Product Spotlight: Taking Tonometry Into the 21st Century

New prism is 94% more accurate than the legacy method.

For more than 50 years, the Goldmann applanation tonometer (GAT) has been the gold standard for measuring intraocular pressure (IOP). But the new CATS Tonometer Reusable Prism (CATS) is looking to change that with its optimized concave-convex prism surface that corrects for corneal biomechanical and tear film errors, unlike the GAT.

“Significant errors exist in IOP measurements with the GAT, which prompted me to create CATS,” says Sean McCafferty, MD, MS, FACS; CEO, CATS Tonometer Prism; partner, Arizona Eye Consultants; and clinical assistant professor of ophthalmology at the University of Arizona. CATS, which stands for “correcting applantion tonometer surface,” measures true IOP to within ±2 mmHg in 97% of the population compared to 50% of the population with greater than 2 mmHg GAT error — improving accuracy by 94%.

IOP errors result from limitations with the legacy prism’s flat design, which doesn’t correct for corneal biometrics issues resulting from center corneal thickness (CCT), corneal hysteresis, corneal curvature, and tear film, all of which affect 50% of the population.

“Current tonometry mixes corneal geometry, corneal biomechanics, keratometry, and tear film information, but it doesn’t tell you how each factor contributes to the IOP measurement,” says Nathan Radcliffe, MD, ophthalmologist and glaucoma specialist at New York Eye Surgery Center and associate clinical professor of ophthalmology at Mt. Sinai Health System, New York, New York, who tried CATS because of his longstanding clinical and research interest in how corneal properties affect glaucoma.

Hans Goldmann designed his device for patients with “normal” corneal biomechanics, even though he knew that variations existed. “Goldmann standardized GAT for standard eyes,” Dr. McCafferty says. “But people can have thick and thin corneas, flat corneas, and scarring; have undergone surgeries such as LASIK; or have had trauma that changes the way their corneas work. All of these factors affect IOP. “Goldmann recognized this, and stated that errors would occur in IOP measurements with his tonometer as a result of these variations.”


The CATS prism integrates seamlessly with current tonometer equipment and is designed for easier and more sterile prism installation. Additionally, users don’t have to recalibrate their existing tonometer or change measurement techniques; they can interpret results the same way. “You simply replace the tip of your current tonometer with CATS; it’s ingenious,” Dr. Radcliffe says. “After one day, I was comfortable using CATS. It can be used on any Goldmann model.”

“It hasn’t changed my workflow at all,” says John Berdahl, MD, ophthalmologist, Vance Thompson Vision, Sioux Falls, SD, who uses the CATS tonometer. As a glaucoma specialist, Dr. Berdahl performs many surgeries based on IOP. “I measure IOP in a few different ways to make sure I’m measuring it as accurately as possible,” he says. “When I heard that the CATS tonometer was an easy way to get a more accurate reading, I wanted to try it.”

The CATS tonometer is universal to all eye clinicians and all patient populations. “I use it for every patient,” Dr. Berdahl says. “It is crucial when making a therapeutic decision based on IOP, especially if that decision is a surgical one. Knowing a patient’s true IOP helps me to more accurately recommend the proper procedure or treatment. In some cases, it has shown that patients didn’t need treatment, and in other cases, patients were at a higher risk for glaucoma progression than I initially suspected.”

Dr. Radcliffe had similar sentiments, and states that when medicating patients with high IOP and thick corneas, often their IOP only appeared to change slightly with GAT. But when he measured those same patients with CATS, their IOP dropped more substantially because its measurements are more independent of corneal thickness. “CATS provides a purer IOP measurement without contaminating it with corneal hysteresis and thickness biometrics,” he says.

The device also comes in a disposable version. The curved finger/thumb hold of the prism body allows for single-hand, no-touch sterile operation. The CATS prism body engages the sterile prism tip in the tray with a tactile “click.” After IOP measurement, the tip is thumb released into the trash, and the process is repeated.


The CATS tonometer was designed to indicate prism alignment and centration with the corneal surface, whereas the GAT prism is subjectively centered. “Only when the prism is centered on the cornea is a measurement possible, allowing the mires to intersect,” Dr. McCafferty says. “At that point the visualization appears like the normal hemicircular mires, which clinicians are familiar with. By having an objective centration indicator, the CATS prism improves accuracy and repeatability.”

Furthermore, mire thickness doesn’t affect IOP measurement. “The CATS design (where the surface curves away from the cornea) minimizes tear film error,” Dr. McCafferty says. “Clinicians will visualize consistent mire thickness using the CATS with every measurement regardless of the amount of tears in a patient’s eye.”

Mires will appear brighter and easier to visualize because the clear flange at the base of the CATS prism tip works like a light pipe, concentrating the cobalt blue filtered light to the prism’s applanation surface — which illuminates the mires. “This makes it easier to get a confident measurement,” Dr. Berdahl says.

When he used CATS on patients who were tough to treat, Dr. Radcliffe says their IOP measurements made much more sense.


FDA cleared, the CATS Tonometer prism’s performance is backed by 7 peer-reviewed journal articles. “All of the studies support the decreased sensitivity to human biomechanical corneal-related errors with the use of the CATS prism compared to the GAT prism,” Dr. McCafferty says.

One of the studies included an intracamera comparison performed by the National Institutes of Health, in which a pressure transducer was placed in 58 patient eyes and their IOP was measured. Results were compared between the Goldmann and CATS tonometers. The study showed that the CATS prism was significantly more accurate when compared to true intra-cameral pressure.1

If clinicians want to do their own assessment, Dr. McCafferty recommends they compare the GAT IOP to the CATS IOP measurement on patients in whom they question the true IOP, such as those with thin corneas or who have undergone LASIK surgery. “The CATS IOP will generally measure the expected CCT corrected GAT IOP,” he says. “Because the CATS also simultaneously corrects for 3 other corneal-related errors in addition to CCT, some variation in its correction will exist.”

Given its capabilities, Dr. McCafferty concludes by saying, “I believe CATS will rapidly supplant the legacy prism and become the new standard of care for IOP measurement on a worldwide basis for all ophthalmology and optometry clinicians and all patient populations.”

Dr. Radcliffe concurs, adding, “CATS is inexpensive. I believe in 5 to 10 years that it will become the standard prism.” GP


  1. McCafferty S, Levine J, Schwiegerling J, Enikov ET. Goldmann and error correcting tonometry prisms compared to intracameral pressure. BMC Ophthalmology. 2018;18(1):2.