Partial Thromboplastin Time (PTT), Activated
Also known as: Activated Partial Thromboplastin Time (aPTT)
Use
The aPTT is sensitive to deficiency or inhibition of factors in the intrinsic pathway. These include the contact factors; high molecular weight kininogen (HMWK), prekallikrein, and factor XII along with procoagulant factors XI, IX, VIII. The aPTT is less sensitive to deficiencies of X, V, prothrombin, and fibrinogen.1 Nonspecific, lupus-type anticoagulants can also extend the aPTT, but the more sensitive aPTT-LA test should be used to screen for this condition.
Special Instructions
This test is for screening purposes only and should not be used for monitoring therapeutic unfractionated heparin. For therapeutic monitoring, please refer to the Heparin Anti-Xa test. It is important to adjust the volume of citrate in the collection tube if the patient's hematocrit exceeds 55%. Follow 'Coagulation Collection Procedures' for guidance. Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate, ensuring proper blood-to-anticoagulant ratio by filling tubes to completion and mixing adequately.
Limitations
The aPTT test has limitations for identifying lupus anticoagulants, where a more sensitive aPTT-LA test may be preferred. Individual factor deficiencies may not significantly extend the aPTT unless levels fall below 15% to 45%, depending on the factor. Multiple factor deficiencies can cause an extended aPTT, but the test is more sensitive to intrinsic pathway factor deficiencies than common pathway deficiencies. Conditions that elevate factor VIII, such as acute phase reactions, can normalize an extended aPTT. Several factors such as aging, obesity, liver or renal disease, and variations in heparin response can complicate its use in heparin therapy monitoring. The test should not be used for monitoring low molecular weight heparin or danaparoid.
Methodology
Other
Biomarkers
LOINC Codes
- 14979-9
- 14979-9
Result Turnaround Time
1 day
Related Documents
For more information, please review the documents below
Specimen
Whole Blood
Volume
4.5 mL, 2.7 mL, 1.8 mL
Minimum Volume
90% of full draw
Container
Blue-top (sodium citrate) tube
Collection Instructions
Collect blood in a 3.2% buffered sodium citrate blue-top tube. Fill tubes to completion to ensure proper blood-to-anticoagulant ratio. Mix by gentle inversion at least six times. If using a butterfly collection set, draw a discard tube first.
Patient Preparation
Draw specimen one hour before the next dose of heparin if given by intermittent injection. Do not draw from an arm with a heparin lock or heparinized catheter.
Storage Instructions
Specimens stable at room temperature for 24 hours; if not tested within this time, centrifuge and freeze plasma. For patients on heparin, if not tested within one hour, submit frozen plasma.
Causes for Rejection
Gross hemolysis, clotted specimen, thawed frozen specimen, tubes <90% full, improper labeling, use of incorrect tube type, refrigerated sample
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 24 hours |