Discrepancy in INR reading

[Tonyj]

Hi
Had surgery 8 May, mechanical aortic valve. Been on Warfarin under the care of the Heart Hospital (where my surgery was done). Last reading from them was Monday (21 May) which was 1.209 (I was told to then take 9mg, the highest dose I've taken so far). My INR has been consistently creeping up; albeit seemingly slower than expected.

Today (23 May) attended my local hospital clinic for the first time (not impressed at all by them in several aspects). Blood was taken by pricking my finger. I was given an INR of 1.8. This seemed VERY low to me and unexpexted. I tried to defer to the nurse's expertise, but did not find her explanation at all satisfactory. We agreed in the end I should have a further test with blood taken from the vein. I've since had a phone call saying the result of this test was 1.26 (my range is 1.2 to 1.3 target 1.25).

On the basis of the first result (1.8) I was advised to take 10mg for 4 days; then 9mg for 2 (the following day having another appointment, Nurse kept saying dose of 9mg on this day as well. I stated it should depend on my INR on that day!). This has been reviewed to 9mg each day. Logically this seems slightly on the high side as I am already over my target (though within range). However I am prepared to properly follow the advice I am given unless I have VERY good reason not too.

What concerns me is the massive difference in results between the 2 tests, which the nurse did not seem to perceive as an issue. To my untrained mind this seems way beyound acceptable margins of error. As the differnce could mean being regarded as on target by one test and above range (fairly significantly) on the other test.

Any coments or advice on the best way for me to deal with this in the most constructive manner would be very much appreciated.

Thanks
Tony

 

[dick0236]

Hi Tony and welcome. It is entirely possible, and probable, to get different INR results from venus draws and finger stick tests. You indicate that "your range is 1.2-1.3 and target is 1.25"??????? I believe you must have misunderstood these numbers. It is impossible to maintain a range of 1.2-1.3. I think they meant to tell you a "range of 2-3 and a target of 2.5.....which is standard for many new patients. A 1.8 would seem to indicate that you are moving close to your range and the dosage increases are proper. My personal belief is to manage my dose based on one of the test method, either venus draw or finger stick....but not interchangably.

 

[ElectLive]

Tony - Surprisingly, the acceptable margin of error for point of care machines (there is an ISO standard they must meet) is pretty wide: at least 90% of the readings must be within only +/- 30% of lab value "standard". Futher, the self published accuracy of one device (CoaguChek XS - but others are similar) shows that only about 2/3 of the readings are within +/- 10%.

Generally the higher the INR the worse the accuracy, though, so despite all of this, your instance seems surprising in that regard. For what it's worth, many a member here report very reliable accuracy between lab and home machine, so perhaps there could be individual patient factors (hematocrit level is one) in play also. Unfortunately, probably the only way to prove anything definitive in your specific case is with mutiple side by side comparisons.

I'm sure others will be along to offer direct experience...welcome aboard, and best wishes getting to the bottom of this.

 

[Eva]

My INR was very low after my surgery? I was kept in the hospital for 12 days until it went up to 2.? My dose was increased until I alternated between 10 mg and 9 mg. My surgeon recommended my INR be at 2.5-3.5 range (not minding it going to 4). since then, it has stabilized at 7.5 mg (4 days) and 8.5 mg (3 days).

I would follow the instructions recommended, and not worry about the discrepancy. And double-check the recommended range!

Keep us posted.

 

[Protimenow]

I'm with Dick on this -- as a 'valver', your INR range should NOT drop below 2.0. The clinic is probably trying to get you there.

Do you know which type of meter was used?

The important thing is to get your INR in range - the sooner the better. Also, realize that warfarin is very slow acting -- the dose you take today will have little effect on tomorrow's INR. It takes a few days to see much effect. Warfarin isn't like most other drugs -- you just won't see any quick results from it.

The test values - lab versus finger stick - may get closer together when you're in range (and please confirm with your doctor what your CORRECT range should be - the one you quote is obviously way too low). And, as others have said, when your INR heads to 3.5 and above, some meters are considerably less accurate and a blood draw will probably be required.

(I had issues with large variance from meter to lab and Alere, which now manufactures InRatio told me, in three words, 'trust the lab.' That's good enough for me.)

 

[Bina]

Tony, please confirm your INR range with your doctors.....it is probably going to be 2.0---3.0 or 2.5---3.5
My hospital kept me in for 2 weeks getting my INR to increase slowly and steadily.

If your hospital is using a finger stick tester for INR, chances are good that it is a Coaguchek XS, they are excellent testers when
used correctly. Mine has tested me perfectly for 5 years !!

 

[Tonyj]

Hi
Thanks for all the replies so far. I am beginning to see this as a bit of a dark art. Firstly yes my range is 2.0 to 3.0 target 2.5 (I dont understand why when a reading is below 2 people seem to have the decimal point in a different place [i.e. "your reading is 1.18"as is recorded in my yellow book then "your reading is 2.6"]).

My concern is the Massive difference between the 2 readings. On the basis of the 1st reading I was advised to inject Fragmin. Wheras the 2nd reading puts me very slighly over my target (I know it is well with range). So the difference has a real effect on medication. On the basis of this; if I were to go by the first reading. By the time I got to 2.5 a reading from the vein would be 3.3 or 3.4 (assuming the difference is consistent) this just seems to be too wide a descrepancy to be reliable (if I were infating my cars tires and suspected the meter had that degree of error. I would go to a different garage).

Regarding coments so far. According to vein test I am at range, not moving closer to it. Inaccuracy increases when reading gets higher. I am not in the higher range. I want to follow instructions. The discrepancy altered the advice and this is my point.

 

[dick0236]

Tony, it really isn't a "dark art", although I can see why newbies can think so. It gets easier with time. I got a little lost in your post, but I'm pretty old LOL. The different readings does not alarm me greatly and I can't tell you why, but I try to stay with one test method (venus draw or finger stick) and not switch back and forth unless there is a suspected bad test. An INR of 3.3 for a range of 2-3 would give me little concern. I have never been able to predict how much my INR will change in relation to any dosage change. I understand that both tests become increasingly more innacturate as you move farther from the "mean" (in your case 2-3). I can tell you, from experience, that this is not "rocket science" and once you get the hang of it, it's pretty easy to maintain your INR.......hang in there.

 

[Protimenow]

Both methods (venous blood draw and lab test) and testing with a meter can be prone to error. Blood can be improperly handled, and the lab can have an incorrect value for its reagent or set their machines to the wrong values. Blood for the meters can be incorrectly handled (newer meters want the first drop, within fifteen seconds, and you aren't supposed to 'milk' your finger; other meters say to discard the first drop). In some cases, even though strips have built-in quality controls, it's possible that your meter can be off by quite a few tenths. In one case, I suspect that an OTC drug that I was taking may have caused my meter to report an incorrectly elevated INR.

It makes sense to stay within range - whichever method you use for getting your INR. It also makes sense, if possible to occasionally test a finger stick INR against a lab INR -- if you can get a prescription for the lab INR and can afford the test.

The deviation between meter and lab can be fairly significant (but often is very close).

What I'm currently doing (though my testing shouldn't be considered the standard for everyone) is testing weekly using my InRatio or InRatio2. When I can afford some cuvettes (ProTime's word for strip), I'll compare my InRatio results to the ProTime (which I suspect may be closer to the lab results). I also get a blood draw, once a month, for the next two months. When I go for the blood draw, I'll test on my InRatio within an hour or so of the blood draw, so I can compare results.

If I didn't have lab results to compare with, I'll try to keep my INR in the middle of the range, and be fairly confident that I'm okay.