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Icare ic100 Tonometer User Guide

2 CPD in Australia | 0.25G in New Zealand | 20 June 2017



This module introduces the optometrist to Icare tonometry and particularly the Icare ic100 tonometer. In this module, you will explore:

  • Features of the Icare ic100 tonometer
  • Mechanism on how the tonometer operates
  • Proper use and care of the tonometer
  • Comparisons between the gold standard in tonometers versus Icare tonometry.

At the end of this module you will:

  • Be able take IOP measurements of patients with proper instruction
  • Recognise the role of the Icare ic100 tonometer as a screening tool
  • Understand how the Icare ic100 tonometer compares with current gold standard tonometers
  • Be aware of errors encountered and how to troubleshoot.


Tonometry began in 1863 when von Grafe demonstrated his first instrument to measure intraocular pressure (IOP).1 Numerous methods have been used over the years with the Goldmann applanation tonometers remaining the clinical standard. The need for corneal anaesthesia remains the main drawback of applanation tonometry. The AO air puff (1975) and the Keeler Pulsair (1988) tonometers were successful non-contact tonometers that worked well in general optometry.

Another development to work around the anaesthesia problem was the rebound tonometry, also known as impact or dynamic tonometry, was first introduced by Obbink in 1931 and later by Dekking in 1967,2,3 but it was not generally accepted and never became popular and no commercial device was ever developed from it. A more advanced rebound tonometer was introduced in the year 2000 by Kontiola,4,5 a commercial version became available as the Icare (Tiolat) tonometer in 2003. Development has continued and there are now four different Icare tonometer models (Icare TA01i, Icare ic100, Icare PRO and Icare HOME) for human use and three for veterinary use (Icare TONOVET, Icare TONOVET Plus and Icare TONOLAB).

Where it all began:

1826      Bowman recommended digital palpation as part of routine eye examination. 
1863   von Graefe developed the first instrument to measure IOP.6
1865   Donders invented scleral indentation tonometer.6
1867   Weber invented the first applanation tonometers.6
1884   Koller introduced topical anaesthesia allowing easier IOP measurement.6
1885   Imbert-Fick Law proposed for applanating tonometers (IOP=F/A).6
1892   Maklakov tonometer.6
1905   Schiotz tonometer.6
1955   Goldmann tonometer.7
1959    Mackay-Marg tonometer.6
1967    Dekking carried out research with his impact tonometer.3
1968    Pneumatonometer.8
1972    Air puff non-contact tonometer.9
1987    TonoPen tonometer.10,11
1987    Keeler Pulsair tonometer.12
1998    Proview tonometer.13
2000    Icare tonometer.5
2003    Pascal dynamic contour tonometer.14,15
2005    Reichert Ocular Response Analyser.16

What is an Icare Tonometer?

Icare tonometers are portable, handheld devices used to measure IOP. They assess the deceleration and rebound time of a small, lightweight probe, which makes brief contact with the cornea and can be used without local anaesthesia.51

Mechanism of Action

How does it work?

Icare’s Research and Development Director, Mr. Mika Salkola explains: “The probe of the Icare ic100 tonometer consists of a magnetised steel wire shaft covered with a round plastic tip at the end (0.9mm radius). The probe is placed in the tube-like probe base surrounded by a solenoid. When the measurement button is pressed, the probe briefly pops out, touches the cornea and rebounds back. This movement of the magnetised wire through the solenoid induces a small current which is measured. The rebound behaviour of the probe is used to calculate the IOP. The probe will contact the cornea for about 0.1 seconds when the measurement button is pressed. The force applied is minimal and does not elicit a blink reflex. This eliminates the need to anaesthetise the cornea. All the Icare tonometer models have the same rebound technology.”

Do the results match with the Goldmann?

IOP measurements done by rebound tonometry are highly correlated to those measured using the Goldmann tonometer.17 Most studies state that IOP measured with an Icare tonometer differs from the Perkins and Goldmann tonometers by 3 mmHg or less.17-26

Do the results match with the Tonopen and air puff (NCT)?

