Laboratory Monitoring of Heparin Therapy
Recommended Laboratory Test
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 =
|0.1 Units||=||47 seconds (aPTT)
Frequency of aPTT Monitoring
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.
Following attainment of a stable aPTT prolongation within the therapeutic range,
the aPTT test should be ordered once a day until heparin therapy is
- 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
- 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
- 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:12/09/13 by: A.P. Huerta, MT (ASCP)