My Home Tonometer Does Not Agree Exactly With My Doctor's Eye Pressure Measurement. What Should I Do?
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21 days ago
@member_31

I purchased an iCare HOME tonometer recently. I took it to my doctor's office to compare the eye pressure measurements side-by-side. I know from reading the published studies that the iCare is very accurate and that it closely agrees with the Goldmann tonometer.

However, in my case, the measurements between the two tonometers did not agree exactly. My doctor thinks it is due to my thick corneas.

What should I do?

icare-tonometer self-tonometry tonometer reichert-tonometer goldmann-tonometer • 126 views
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Entering edit mode
21 days ago
david 3.9k
@david_fe

For this discussion, I will assume your doctor is using a Goldmann tonometer and you are using an iCare or Reichert.

First, find out exactly what you are comparing against. Start by asking your doctor when the Goldmann tonometer was calibrated and how close it is to the recommended calibration accuracy. Why?

Because most Goldmann tonometers in clinical practice are out of calibration. It makes little sense to test your new home tonometer against a device that is likely out of calibration.

Second, compare the tonometers side-by-side. Many people do not do this step correctly, and they are misled about the accuracy of their tonometer. Why?

Because for many (maybe most) people, eye pressure is dynamic (it's constantly changing by a few points). It can change 3 to 5 mmHg with each heart beat. (Goldmann tonometry averages the cardiac cycle out of the reported IOP, but some home tonometers give you the exact, non-averaged, instantaneous IOP.) IOP can change very quickly for a wide variety of reasons including stress.

When I last took my home tonometer to my doctors office, we did the side-by-side comparison as follows:

  • he dedicated an exam room for our little experiment. We set it up so that he could do Goldmann tonometry on me and I could check my IOP with my tonometer with no delay and without me moving around. Getting up and walking can change one's IOP. Your IOP can change within a couple minutes even if you don't move around. So the testing needs to truly be side-by-side. Ideally, you should remain seated in the same place and there should be no delay between tests.

  • we repeated the side-by-side comparison three times throughout the office visit.

  • we randomized the order; sometimes he tested first, sometimes I tested first.

  • we recorded the exact IOP values and the exact clock times for each measurement.

Third, I suggest you write down all the following information.

  1. the actual IOP values and the exact clock time of each IOP measurement. Make a nice table or chart if you can.
  2. the type of tonometer your doctor used (and its calibration status)
  3. the type of tonometer you used. Home tonometers are self-calibrating, but you do need to keep them cleaned. So include the last time you cleaned your device and how.
  4. who performed each measurement (you, your doctor, staff, etc.)
  5. the steps you and your doctor took to make this a true side-by-side comparison (as per my example above)

If you want our opinion on your side-by-side comparison, share all the above information with us.

Next, let's assume you did the side-by-side comparison correctly. You found a disagreement between your tonometer's results and your doctor's eye pressure measurements. What are your next steps?

Understand the limitations of Goldmann tonometry. Many doctors think it is the "gold standard." It's not. Read this article to understand why:

Is Goldmann Applanation Tonometer (GAT) the Gold Standard? | Ask FitEyes

Without a true reference standard, manometric IOP, it is impossible to determine the accuracy of one instrument versus another, because the errors of both are either contrasting or compensating while the true pressure remains unknown.

Therefore, you should not and cannot simply assume that your home tonometer is incorrect if it disagrees with the intraocular pressure value obtained by your doctor.

Of course, you should be sure your device is in good working order. Usually this just means that it is clean, and if it is portable, the batteries are fresh, the probe is in good condition, etc.

Also familiarize yourself with the studies. Most published studies show very close agreement between the iCare, Reichert 7CR and Goldmann tonometers for the vast majority of patients. One informal comparison in your doctor's office does not invalidate all that research!

As mentioned, most people don't even do the side-by-side comparison correctly. But let's assume you do -- and that you also believe you are one of the rare individuals for whom the home tonometer is not consistent with Goldmann. You still need more than one well-done side-by-side comparison to get a clear picture. Therefore, you could repeat your side-by-side comparison a few times (such as on subsequent office visits). Make your own small research study and do it carefully.

If you don't do that, you should not conclude from one informal test that your tonometer is inaccurate (or that your eyes are highly unusual). Trust the science (to a point). The evidence shows home tonometers are generally accurate for most people.

As a true scientifically-minded person, also recognize the shortcomings in the science. I must again emphasize the common shortcoming in most of the scientific research comparing tonometers which is the absence of any true reference standard. All non-invasive tonometers have limitations. Also a small percentage of people have corneal characteristics that are far from normal; these would cause issues for most tonometers (including Goldmann).

Because we know that iCare and Reichert tonometers are generally about as accurate as Goldmann (often better), I suggest that doing these side-by-side comparisons is not needed in most cases. A poorly done comparison will be misleading. Yet we still want to do them. (I understand. I wanted to do the comparison too!)

