Blood tests that measure clotting function. Used to monitor warfarin therapy (PT/INR), heparin therapy (aPTT) and for preoperative evaluation.
Indication
- Monitoring warfarin (coumadin) therapy (target INR 2-3, mechanical heart valve 2.5-3.5)
- Adjusting heparin dose with aPTT during therapy
- Preoperative coagulation assessment
- Evaluation of liver function (the liver produces clotting factors)
- Investigation of bleeding tendency (frequent bruising, nosebleeds)
- Suspected vitamin K deficiency in newborns
- Diagnosis of disseminated intravascular coagulation (DIC)
Preparation
- No special fasting required (although 8-12 hours of fasting may be needed for combined tests)
- Disclosure of all blood-thinning medications used
- Foods containing vitamin K (leafy green vegetables) should be consumed in a balanced manner
- Use of aspirin or NSAIDs should be reported
How it's performed
- Venous blood is drawn from the arm into a special citrate tube
- The tube is gently inverted to mix with the anticoagulant
- Plasma is separated and measured on an automated coagulometer
- PT result is reported in seconds and as INR; aPTT is reported in seconds
- Results are used by the physician to adjust the dose
Post-procedure
- INR is checked frequently at warfarin initiation (every 2-3 days), and every 4-6 weeks once stable
- aPTT may be checked every 4-6 hours during heparin therapy when needed
- Repeat testing after dose changes
- Monitoring for bleeding/thrombosis when values are outside target range
- Patient diary keeping and dietary regulation
Risks
- Minor bruising or tenderness from the blood draw
- Inaccurate results if the tube is not mixed properly
- Test repetition due to hemolysis or insufficient sample volume
- Risk of serious bleeding when INR is high; risk of thrombosis when low
FAQ
What does INR mean?
INR is a standardized measure of dosing during warfarin therapy. The target is between 2 and 3 for most indications; your physician will determine your individual target.
Should I avoid leafy green vegetables?
It is not necessary to avoid them entirely. What matters is that intake remains balanced and consistent; sudden increases reduce the effect of warfarin.
Are PT/INR tests required for direct oral anticoagulants (DOACs)?
No. Routine INR monitoring is not required for newer agents such as rivaroxaban or apixaban.
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