Unfractionated heparin (UFH) remains widely used because it is effective, safe and inexpensive; it is the preferred anticoagulant when rapid and reversible anticoagulation is required because of its short half-life and its complete reversal of its anticoagulant effect with protamine. In patients with renal failure it is favoured because of non-renal clearance.
Although the APTT ratio is most widely used to monitor UFH therapy it is affected by analytical and biological variables independent of the anticoagulant effect of UFH, including lupus and low-level coagulation factors. In acutely ill patients, the APTT response may be lessened by elevated levels of fibrinogen and FVIII (acute phase reactants); in such patients, UFH therapy may be monitored using the anti-Xa assay.
It should be noted that UFH exerts an anticoagulant effect by potentiating the action of the serine protease inhibitor antithrombin (AT). The UFH assay relies on patient AT function; therefore patients with AT deficiency may remain at a sub-therapeutic UFH level following appropriate dosing.
The therapeutic range for UFH anti-Xa activity is 0.3 to 0.7 IU/mL. Initial heparin therapy monitoring should occur every 6 hours. Once the patient has reached a therapeutic UFH level (i.e. 0.3 to 0.7 IU/mL), monitoring may be extended to every 24 hours provided the UFH level remains therapeutic and the patient is clinically stable.
The volume of blood in coagulation samples must lie within the volume range as indicated by the size of the black fill arrow present on tubes. Volumes above or below the arrow will result in sample rejection to ensure validity of results.
Age of sample
Samples will be rejected if received more than 2 hours post venepuncture.
- 2 blue top (sodium citrate) samples
- All coagulation tubes must be adequately filled (see above)
- All coagulation tubes must be mixed several times by gentle inversion immediately after venepuncture. Mixing the sample with the anticoagulant stops the sample clotting within the tube.
- It is not possible to provide results on clotted, insufficient, lipaeamic, or haemolysed samples. These will be rejected with the appropriate comment.
- Samples tubes that have expired cannot be accepted.
Coagulation Lab, James Cook University Hospital, Marton Road, TS4 3BW