aPTT (Partial Thromboplastin Time (PTT), Activated)
Activated Partial Thromboplastin Time (aPTT)
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.
The aPTT is often ordered, along with the prothrombin time, to diagnose the cause of patient bleeding or as part of a presurgical screen to rule out coagulation defects.9-11 The aPTT can be prolonged when the activities of any of the factors of the intrinsic pathway are significantly diminished. Deficiencies or inhibition of high molecular weight kininogen (HMWK), prekallikrein, or factors XII, XI, IX, and VIII can result in an extended aPTT with a normal protime (PT) since these factors are not part of the extrinsic pathway. Significant deficiencies of factors that are common to both the intrinsic and extrinsic pathways (factors X, V, prothrombin, or fibrinogen) can extend both the aPTT and PT.
An extended aPTT can be seen in acquired deficiencies of factors II, IX, and X that result from vitamin K deficiency or the use of anticoagulants that block vitamin K-dependent production of procoagulant factors. These conditions also affect the level of factor VII, an extrinsic pathway factor. Since factor VII has a short half-life relative to the vitamin K-dependent factors of the intrinsic pathway, nutritional or therapeutic vitamin K-dependent factor deficiency can sometimes result in an extended PT with a normal aPTT. Consumption coagulopathies, such as disseminated intravascular coagulation (DIC), can produce an extended aPTT due to depletion of intrinsic factors. The aPTT can also be extended in conditions that reduce the production of procoagulant factors (ie, severe liver disease or malnutrition). Inhibitors, both factor specific and nonspecific, can also prolong the aPTT. A description of the many potential causes of an extended aPTT is described in more detail in the online Coagulation Appendix: Lupus Anticoagulants.
Unfractionated heparin is commonly used to limit fibrin clot formation in individuals with increased risk of venous or arterial thrombosis.12 Overdosing with heparin can increase the risk of hemorrhage and inadequate dosing decreases the efficacy of anticoagulation. Heparin works as an anticoagulant by enhancing the ability of plasma antithrombin to bind and inactivate the serine proteases XII, XI, IX, X, and thrombin. Therapeutic monitoring with the aPTT is commonly used because of the wide interindividual variation in response to this therapy; however, this application of the aPTT test can be less than optimal in a number of clinical circumstances
Whole blood or plasma
4.5 mL, 2.7 mL, 1.8 mL
90% of full draw
Blue-top (sodium citrate) tube
Draw specimen one hour before next dose of heparin if heparin is being given by intermittent injection. Do not draw from an arm with a heparin lock or heparinized catheter.
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.
Storage & Stability 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 transpak frozen purple tube with screw cap Freeze immediately and maintain frozen until tested.
Requirements for patients receiving heparin are different: If testing cannot be performed within one hour of collection, frozen plasma must be submitted. Specimens should be centrifuged for at least 15 minutes at 1500xg to produce platelet-poor plasma and the plasma quick frozen and maintained in this condition until tested.
Gross hemolysis; clotted specimen; frozen specimen thawed in transit; tubes <90% full; improper labeling; specimen collected in tube other than 3.2% citrate
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