LABORATORY MONITORING OF HEPARIN THERAPY
RECOMMENDED LABORATORY TEST
Currently, the Activated Partial Thromboplastin Time (aPTT) is the laboratory test most commonly used to monitor UNFRACTIONATED HEPARIN THERAPY. However, some patients receiving heparin but not demonstrating an adequate aPTT prolongation may require further laboratory evaluation for heparin resistance. The AntiXa test can quantitatively determine the plasma level of Unfractionated Heparin as well as Low Molecular Weight Heparin.
aPTT THERAPEUTIC RANGE
Historically, the laboratory has recommended an aPTT prolongation of 1.5 – 2.5 times the MEAN NORMAL REFERENCE INTERVAL. (The MEAN NORMAL value is recalculated with each change in reagent lot number, approximately once per year.) The current MEAN NORMAL is reported with each patient test result.
Example: aPTT MEAN NORMAL = 30 seconds (10/16/06 to present)
Therapeutic Range = 45 – 75 seconds (1.5 – 2.5 x MEAN NORMAL)
Recently, another method of determining the Heparin Therapeutic Range was developed utilizing a procedure derived from Brill-Edwards, et al, in which aPTT values and heparin levels are obtained from patients actually receiving heparin. Using linear-regression, a graph is prepared that correlates the aPTT in seconds to the heparin Anti-Xa units. The ranges established, are the time in seconds equivalent to 0.1 to 0.3 and 0.3 to 0.7 Anti-Xa units of heparin.
Therapeutic Range =
FREQUENCY OF aPTT MONITORING
INITIATION PHASE: Upon initiation of heparin therapy, the aPTT test should be ordered every 6 hours until the result falls within the target therapeutic range. Whenever the heparin dosage is changed, the aPTT should be reevaluated every 6 hours until the desired therapeutic range is reached.
STABLE PHASE: Following attainment of a stable aPTT prolongation within the therapeutic range, the aPTT test should be ordered once a day until heparin therapy is discontinued.
· The daily collection time should be standardized (preferably prior to 10 AM) to avoid any diurnal variation in aPTT results (despite constant heparin infusion rates).
· To avoid falsely prolonged aPTT results due to heparin contamination, specimens for monitoring heparin therapy should not be collected from the same extremity used for heparin infusion or from an indwelling catheter.
· Prolongation of the aPTT does not necessarily indicate that the blood is effectively anticoagulated in vivo. A variety of conditions can complicate the administration and monitoring of unfractionated heparin therapy (liver disease, renal disease, obesity, aging, etc.).
· Patients receiving heparin but not demonstrating an adequate aPTT prolongation may require further laboratory evaluation for heparin resistance. Contact the laboratory for assistance (409-772-3314).
· Patients with anti-phospholipid antibodies ("lupus anticoagulants") may exhibit a significant prolongation of the aPTT yet still be at increased risk of thrombosis. Specialized assays are required for heparin monitoring in patients with anti-phospholipid antibodies [as well as contact factor (factor XII, prekallikrein, HMW kininogen) deficiencies]. Contact the laboratory for assistance.
· The aPTT is NOT recommended for monitoring low molecular weight heparin, danaproid or direct heparin inhibitors such as hirudin.
· Call lab for further recommendations on monitoring anticoagulation.
Reviewed 4/5/2010 by W.Russo, MT (ASCP)