In the research publications on this topic, there is no significant difference between the Icare, Reichert and Goldmann IOP values over a large patient population. There will be individual differences, however. In particular, if your corneal biomechanics are unusual in any way, Goldmann will not be as accurate for you as the Reichert 7CR. (My opinion is that the Icare is also less affected by corneal biomechanics than Goldmann, but I cannot point to a study to support that opinion.)
Our IOP fluctuates a few points all the time, even with breathing, physical activity levels, thoughts/emotions/stress, etc. The only sure-fire way I know to compare these tonometer for yourself is to take your tonometer into your doctor's office and measure with each tonometer more than once. (However, as I will explain further below, it is not necessary to do this and it is debatable what value there is in doing so, if your goal is to simply compare tonometers.)
I have done this comparison several times with my friend Dr. P. The value of what I did was not comparing the accuracy of tonometers. The title of the blog post below states the valuable insight that resulted from my experiment:
White Coat Syndrome for Eye Pressure | FitEyes.com
Here are two of the measurements:
Dr. P Office 3:12 PM 17.3 OD and 15.3 OS Reichert AT555
Dr. P Office 3:14 PM 18 OD and 16 OS Goldmann (GAT)
The 7CR is more accurate than the AT555, but you see how close these measurements were. They are less than 1mmHg apart. Considering that the Goldmann doesn't measure fractions, you can't expect much better agreement than this.
However, if you notice in the blog post above, 2 minutes prior to this comparison my IOP was higher. That's due to white coat syndrome. As soon as I relaxed, my IOP was 2 points lower in each eye. Therefore, if I had delayed even one or two minutes longer when comparing the tonometers, I would have thought there was a bigger difference between the instruments. That would have been the wrong conclusion, yet it is the one most of us would jump to -- partly because the doctor's themselves don't realize how much different a couple minutes can make in one's IOP.
Also notice how much lower my IOP was at home. If I had not taken my tonometer into the doctor's office and measured carefully (more than once), I might have concluded that there is an even larger difference between my tonometer and my doctor's Goldmann tonometer. That is wrong. The tonometers agree closely, but my IOP reacts to being in the doctor's office -- even with a very friendly doctor like Dr. P.
I followed that up with this similar comparison where I again took my tonometer into Dr. P's office.
White Coat Ocular Hypertension In One Eye | FitEyes.com
I did these experiments in 2007. What I reported there is still valid today. The main differences are that the Reichert 7CR is significantly more accurate than the AT555, and that the newer Icare tonometers are also more accurate than the older ones.
Any disagreement between your Icare and your doctor's Goldmann, it will likely be explained by one of these factors:
- your own measurement technique when using the Icare. It is possible, even with the green LED ring at the base of the probe (on the newer models) to misalign the tonometer slightly. If you feel any slight discomfort from the probe, chance are you have it misaligned -- even if the Icare tells you the measurement was valid. So #1, be sure you are measuring correctly. Have someone watch you. Or, like Antonio suggested, video yourself taking measurements to see that the probe is contacting the center of your pupil and is perpendicular to the surface (as well as being close to horizontal).
make sure your Icare probe tube is clean and your batteries are not low. If the probe or probe tube are dirty or your batteries are low, the Icare may not be accurate.
compare side by side (as I tried to do above) or recognize that measurements at different times are not directly comparable. In that case, you are not comparing tonometers, you are comparing changes in your IOP over time.
- If you can check your IOP even once on any Reichert ORA or 7CR, you will have an idea of your corneal biomechanics. If your CH, for example. (one of the main corneal biomechanical parameters) differs from normal, than any disagreement of Goldmann IOP with your IOP as measured by another tonometer is not a test of the accuracy of your other tonometer. In fact, I quoted Dan Eisenberg, MD, in the article below, saying that it is not such a good idea to compare any tonometer against the Goldmann because the Goldmann is an inherently flawed tonometer.
Is Goldmann Applanation Tonometer (GAT) the Gold Standard? | Ask FitEyes
The need to compare everything to the GAT is completely illogical.
If you own an Icare, know that it is at least as accurate as a Goldmann (GAT) if you use the proper technique and keep your tonometer maintained (as I said above). I agree with Dr. Eisenberg that comparing your Icare to your doctor's Goldmann is of very little value, especially with all the research that has been done since 2007.
When I did my experiments with Dr. P in 2007, there was not as much published research. And the main take-away from my experiments was the documentation of white coat syndrome for IOP rather than a comparison of the difference in the tonometers. I had to do the comparison in office so that I could demonstrate that the large reduction in my IOP outside the doctor's office was valid, since, at that time, nobody had demonstrated the existence of a white coat syndrome for intraocular pressure.