Activated Partial Thromboplastin Time: (089-0220)

Clinical Indication:

Useful in monitoring unfractionated heparin therapy and detecting abnormalities of the intrinsic clotting pathway.

Test Mnemonic:

 APTT

Methodology:

 Electromagnetic Viscosity Detection

Performed:

 Samples are accepted 24 hours per day at Sample Management, 5.136 McCullough Bldg.

Turnaround Time:

 STAT: 1 hour; ROUTINE: 4 hours

Specimen Requirements:

Collect: Collect:   Blue-topped (3.2% sodium-citrate) tube. Routine venipuncture; discard 1st mL of blood by collecting a discard tube prior to collecting the blue-topped (3.2% sodium-citrate) tube.  For collections with butterfly blood collection sets, a discard tube should also be collected prior to collection of the blue top to ensure sufficient sample volume. Drawing a discard tube will displace the air from the blood collection set tubing to ensure proper blood draw volume. For patients on heparin therapy, the whole blood sample must be centrifuged within 1 hour of collection.

 Specimen PreparationAcceptable up to 4 hours after draw, at ROOM TEMPERATURE (20 +/- 5°C).

Storage/Transport: NOTE:  If delivery time is to be greater than 4 hours from time of draw, centrifuge the specimen for 15 minutes at 2000-2500g.  Separate the plasma from the cells and freeze at -200C. Stability:  Plasma 4 hrs at 20 +/- 5°C.   Patients on heparin therapy -plasma is stable for 2 hrs at 20 +/- 5°C when collected in 3.2% sodium citrate (blue top).   Frozen plasma 2 Weeks at   -20°C.  Whole blood is stable for 4 hours at room temperature.

 Causes for Rejection: QNS, clotted, severely hemolyzed, greater than 4 hours old, wrong tube (3.8% sodium citrate), high hematocrit (> 55%), sample identification error, processed and/or stored improperly.

Clinical Information:

Currently, the activated Partial Thromboplastin Time (aPTT) is the laboratory test most commonly used to monitor unfractionated heparin therapy. The Heparin Therapeutic Range is developed utilizing a procedure derived from Brill-Edwards, et. al., in which aPTT values and heparin levels are obtained from patients actually receiving heparin. 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 = 44 seconds (aPTT)
                                            0.3 Units = 63 seconds (aPTT)
                                            0.7 Units = 101 seconds (aPTT)

However, 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. Refer to Monitoring Heparin Therapy in Lab Survival Guide http://www.utmb.edu/lsg/hem/ACTIVATED_PARTIAL_THROMBOPLASTIN_TIME.htm or contact laboratory [409-772-3314] for further recommendations.

Critical Values: > 70 seconds  (if patient is not on anticoagulant therapy)

Interfering  Substances:  Anticoagulant therapy, circulating anticoagulants

Reference Range:

23 - 38 seconds

CPT 4 Code:

 85730  Limited Coverage Test (NCD)

Reviewed:   3/20/13                  

 W. Russo, MT(ASCP)

When ordering tests for which Medicare or Medicaid reimbursement will be sought, physicians should only order tests that are medically necessary for the diagnosis or treatment of the patient.  Components of the organ or disease panels may be ordered individually.  The diagnostic information must substantiate all tests ordered and must be in the form of an ICD-9 code or its verbal equivalent.