Studies have shown that measurements performed with the Icare ic100 are in agreement with those of the Tonopen tonometer.27,29 In other studies the Icare was found to differ less than 3mmHg as compared to Tonopen and NCT.28-30

Does it cause discomfort?

The optometrist’s technique is important when trying to obtain a reliable reading. Some patients don’t even notice the measurement.

While the aim is to be delicate, some optometrists, are not as sensitive with patients. However, it is really important to not touch the patient’s eye at all. Make sure the patient relaxes and breathes while you’re taking the pressure. To help the patient relax, ask them if they can reposition the way they sit or loosen their collar, or take off a warm piece of clothing, for example.52

Most patients feel or see something touching the eye but it does not create any sensation of pain or otherwise bother them. No topical anaesthetics are needed. The Icare tonometer is well tolerated by patients.21,31,32 The Icare tonometer has been found more comfortable than for instance the Goldmann and Pulsair (NCT) tonometers.28,40

In addition, Icare’s R&D Director Mr. Mika Salkola states: “Rebound tonometry demonstrated an excellent safety profile with no adverse events observed in an unpublished clinical study of 383 subjects conducted by Icare. Importantly, there was no clinically meaningful change from baseline in corneal epithelial staining nor change from baseline in discomfort score during the study even though the corneas underwent many tonometer measurements.”

Use With Population Groups

Is it good for children?

Icare tonometers have been used extensively in studies of IOP in children.31-33 The Icare is better tolerated than the air puff non-contact tonometer in children below the age of three years.30 The Figure 1 shows measuring of a child.


Figure 1. Measuring a child.

It is useful in regional and third world locations?

The Icare tonometry is used extensively in remote clinical settings in Australia and Malawi due to its ease of use, minimal infection risk and no need for anaesthesia.34,35

Does it need extensive training?

There is no significant difference between IOPs measured by experienced and inexperienced tonometrists using the Icare tonometer.36

What batteries should I use?

Always use non-rechargeable batteries. Rechargeable batteries are not compatible with the Icare tonometers as they supply lower voltage.


Using the Icare ic100 Tonometer

How do I store the Icare tonometer?

Place the tonometer in the case when not in use.

Can I load the probe incorrectly?

The design of probe makes it impossible to insert it incorrectly.

How do I clean the body of the tonometer?

The tonometers body can be cleaned with a damp cloth containing disinfectant.

Do I need to disinfect the forehead support?

Disinfect the forehead support with an alcohol wipe between each patient.

What is the infection risk?

There is minimal infection risk. The probes are clean when taken from an intact, original packaging. The probes are for single-use only.

Can I use the probe on both eyes of one patient?

The same probe can be used to measure both eyes of one patient if the patient's eyes are healthy.

Can I use the probe on an infected eye?

The probe can be used on an infected eye. However, measure the healthy eye first if the patient has infection in one eye. A probe used to measure an infected eye should never be used to measure a healthy eye. If in doubt, always use a new probe.

Disinfection protocol, Influences on IOP measure, Calibration and Veterinary Use

Can the probe be disinfected and reused?

The probes are for single-use only. Reuse of the probe could result in incorrect measurement values, in the breakdown of the probe, cross-contamination of bacteria or viruses, and infection of the eye. Reuse will void all responsibilities and liabilities of the manufacturer concerning the safety and effectiveness of the tonometer.

Does corneal thickness influence the Icare tonometer?

The IOP measured by the Icare tonometer increases by 0.1 to 1.0mm Hg for every 10μm increase in corneal thickness.37-41

Does refractive surgery influence IOP readings?

Refractive surgery does influence IOP readings. LASIK and similar surgeries can change corneal thickness and measured IOPs (thinner corneas result in lower measured IOP).

Does corneal oedema influence the Icare tonometer?

Corneal oedema can influence the reading, but very little (+/-2 mmHg) as the Icare tonometer probe has a relatively small contact area with the cornea. Consequently, greater independence from corneal biomechanical properties than applanation tonometers.42

Does corneal curvature and shape influence IOP readings?

No the corneal curvature and shape does not influence IOP readings. The contact area is so small that the measurement is not influenced. However, the contact point should be in the centre of the cornea for the most accurate and repeatable result.

