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No changes in diet or activity are necessary before an INR/PT test. However, a medical history will be updated before the test to determine if any of the patient’s medications might interfere with the accuracy of the test or cause the INR to be too high (blood takes too long to clot) or too low (blood is more prone to clotting). Substances that may affect the INR/PT include:
- Antibiotics. Medications that harm or kill microorganisms and are commonly used to treat infections. These medications may cause a deficiency in vitamin K, which is used as part of the clotting pathway.
- NSAIDs (nonsteroidal anti-inflammatory drugs). Anti-inflammatory drugs like NSAIDs prolong bleeding by inhibiting the function of platelets and may directly alter the INR/PT measurement.
- Aspirin. While typically used as a pain-reliever (analgesic), aspirin is also an antiplatelet agent that will influence the risk of bleeding.
- Progestin/estrogen combinations. This combination of hormones is often used in oral contraceptives (“The Pill”) and hormone replacement therapies.
- Vitamin K deficiency. Whether due to poor nutrition or prolonged use of antibiotics, vitamin K deficiency may prolong PT by decreasing the blood’s ability to clot.
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