What Is INR

by Bobski606

INR is used to monitor the clotting time in patients on oral anticoagulation therapy.

INR, also known as the International Normalized Ratio, is a calculation medical professionals use to standardize Prothombin Time (PT). INR is based on the ratio of the patients PT and the normal mean PT of a healthy person. By testing PT you will learn how fast the blood clots in patients on anti-coagulation therapy such as Warfarin, Coumadin, or other oral blood thinners.

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What Is INR Used For?

Therapeutic Prothombin Time

INR is a great tool to allow patients and their medical care givers monitor their clotting time as part of their coagulation profile (various aspects that make up the contents and functionality of your blood). Most commonly a patient will need to have a regular INR test at their doctor or hospital lab to see if they are within acceptable parameters for their condition and therapy.

For those patients such as myself who are on a permanent oral anticoagulation therapy it is a nice and simple test to monitor our daily dosage of medication such as Warfarin (in the UK) or Coumadin (USA based drug). That's not to say that you will need to test your INR daily but your doctor will used the INR result to work out the daily dose needed for a set amount of time. For example I am on 24mg per week (3mg or 4mg per day) and I'm being test once every four to five weeks.

What Is A Normal INR?

Normal Therapeutic Ranges

To have a basis to work from you need to understand what a "normal" range is in someone who is not on blood thinners. The normal range for a standard blood sample will be between 0.8 - 1.0 for their Prothombin Time (time taken to clot). 

For a patient needing to be on blood thinners the therapeutic range will be between 2.0 - 3.0 with the aim being to get 2.5 overall. The range will differ for various medical conditions and also your own personal needs so your primary medically professional will be the one to set the INR goal for your therapy.

For those who have had a failed therapy previously - developing clots while still taking blood thinners - their doctor will normally have them on a higher therapeutic level of 3.0 - 4.0.

What Causes High INR Levels?

INR Readings Over 3.0

When you first begin your oral anticoagulant therapy your hematologist will talk you through all of the possible factors that will affect your INR. However, there are some common consumables that can have an elevate your levels on a short-term and long-term basis:

  • NSAID such as Ibuprofen, even at lower doses
  • Antibiotics
  • Alcohol
  • Steriods
  • Herbal preparations or food supplements such as cranberry and grapefruit juice, mango, fenugreek and fish oils
  • Certain anti-depressant medication such as SSRIs
  • Taking too much anti-coagulant medication

Before starting any new medication you doctor will need to check for any potential interactions and adjust your anti-cogulant as needed. You'll also need to have an INR test approximately one week later to ensure you don't need any adjustments.

Food and herbal supplementation is much more difficult to work with which is why having regular INT tests is so important. If you are planning on changing your diet then you will need to discuss your plan in detail with your hematologist to try and preempt any changes ahead of time.

If you have also been sick and not able to eat your usual diet then you will need to speak with your hematologist as soon as possible and arrange for a blood test.

Alcohol can be a tricky vice to deal with sometimes because of the drastic effect it can have on your INR levels. If you like to have a drink on a regular basis it is possible to manage your INR levels as long as you are consistent (alcoholism can be adjusted for as long as both patient and doctor are honest about the situation).

You may be able to spot when your INR is too high as you may experience bleeding gums, nosebleeds and increased bruising. If you notice any of these then speak with your doctor immediately to arrange a blood test.

What Causes Low INR Levels?

INR Readings Below 2.0

Just how INR levels can be increased, so too can they be lowered. If your INR readings are below 2.0 then your blood will be thicker and you'll be at an increased risk of clotting, which for some patients (like myself) can be worse. Making sure that your INR doesn't drop below 2.0 is vital in having a consistent therapeutic level. Some of the common reasons your INR will drop is:

  • Alcohol consumption
  • Foods with a high vitamin K volume: kale, salad greens, avocado, broccoli, peas, leeks and pretty much anything else that's green
  • Birth control pills
  • Taking too little anti-coagulant 

Just like I mentioned before you will need to be consistent with alcohol consumption as it will effect people differently, and if you are going for a night out then speak with your doctor first and make them aware so they can help you monitor your levels.

You'll find that the only symptoms for INR being too low will be when it's too late and you suffer from a blood clot. This is why it's so important to be consistent with your eating, lifestyle and blood tests.

What Causes Inconsistencies With INR

The Importance Of A Food Diary

As I've discussed above there are various things that will cause your INR to come out of the therapeutic range. If you find that your levels are fluctuating each time you visit your doctor then they will need to reassess your lifestyle and eating habits and you may be asked to keep a diary.

Using a daily diary or calendar to keep a note of what you are eating, exercising (as this can impact your levels too), and taking other drugs (whether prescribed or not) will help your doctor get a detailed picture of your day to day life. You may even notice the problem yourself, like spotting that you've started having regular salads or having an extra glass of wine, and be able to adjust your lifestyle causing your INR levels to go back to normal.

If over a course of many months you are still not getting consistent therapeutic levels on Warfarin of Coumadin then your doctor may switch you to another medication such as Clexane or another form of heparin through an injection as this can be a better method for some people.


Although I have been on Warfarin since 2014 and have a lot of personal experience in managing my own personal levels with the assistance of my medical team, I am in no position to be given medical advise to others. The purpose of this article if for informational use only and is designed to assist those who are experiencing fluctuations in their INR levels and want to know why; only their doctors are in a position to make any adjustments to medication and should be the only source of knowledge when it comes to making changes.

Updated: 03/16/2017, Bobski606
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