- Light blue top (citrate) tube
- Fill tube to maximum draw
Additional information on specimen collection:
- Mix well. Do not centrifuge
- Reject:
- Grossly hemolyzed specimen
- Frozen specimen that has thawed during transport
- Clotted specimen
- Specimen collected in tube other than that containing 3.2% citrate as anticoagulant
- Specimen received more than 2 hrs after collection
Activated Partial Thromboplastin Time (aPTT) test is done to evaluate blood coagulation disorders; especially of the intrinsic coagulation pathway. It measures the length of time (in seconds) required for clotting to occur when certain reagents are added to plasma.
This test is most commonly used to monitor heparin therapy; but can also be used to investigate a bleeding or thrombotic condition.
The aPTT test should not be used as a routine general screening test. It should be performed to:
- Evaluate patients with bleeding episodes
- Evaluate patients with thromobotic episodes
- Evaluate risk of excessive bleeding (before a surgery)
- Monitor anticoagulant therapy (heparin therapy)
aPTT evaluates all clotting factors except the factor VII and factor XIII (intrinsic pathway). It does not however, reflect the platelet function.
Monitoring heparin therapy:
- When used for monitoring heparin therapy, aPTT should typically be performed every 6 hours, until the desired therapeutic range is reached (2-3 times control)
- aPTT values should be maintained at 3 times the control value by heparinization for 18 to 24 hours after a percutaneous transluminal coronary angioplasty (PTCA)
- When heparin is given in large volumes, as during cardiopulmonary bypass, the activated clotting time (ACT) is a preferred test for monitoring heparin therapy.
In order to differentiate between coagulation factor deficiencies and the presence of coagulation factor inhibitors like antifactor antibodies, mixing studies may be performed.
The clotting time can be corrected by mixing the patient's clotting factor deficient plasma with normal plasma. However, if the prolonged aPTT is due to coagulation factor inhibitors, there will be no correction in mixing studies.
Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.
Quantitative
- Normal: 25-39 seconds
- Therapeutic: Depending upon indication, typically 2-3 times the control value
Increased aPTT will occur in the following conditions:
Drugs that increase aPTT include:
- Therapeutic anticoagulants
- Other drugs such as:
- Anistreplase
- Antihistamines
- Ascorbic acid
- Chlorpromazine
- Salicylates
Critical Values:
- >70 seconds
- Important signs:
- Bleeding gums
- Blood in stool
- Hematoma at the puncture site
- Hemorrhage
- Prolonged bleeding
- Shock
- Administration of protamine sulfate can be used to reverse heparin and to some degree heparinoids or LMWH's
The aPTT value may be decreased in the following conditions:
- Hypercoagulable states
- DIC
- Inadequate centrifugation