Partial ThromboPlastin Time

Partial ThromboPlastin Time (PTT),
Activated Partial ThromboPlastin Time (APTT)

Normal Values
PTT: 30–45 sec
APTT: 21–35 sec
Check with your laboratory concerning therapeutic range values for heparin therapy.

Background
The PTT, a one-stage clotting test, screens for coagulation disorders. Specifically, it can detect deficiencies of the intrinsic thromboplastin system and will also reveal defects in the extrinsic coagulation mechanism pathway.
NOTE: The PTT and APTT test for the same functions. APTT is a more sensitive version of PTT that is use to monitor heparin therapy.

Explanation of Test
The APTT is the preferred test to monitor heparin therapy. It can also detect circulating anticoagulants.

Procedure
1. A 7-ml venous blood sample is anticoagulated with sodium citrate and put on ice until the test can be run. Use the two-tube method.
2. Do not draw blood samples from a heparin lock or heparinized catheter.

Clinical Implications
1. The APTT is prolonged in
• All congenital deficiencies of intrinsic system coagulation factors, including hemophilia A and hemophilia B
• Congenital deficiency of Fitzgerald factor and Fletcher factor (prekallikrein)
• Heparin therapy
• Warfarin (Coumadin)-like therapy
• Vitamin K deficiency
• Hypofibrinogenemia
• Liver disease
• Disseminated intravascular coagulopathy (DIC)—chronic or acute
• Fibrin breakdown products (FBPs)
2. The APTT and PT will detect approximately 95% of coagulation abnormalities. When APTT is performed in conjunction with a PT, a further clarification of coagulation defects is possible. For example, a normal PT and an abnormal PTT means that the defect lies within the first stage of the clotting cascade (factors VIII, IX, X, XI, and/or XII). A normal PTT and abnormal PT suggests a possible factor VII

deficiency. The pattern of a prolonged PT and PTT suggests a deficiency of factors I, II, V, or X.
3. Shortened APTT occurs in:
• Extensive cancer, except when the liver is involved
• Immediately after acute hemorrhage
• Very early stages of disseminated intravascular coagulopathy (DIC)
4. Circulating anticoagulants (inhibitors) usually occur as inhibitors of a specific factor (eg, factor VIII). These are most commonly seen in the development of antifactor VIII or antifactor IX in 5% to 10% of hemophiliac patients. Anticoagulants that develop in the treated hemophiliac are detected through prolonged APTT. Circulating anticoagulants are also associated with other conditions such as
• Following many plasma transfusions
• Drug reactions
• Tuberculosis
• Chronic glomerulonephritis
• Systemic lupus erythematosus
• Rheumatoid arthritis

Clinical alert
APTT > 100 sec signifies spontaneous bleeding.
5. Heparin therapy: Heparin is a direct anticoagulant.
• In the blood, heparin combines with an alpha-globulin (heparin cofactor) to form a potent antithrombin.
• Intravenous heparin injection produces an immediate anticoagulant effect; it is chosen when rapid anticoagulant effects are desired.
• Because the half-life of heparin is 3 hours, APTT is measured 3 hours after heparin administration.
• Therapeutic APTT levels are ordinarily maintained at 2 to 2 1/2 times normal values.
• To evaluate heparin effects blood is tested
(1) For baseline values before initiating therapy
(2) One hour before next dose is due (when a 4-hour administration cycle is ordered)
(3) According to the patient’s status (bleeding)

Patient Preparation
1. Explain test purpose and procedure.
2. Draw blood sample 1 hour before next dose of heparin. The heparin dose given relates to the APTT result.






Patient Aftercare
1. Interpret test outcome and monitor appropriately. Protamine sulfate is the antidote for heparin overdose or for reversal of heparin anticoagulation therapy.
2. Follow Chapter 1 guidelines for safe, effective, informed posttest care.

3. Watch for signs of spontaneous bleeding, notify physician, and treat accordingly.
4. In case of spontaneous bleeding, notify physician immediately.
5. Alert the patient to watch for bleeding gums, hematuria, oozing from wounds, excessive bruising.
6. Instruct the patient to use electric shaver instead of blade and to exercise caution in all activities.
7. Avoid use of aspirin or ASA-like drugs as these contribute to bleeding tendencies (unless specifically prescribed).