With the Doctor' Coaguchek S and the hospital lab results being nearly Identical - I wonder if it's possible that the Doctor's coaguchek's have been calibrated to reflect the hospital lab inr's in a quality control procedure.
At the risk of becoming a pin cushion wonder what would happen if you got yet another test from a lab not connected with your doctor or the hospital.
What a mess that both sets of results would be out of range for a 2.5 to 3.5 therapueutic range - one too high, one too low - what do you do increase or decrease?
This seemed liked it'd be an interesting article where they say even with the INR - different thronboplastin reagents can give significantly different INR's for the same sample of blood, even when they used the same testing machine.
From:
http://www.csanz.edu.au/abstracts/49abstracts/131.htm
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CLINICAL SIGNIFICANCE OF INTERNATIONAL NORMALISED RATIO (INR) VARIABILITY WITH THE USE OF DIFFERENT THROMBOPLASTINS
J.E. Bray*, E. Malan, T.E. Gan, R.E. Peverill.
Centre for Heart and Chest Research, Department of Medicine, Monash University and Monash Medical Centre and Haematology Department, Monash Medical Centre, Clayton, Victoria.
Commercially available thromboplastin reagents used for the determination of the prothrombin time vary in their response to coagulation factor depletion and each is assigned a value of the International Sensitivity Index (ISI) which is used for calculation of the INR. The INR/ISI system was designed to correct for differences between thromboplastins and coagulometers but it is now clear that complete normalisation cannot be achieved. The aim of this study was to investigate the variability of INR levels with some thromboplastins in common use and to determine the clinical significance of such variability.
METHODS: We compared INR values using three thromboplastins, Innovin (I), Thromborel S (T), and RecombiPlasTin (R), in 65 patients with mechanical prosthetic valves. To control for differences due to coagulometers, all assays were performed on an `Il Futura' instrument. The ISI of each thromboplastin was calculated manually using calibration plasmas. INRs were divided into three ranges <2.5, 2.5-3.5, and >3.5, with a difference between INRs defined as `clinically significant' if the INRs fell into different ranges. RESULTS: Comparing I and T, there was a large difference in mean INR (3.5 vs 2.8, p<0.001), only a moderate correlation between the INRs (r=0.75) and 32 (49%) `clinically significant' discrepancies. Comparing T and R, there was a moderate difference in mean INR (2.8 vs 3.2, p=0.001), a moderate correlation between INRs (r=0.81) and 20 (31%) discrepancies. While there was a strong correlation between INRs using I and R (r=0.96), there were still discrepancies in 12 (19%) and a moderate difference in mean levels (3.5 vs 3.2, p=0.03). In individual patients INR levels with different thromboplastins differed by as much as 2.5.
CONCLUSION: In conclusion, these results confirm that the INR does not adequately standardize for different thromboplastins even with the use of a single instrument and manually calculated ISIs. Clinically significant differences were evident in up to 50% of cases and this could be even higher in the community with the use of different coagulometers.
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here were a couple more articles that mentioned variability-
From:
http://www.4s-dawn.com/dawnac/dawnug2001/benchmark.html
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All oral anticoagulant management clinics rely on some form of Prothrombin Time measurement, usually expressed as an International Normalised Ratio (INR). Different testing systems exhibit different characteristics in terms of sensitivity to various clotting factors. Local variations in the choice, calibration and standardisation of INR systems can impact on patient time in range.
The cross-calibration of different INR methods at the same site can be a particular problem.
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From:
http://www.blackwellpublishing.com/abstract.asp?ref=0007-1048&vid=122&iid=6&aid=11&s=&site=1
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British Journal of Haematology
Volume 122: Issue 6
European Concerted Action on Anticoagulation. Correction of displayed international normalized ratio on two point-of-care test whole-blood prothrombin time monitors (CoaguChek Mini and TAS PT-NC) by independent international sensitivity index calibration
Summary
The international normalized ratio (INR) on two widely used point-of-care test (POCT) prothrombin time (PT) monitors (CoaguChek Mini and TAS PT-NC) differed considerably and also differed from the â??trueâ?? INR obtained on the same samples using a manual PT and the same species thromboplastin international reference preparation. Agreement between the displayed INR and difference from â??trueâ?? INR has been reassessed following an independent international sensitivity index (ISI) calibration of the two systems. The displayed INRs taken at seven centres were compared with â??trueâ?? INRs from the same blood donations and INRs based on the resulting ISI. The overall difference between the displayed INRs on the two monitor systems was reduced from 21·0% to 3·5%. The overall difference in mean INR of system A from the â??trueâ?? INR was reduced from 19·0% to 9·5% and of system B from 6·8% to 0·3%, but individual centre's results still showed considerable mean INR variability. Differences between overall displayed INR with the two monitor systems have been reduced by an independent multicentre calibration, and agreement with â??trueâ?? INR on the same blood samples improved. However, marked variability in mean INR at individual centres remained after ISI correction, which demonstrates the need for external quality control of individual POCT whole-blood PT monitors.
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My levels are sure to be low. Hopefully not! 
