PTT
Test:
158
CPT:
85730
Synonyms
Activated Partial Thromboplastin Time (aPTT)
Special Instructions
This test is for screening purposes only; it should not be used for therapeutic unfractionated heparin monitoring. Please refer to test Heparin Anti-Xa [117101]. If the patient’s hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions.
Expected Turnaround Time
Within 1 day
Specimen Requirements
Specimen
Within 1 day
Volume
2.7 mL-sized tube filled to 90% capacity or 1.8 mL-sized tube filled to 90% capacity.
Minimum Volume
90% of full draw (Note: This volume does not allow for repeat testing.)
Container
Blue-top (sodium citrate) tube; do not open tube unless plasma is to be frozen.
Collection
Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood-to-anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples unless the sample is collected using a winged (butterfly) collection system. With a winged blood collection set a discard tube should be drawn first to account for the dead space of the tubing and prevent under-filling of the evacuated tube.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternative anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes.
Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.
Storage Instructions
Specimens are stable at room temperature for 24 hours. If testing cannot be completed within 24 hours, specimens should be centrifuged for at least 10 minutes at 1500xg; plasma should then be transferred to a Labcorp PP transpak frozen purple tube with screw cap (Labcorp No. 49482). Freeze immediately and maintain frozen until tested. Refer to Coagulation Collection Procedures for directions.
Causes for Rejection
Clotted specimen; hemolysis; tube <90% full; improper labeling; specimen collected in tube other than 3.2% citrate; refrigerated specimen
Test Details
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.