Prothrombin time and INR
Clotting tests that show how quickly blood plasma forms a clot
Also known as
Definition
Prothrombin time (PT), the INR and the Quick value all come from the same measurement: the lab times in seconds how long plasma takes to clot after a clotting reagent is added. The INR turns this into an internationally comparable ratio; the Quick value, common in Germany, expresses it as a percentage of normal plasma. The test mainly reflects vitamin K-dependent clotting factors made by the liver. The longer clotting takes, the higher the INR and the lower the Quick value.
| Parameter | Value |
|---|---|
| Unit | PT in seconds, INR without unit, Quick in % |
| Reference Range | Without blood-thinning medicines: PT about 11–13.5 seconds, INR about 0.8–1.1 (MedlinePlus), Quick about 70–120% (KBV). Labs differ; the range printed on your report is what counts, and your doctor interprets the result. |
Agree on your personal target range with your doctor.
According to the KBV, Quick values are hardly comparable between labs because reagents differ in sensitivity. The INR corrects for this mathematically and is used mainly to monitor treatment with vitamin K antagonists; internationally it is the usual way to report the result.
↓ What a low value means
A low Quick value, that is a prolonged PT and a raised INR, means that blood clots more slowly. Without blood thinners, this can be related to an inherited or acquired bleeding disorder, liver disease, vitamin K deficiency or disseminated intravascular coagulation. With vitamin K antagonists such as warfarin, a raised INR is intended; if it is above the target range set by your doctor, the risk of bleeding rises, according to MedlinePlus. Discuss any deviation with your doctor.
↑ What a high value means
A high Quick value, that is a short PT and a low INR, means that blood clots faster. According to MedlinePlus, this can be related to a clotting disorder with a tendency to form clots, a diet high in vitamin K or supplements containing vitamin K, and certain medicines. With vitamin K antagonists, an INR below the target range can come with a higher risk of blood clots. Discuss any deviation with your doctor.
When to test
The test is ordered regularly to monitor treatment with vitamin K antagonists, and also before surgery, for unexplained bleeding, bruising or clots, to assess the liver and when vitamin K deficiency is suspected, often together with the aPTT. Alcohol, foods high in vitamin K, high-fat diets, medicines, supplements and herbal remedies can affect the result. Your doctor will tell you whether to take your daily dose before or after the blood draw.
Frequently asked questions
What is the difference between the Quick value and the INR? +
Both come from the same measurement. The Quick value gives clotting activity as a percentage of normal plasma and is widespread mainly in Germany; according to the KBV, it should be reported there for all uses other than monitoring vitamin K antagonists. The INR removes differences between lab reagents so that values are comparable internationally. A low Quick value corresponds to a high INR.
What INR applies when taking blood thinners? +
Your doctor sets the target range. It depends, among other things, on the reason for the treatment.
Can I measure my INR myself? +
According to MedlinePlus, people who take warfarin long term can, after training, test themselves with a device that uses blood from a fingertip. The results are reviewed with the treating practice.
Does a normal result rule out every clotting disorder? +
No. According to the KBV, these screening clotting tests only pick up more severe disorders; a single clotting factor, for example, has to fall below about 45 percent activity before the Quick value becomes abnormal. The result is therefore always assessed together with medical history and symptoms.
Prothrombin time and INR
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.