heparin (UFH) therapy. It is a clot-based test that requires citrated patient blood to produce platelet-poor plasma, or PPP, for testing. Phospholipid and an activator are added to the patient’s PPP and this mixture is allowed to incubate. Calcium is added to reverse the citrate and the clotting time is measured.
The PTT offers many advantages. It is quick, cheap, and widely available.
However, it does not directly measure heparin.
A common situation is concurrent administration of anticoagulation, usually a vitamin K antagonist. Patients with INR greater than 1.3 due to warfarin may have unreliable PTT monitoring of heparin.
Another commonly encountered clinical condition is patients with antiphospholipid antibodies, which interfere with the clotting test. Furthermore, factor deficiencies, for example, as a result of liver disease or consumptive coagulopathy in disseminated intravascular coagulopathy, will alter the PTT results, which will also make it a poor reflection of heparin anticoagulation.
On the opposite side of the spectrum, clinical conditions may also result in a blunted PTT response. Acute-phase reactants factors VIII and fibrinogen are the most important confounders, and may be dramatically elevated and shorten the baseline PTT. At the same time, acutely ill patients are also known to have antithrombin deficiency. These conditions can result in potential overanticoagulation when monitoring heparin by the PTT.
The antifactor Xa assay is a chromogenic assay. A known amount of factor Xa is added to platelet-poor plasma, which includes therapeutically administered heparin. The heparin enhances factor Xa inhibition by antithrombin and the remaining uninhibited Xa cleaves an added chromogenic substrate. This process creates a colored compound that can be detected by a spectrophotometer and is directly proportional to the amount of factor Xa present.
By measuring the activity of only Xa, the anti-Xa assay is more specific than the PTT for measuring heparin activity.
However, the anti-Xa assay is not perfect. Being a chromogenic assay, the anti-X assay will be affected by hemolysis, icterus, and hypertriglyceridemia, which affects the instrument’s ability to measure and discriminate the chromogenic reaction. Patients with significantly elevated hemolysis, total bilirubin, and triglycerides, should have their heparin monitored by PTT.