Prothrombin Time (PT): Test Procedure, Uses & Interpretations

Prothrombin Time (PT)

Synonyms:

Protime, PT

🔬 Abstract

The Prothrombin Time (PT) test measures how long it takes for a clot to form in plasma after adding calcium and thromboplastin. It is sensitive to reduced factor levels (II, VII, X) and useful for evaluating warfarin therapy and extrinsic coagulation.

🧪 Specimen Requirements

• Specimen: Plasma
• Container: Blue top (sodium citrate) tube

🛠️ Collection & Handling

• Draw venipuncture; include PT last when taking multiple tubes.
• Use a two‑syringe technique if drawing only PT to prevent tissue thromboplastin contamination.
• If not tested immediately, separate plasma and refrigerate; test within 4 hours.

❌ Reasons to Reject Sample

• Received 3–4+ hours after collection
• Tube not full, clotted, hemolyzed, lipemic, or icteric

⏱️ Reference Range & Panic Level

• Normal range: 10–13 seconds
• Possible panic: >20 seconds (non‑anticoagulated), >3× control (anticoagulated)

🔍 Clinical Uses

• Monitor warfarin/coumarins
• Identify prothrombin deficiency, dysfibrinogenemia, afibrinogenemia
• Detect liver failure, DIC, vitamin K deficiency
• Evaluate heparin effect

⚠️ Limitations

PT may be prolonged by heparin if measured <2 hours after dosing. Numerous drugs and conditions—like antibiotics, salicylates, anticonvulsants, and contraceptives—can either increase or decrease PT. Individual warfarin response also varies with genetics and medications.

⚙️ Methodology

In sodium citrate plasma, calcium and thromboplastin are added. Clotting time is measured—visually or via automated fibrin detection—and compared to a concurrently run normal control plasma.

ℹ️ Additional Information

• Proper blood-to-citrate ratio is critical—underdrawing can falsely prolong PT.
• Clotting tubes must be fully filled for accurate results.
• Antibiotics like cephalosporins may lower vitamin K levels—vitamin K prophylaxis may be advised.
• Warfarin effect typically achieved within 3–7 days; reversed with oral or intramuscular vitamin K.

📚 References

1. Andrew M, Paes B, Blood, 1988;72:1651–7.
2. D’Angelo A, Am J Clin Pathol, 1989;92(3):321–8.
3. James AH, J Clin Pathol, 1992;45:704–6.
4. Peterson CE, Arch Intern Med, 1986;146:581–4.
5. Poller L, Am J Clin Pathol, 1989;92(1):124–6.
6. Jacobs et al., Laboratory Test Handbook, Lexi‑Comp Inc, 1994.