Can the IOP be measured through soft contact lenses?

IOP can be measured through soft contact lenses. For the most accurate measurement, it is recommended that contact lenses be removed. However, an estimate of IOP can be taken through silicone hydrogel lenses and some soft two week or monthly lenses. Rigid gas permeable (RGP) lenses should not be worn when using the Icare tonometer.43,44

What do I do if the Icare tonometers IOP measures high?

Confirm the measurement using a Goldmann or Perkins applanation tonometer.

Can I use the Icare tonometer to manage glaucoma patients?

The Icare ic100 can be used to manage glaucoma patients’ IOP. However, if the IOP value seems high or extremely low, it should be confirmed with Goldmann.

How do I dispose of a used probe?

Dispose of the single use probe in the rubbish bin like a used contact lens.

Does the Icare tonometer need calibration or service?

Icare tonometers do not require any calibration or regular service. You should contact your local distributor for service if you doubt the measurement results or if you drop the tonometers on the floor.

Can the Icare tonometry be used at home to self-measure IOPs?

The Icare HOME self-tonometer, which has the same rebound technology as the Icare ic100 tonometer, allows patients to measure their own IOP at home. Like finger prick blood glucose measurements in diabetes, this gives the patient increased engagement with their disease. Each patient needs to be trained to use Icare HOME.

Mr. John Floyd, President and CEO of Icare USA says this about the Icare HOME: “The fact that we can put a tonometer of this calibre (sic) in the hands of patients is truly unprecedented… The Icare HOME makes doctors [and patients] aware of dangerous spikes [in IOP] that they may never have known about otherwise. The impact and level of care that this provides cannot be overstated.”53

Studies show that the mean IOP differences between the Icare HOME rebound tonometer and Goldmann are within 1mmHg and most of the Icare HOME measurements are within 3mmHg of the GAT measurements.41,45-48 Acceptability questionnaire responses of one study (N=76) showed high agreement that the Icare HOME was easy to use (84 per cent), the reading was quick to obtain (88 per cent) and the measurement was comfortable (95per cent).46 Acceptability questionnaire responses in another Icare HOME study (N=43) showed that 39.5 per cent of the subjects answered "easy to use," 46.5 per cent answered "normal," and 13.9 per cent answered "difficult to use”.47 Figure 2 below, shows a self-measurement situation.


Figure 2. Self-measuring.

Can the Icare tonometers be used on animals?

The Icare TONOVET, TONOVET Plus and TONOLAB rebound tonometers are designed for animal use. These have been found to give results that are similar to or better than the results of TonoPen XL/TonoPen Vet.49,50 The Figure 3 shows measuring of animals with the Icare TONOVET Plus tonometer.

Figure 3. Measuring animals.

The full range of Icare tonometers can be viewed at http://www.icaretonometer.com.

Using the ic100 Tonometer


Figure 4. Parts of the Icare ic100 Tonometer



1. Press the ‘Select’ or ‘Measure’ button to turn the tonometer on (Figure 5).
Pressing the Select button Pressing the Measure button

Figure 5. Two ways of turning on the tonometer.

2. Open the probe tube by removing the cap and insert the probe into the probe base as shown in Figure 6.


Figure 6. Probe loading

3. When the ‘Play-symbol’ appears on the display, the tonometer will be ready for measurements (Figure 7).

Figure 7. Ready to measure.

4. Clean the forehead support with an alcohol wipe and ask the patient to relax and look straight ahead at a specific point.

5. Bring the tonometer near the patient’seye. Keep the probe horizontal and pointing perpendicularly to the centre of the cornea. The distance fromthe tip of the probe to the patient´s cornea should be 4-8mm.
The probe base light appears in the front and the side of the probe base collar, thus visible to the patient as well as the measurer. Green light indicates the vertical alignment is correct. If the light is red, a measurement cannot be taken. See Figure 8 for correct alignment of the tonometer.

Figure 8. Correct alignment.

