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Introduction ⬇

The PTT, a one-stage clotting test, screens for coagulation disorders. Specifically, it can detect deficiencies of the intrinsic thromboplastin system and also reveal defects in the extrinsic coagulation mechanism pathway.

The aPTT is used to detect deficiencies in the intrinsic coagulation system, to detect incubating anticoagulants, and to monitor heparin therapy. It is part of a coagulation panel workup.

Normal Findings ⬆ ⬇

aPTT: 21.0–35.0 seconds

Check with your laboratory for therapeutic range values during heparin therapy (2–2.5 times normal).

Clinical Alert

Critical ValueaPTT >70 seconds signifies spontaneous bleeding.

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood sample in a light blue–topped tube and anticoagulate with 3.2% sodium citrate. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

  2. Do not draw blood samples from a heparin lock or heparinized catheter.

  3. Although the sample may be transported at room temperature, the tube vacuum must be intact (do not remove stopper). It is stable for 12 hours.

Clinical Implications ⬆ ⬇

  1. Prolonged aPTT occurs in:

    1. All congenital deficiencies of intrinsic system coagulation factors, including hemophilia A and hemophilia B

    2. Congenital deficiency of Fitzgerald factor, Fletcher factor (prekallikrein)

    3. Heparin therapy, streptokinase, urokinase

    4. Warfarin (Coumadin)-like therapy

    5. Vitamin K deficiency

    6. Hypofibrinogenemia

    7. Liver disease

    8. DIC (chronic or acute)

  2. When aPTT is performed in conjunction with PT, a further clarification of coagulation defects is possible. For example, a normal PT with an abnormal aPTT means that the defect lies within the first stage of the clotting cascade (factor VIII, IX, X, XI, or XII). The pattern of a normal PTT with an abnormal PT suggests a possible factor VII deficiency. If both PT and aPTT are prolonged, a deficiency of factor I, II, V, or X is suggested. Used together, aPTT and PT will detect approximately 95% of coagulation defects.

  3. Shortened aPTT occurs in:

    1. Extensive cancer, except when the liver is involved

    2. Immediately after acute hemorrhage

    3. Very early stages of DIC

  4. Circulating anticoagulants (inhibitors) usually occur as inhibitors of a specific factor (e.g., factor VIII). These are most commonly seen in the development of anti–factor VIII or anti–factor IX in 5%–10% of patients with hemophilia. Anticoagulants that develop in the treated hemophiliac are detected through prolonged aPTT. Circulating anticoagulants are also associated with other conditions:

    1. After many plasma transfusions

    2. Drug reactions

    3. Tuberculosis

    4. Chronic glomerulonephritis

    5. SLE

    6. RA

  5. Heparin therapy: With deep vein thrombosis (DVT) or acute MI, the usual protocol requires infusion of heparin (monitored by the aPTT), followed by long-term therapy with oral anticoagulants (monitored by the PT, aPTT, or both).

    1. In the blood, heparin combines with an alpha-globulin (heparin cofactor) to form a potent antithrombin. It is a direct anticoagulant.

    2. IV heparin infusion produces an immediate anticoagulant effect; it is chosen when rapid anticoagulant effects are desired.

    3. Because the half-life of heparin is 3 hours, the aPTT is measured 3 hours after heparin administration, or 1 hour before the next dose.

    4. Therapeutic aPTT levels are ordinarily maintained at 2–2.5 times the normal values.

    5. To evaluate heparin effects, blood is tested:

      1. For baseline values before therapy is initiated

      2. One hour before the next dose is due (when a 4-hour administration cycle is ordered)

      3. According to the patient’s status (e.g., bleeding)

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose, procedure, benefits, and risks.

  2. Follow guidelines in Chapter 1 for safe, effective, informed, pretest care.

  3. Draw blood sample 1 hour before next dose of heparin or per heparin protocol when infusion is used. The heparin dose given relates to the aPTT result.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Protamine sulfate is the antidote for heparin overdose or for reversal of heparin anticoagulation therapy.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

  3. Watch for signs of spontaneous bleeding; notify the healthcare provider immediately and treat accordingly.

  4. Alert the patient to watch for bleeding gums, hematuria, oozing from wounds, and excessive bruising.

  5. Instruct the patient to use an electrical shaver instead of a blade and to exercise caution in all activities.

  6. Avoid use of aspirin or ASA-like drugs (unless specifically prescribed) because they contribute to bleeding tendencies.

  7. Long-term use of heparin can cause development of osteoporosis with fractures.

  8. Thrombocytopenia can also develop with high-dose heparin therapy, along with progressive thromboembolic syndrome. This platelet abnormality quickly reverses when heparin is discontinued.

Interfering Factors ⬆

  1. see Appendix E for drugs that affect test outcomes.

  2. Hemolyzed plasma shortens aPTT in normal patients but not in abnormal (heparinized) patients.

  3. Very increased or decreased Hct.

  4. Incorrect ratio of blood to citrate (“short” fill of blood in collection tube).