Coaguchek "non-linear"

[Marty]

I recently received an E-mail at my office from the director of our lab that stated that she had to suspend point of care testing at three satellite clinics because the Coagucheks did not pass the tests mandated by the CAP ( College of American Pathology). She said the Coagucheks produced non-linear data above INR 3.2 At the central lab they do vein sticks and use a big machine called the CA-1000 and they passed with flying colors. The possibility that these results may not impact on clinical care had no relevance since they must pass inspection.

I communicated this information to Professor Ansell in Boston and this is his reply.

" I think the Coaguchek is more than sufficient to monitor oral anticoagulation. It is less accurate at high INR's but so is the laboratory. I would feel comfortable using it if I was on therapy".

Lance, what do you think of this? Is the ProTime "linear" at the higher levels?

 

[QAS]

Matrx

Matrx

In my experience, there is not an exact point or INR in the high levels where the results become non-linear or not accurate. Depending on a person's range, and INR at 4.0 will be therapeautic to one patient, and out of range to another. Keep in mind that I am not a Dr., and I am just sharing what I've learned. I do know that if someone get's an INR that high, the test should definately be verified with the Dr. or laboratory. At higher ranges, there will be differences between monitor to monitor or even lab to lab. If anyone would like further assistance, or has additional questions, please call 800-298-4515 and ask to speak with one of our nurses. Thanks,

Lance

 

[Johnny Stephens]

My clinic uses a Coaguchek and will throw out any result over 5 because they say it becomes "less accurate" above 5. They'll then do a full needle draw for the lab and report to me the results later in the day. This has only happened once early on when my readings were all over the map...I tested a 7.something on the Coaguchek; they then did an arm draw which came back from the lab at 5.8. Skipped a day of Coumadin and got back in range.

 

[ALCapshaw]

The formula for INR is given in the first Reference cited in Blanche's response to my INR Confusion post (too bad copy and paste doesn't work on the VR.com replies .

The bottom line is that INR is the PT ratio raised to the power given by the ISI number. Alledgedly, the Coaguchek machines use an ISI of 2 so their numbers are clearly NON-Linear. ProTime claims to use an ISI of 1 which of course results in a linear result.

It would be nice to know how these formulas were derived, especially the "mean normal" protime reference number!

'AL'

 

[Marty]

"non-linear"...cont.

"non-linear"...cont.

Al, I dropped out of engineering school and took up pre-med because I had problems with exponents, logs, etc. However if they gave our lab unknowns of INR 2,4,6,8 couldn't the coaguchek still stay on the line with an ISI of 2? I think the readings off the line must be inherent in the reader of the iron filings( the end point for coaguchek is when the iron filings stop wiggling), the thromboplastin, etc. and this gets worse as you get longer PT times.Please enlighten us.

 

[ALCapshaw]

I recently changed Coumadin Clinics and discovered there are TWO models of CoaguChek INR Instruments.

My original clinic used the NEW CoaguChek model and consistently yielded INR numbers 0.5 to 0.8 higher than my Protime instrument along with a couple of much higher (Coaguchek) numbers that seemed questionable (4.2 and 4.5).

My new clinic uses the OLD Coaguchek model which produced an INR number 0.5 LOWER than my ProTime unit. A Lab Draw taken within an hour produced an INR IDENTICAL to the OLD CoaguChek instrument. The CRNP at the new clinic told me they tested the New CoaguChek machine and found it did NOT correlate well with their Lab Draws so they stayed with the OLD model.

'AL'