In favorable ambient lighting, you may find it helpful to utilise the probe base light in the following manner to secure correct alignment:

Step 1.    Align the probe in the center of the cornea; i.e. vertically and horizontally at 90 degrees.
Step 2.    Ask the patient to look in the center of the green light ring.
Step 3.    Verify the green light ring reflected off the cornea appears as a symmetrical, intact circle in the center of the eye (the light will appear as an oval in the presence of horizontal or vertical tilt, avoid such tilt), see Figure 9.

Figure 9. Reflection of the green probe base light off the cornea.

6. Press the Measure button to measure IOP. Probe will contact the eye during measurements: Press and hold the button to collect six measurements in a row OR Press the button briefly six times to collect one measurement at the time. Follow the signals and display (Figure 10).


Figure 10. Signals and IOP display.

Accessing old measurements

Press the ‘Select’ button to access. Scroll through the old values by pressing either of the ‘Navigation’ buttons. Value colours green and yellow are related to Standard deviation (SD). To exit, press the ‘Select’ button. See (Figure 11).


Figure 11. Old measurements.

Cleaning the probe base

  • Clean every six months or when the error message ‘Clean Change’ is displayed.
  • Fill the probe base cleaning container or other clean container with 70-100 per cent isopropyl alcohol or ethanol.
  • Turn the power off.
  • Unscrew the probe base collar.
  • Invert the probe base over the container, drop in the probe base into the container and let soak for 5–30 minutes.
  • Remove the probe base from alcohol.
  • Dry the probe base by blowing clean canned or compressed air into the hole in the probe base. This will additionally remove possible residual dirt.
  • Insert the probe base into the tonometer.
  • Screw the collar in, to lock the probe base.

Replacing the probe base

  • Replace every twelve months or when the error message ‘Clean Change’ is displayed.
  • Turn off the tonometer.
  • Unscrew the probe base collar and put it in a safe place.
  • Remove the probe base by pulling it out of the tonometer with your fingers (Figure 12).
  • Insert a new probe base into the tonometer.
  • Screw the collar in, to lock the probe base.


Figure 12. Probe base removal.

Turning off the tonometer

Press the ‘Select’ button until the display shows the ‘End-symbol’ (Figure13). Alternatively, if you do not use the tonometer, it will switch off automatically after three minutes.


Figure 13. Select button and End symbol.

Error and info messages

Several excellent training videos are found on the Icare website. It is recommended that you view Icare ic100 Tonometer videos: http://www.icaretonometer.com/products/icare-ic100/. There are also many Icare videos on YouTube. Have a search and look at a few for an overview of how the Icare tonometers are used in other practices.

Dr. Peter Herse and Mr. Mika Salkola
DipAppSc (Optom), PhD, FAAO, GradCert Oc Therapeutics and
Peter Herse qualified as an optometrist from the Queensland Institute of Technology in 1980 and practiced in Queensland and Kenya. He completed a PhD at the University of Houston and was admitted as a Fellow of the American Academy of Optometry. He has held academic appointments in optometry schools at the University of Durban-Westville, the University of Auckland and the University of New South Wales and Research Fellow appointments at universities in the USA, India and Thailand. He has a strong interest in low vision, was staff optometrist at Vision Australia Sydney for many years and was a Director of the Macular Degeneration Foundation of Australia. He is currently the Academic Director of the Luxottica Institute of Learning. His role is to promote lifelong learning and professional development for optometrists associated with Luxottica.

Mr. Mika Salkola, Research and Development Director, Icare Finland

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52. Michelle Stephenson, Contributing Editor, Review of Ophthalmology, Can You Trust Your IOP Readings?, Published 13 June 201, quoting Dr. Lehrer https://www.reviewofophthalmology.com/article/can-you-trust-your-iop-readings accessed on 16/5/2017.
53. Med Device Online, FDA Cleared Icare HOME, An Innovative Device Poised To Revolutionize IOP Self-Monitoring, published 22 March 2017, quoting John Floyd, President and CEO of Icare USA, https://www.meddeviceonline.com/doc/fda-cleared-icare-home-innovative-device-poised-revolutionize-self-monitoring-0001 accessed on 18 May 2017.

' The fact that we can put a tonometer of this calibre in the hands of patients is truly unprecedented… '