There are blood issues that could interfere with getting a correct result on the CoaguChek XS. If I recall, a problem with hematocrit level might be one of the issues. Perhaps there is something about your blood chemistry that throws it off. (I'm not an MD, but I wonder if you may have an issue with your platelets - if you have more than you should in your blood, perhaps this would mess up the electrical processes used by the XS -- have you had a CBC lately?).
At one time, I thought that I may have had a problem with MY blood and the CoaguChek XS and InRatio meters because the results were always high, but my CBC was fine.
The Coag-Sense (the company stopped calling it Coagusense for, apparently, some legal reasons) is an interesting device. The strips have a small wheel that starts turning when you add your blood. The blood mixes with the thrombin on the strip, and the blood starts to clot. An optical sensor determines when the clot has formed. There is supposed to be at least one clinic that uses the Coag-Sense as a second test if the other POCs report an INR above 4.0. This may be one of your best options, if the CoaguChek XS and something specifically about your blood chemistry produce erroneous results.
One other thing -- and I mentioned it earlier: the ProTime and ProTime 3 meters, built by ITC, are somewhat older technology but also use clot detection. The company makes a meter that is used at some clinics and even in Operating Rooms for quick, accurate blood component results. I have both types of meter -- unfortunately, I'll probably have to replace the batteries, but they can still be run off the charger. The strips that I have are probably expired.
These machines aren't quite as easy to use as the newer machines, and they require more blood. They also don't take the first drop, so you have to wipe off the first drop of blood. I've bought a few of these over the years, and sent a couple to people who wanted to test but didn't have the funds to buy one. If you're willing to get some cuvettes for one (ITC calls these cuvettes, instead of strips), I can send you one to try out and compare to the labs. If it works for you, we can explore our options.
So -- a CBC and a bit of a search for something in your blood that is throwing the XS off (and perhaps testing with an InRatio, if you can find someone with one) may be the first way to go. You might try to find a Coag-Sense somewhere (I have two), or a ProTime meter.