Continuing: assume you did the side-by-side comparison correctly and you found a disagreement. You understand the limitations above, but you want to dig into this deeper to be rule out the possibility that your corneal characteristics are throwing a wrench into your home tonometry accuracy.

The next thing you could do is to arrange for a well-performed side-by-side comparison at an ophthalmology practice that has a Reichert Ocular Response Analyzer ("ORA") and that will accommodate your testing protocol (as described above). Make sure they will do multiple side-by-side comparisons, and that for each one, they take 4 measurements with the ORA. They should give you a printed report with waveforms for each one of the comparisons.

Comparing your tonometer versus a Reichert Ocular Response Analyzer is a much better comparison than against the Goldmann tonometer. If your home tonometer significantly disagrees with a Reichert Ocular Response Analyzer, see "Worst Case Outcome" below.

Expected Outcome: In 97-99% of the cases (in my experience), your home tonometer is going to be at least as accurate as your doctor's tonometer. Your doctor will likely not admit that, but the evidence supports it and more than a decade of experience with the FitEyes community supports it.

Furthermore, with your home tonometer you can collect hundreds and thousands of intraocular pressure measurements in real life, at all times of the day (or night) and under a wide variety of conditions. You can even create your own research-quality personal IOP database.

It's common sense -- which is the better IOP data set? The few IOP measurements obtained by your doctor each year in an artificial setting (the office) during limited office hours? Or the large number of measurements you obtain in real life? Obviously, you can build a much better IOP dataset than your doctor, if you wish.

What Are Examples of the Different Approaches to Home Eye Pressure Monitoring? | Ask FitEyes

Worst Case Outcome: What if you do all of the above and you find that your home tonometer does not agree with the IOPcc value of the Reichert Ocular Response Analyzer? I call this the "worst case outcome" because it is rare (affects approximately 1-3% of individuals using an iCare, in our experience) and because if you can live with the worst case scenario, then you really don't have a problem.

Here is Terry's take on it:

It is always good to emphasize that the iCare is at its best when looking for fluctuations. Even if it is consistently 3 points above Goldman, we are really looking for 5-10 points (or more) spikes in pressure. As patients, we want to protect against significant pressure spikes, and know when to call the doctor. Excuse my preaching to the choir here, but it is important to remember what is most important.

I agree with Terry's take for most people, most of the time. But let's assume you will not be satisfied until you really understand all IOP accuracy limitations (whether they affect your home tonometer or your doctor's tonometer).

A number of FitEyes members own both a Reichert 7CR and an iCare. The two tonometers compliment each other. I own both and I have for a number of years. I get value from both in many different ways.

Cross checking my iCare against my Reichert 7CR at different times of the day and under a variety of different conditions has given me a lot of confidence in my iCare over time. I believe this may be the ultimate solution for answering all questions about home tonometer accuracy -- provided you use the FitEyes Insight software.

Interestingly, my iCare does not agree with the Goldmann IOP value (IOPg) of my Reichert 7CR. However, my iCare agrees almost exactly with my Reichert 7CR's IOPcc value (which is considered to be closer to true IOP).

If you are determined to address any questions about IOP measurement limitations on your eyes, but are not able to purchase both a Reichert 7CR and an iCare, you could take my recommendation to compare your home tonometer against the Reichert Ocular Response Analyzer a step further. You could, for example, repeat that side-by-side comparison in the early morning and the late evening (the extremes of the clinic's office hours). You could, with multiple comparisons, build your own person correction factor which incorporates your unique corneal biomechanical properties.

Bottom Line: If all this is too much for you, I suggest you skip the informal side-by-side comparison in your doctor's office and just trust your tonometer. In the vast majority of cases, that's your best choice. The Goldmann tonometer is not perfect and most side-by-side comparison testing is not perfect. If you can't do it right, you may be better off skipping it. Just use your tonometer and know that it has been validated by multiple scientific studies and the US FDA (or equivalent for your country) approved it on the basis of that evidence.

Conversely, if this is not enough for you and you absolutely must get to the bottom on any possible inaccuracies in your home tonometry (or prove to yourself or your doctor the accuracy of your measurements), then please reach out to me. You and I are probably very similar. (I know another FitEyes member you may want to reach out to as well.)

My method for getting to the bottom of it was to get a Goldmann tonometer (and slit lamp biomicroscope, etc.) for my home. I was able to test all my other tonometers side-by-side with Goldmann at home as often as I wished. (Note: I no longer have any need to use my Goldmann. My Reichert and iCare are more accurate.) I also did the same with a Pascal DCT, which is highly accurate. In fact, I purchased an Reichert Ocular Response Analyzer for home use too (and I later sold it). I can tell you how to do what I have done, and I can help you with any questions you have. If you do this, your home IOP data can essentially be beyond question by any expert.

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