Tonometry is 7% of the COA® exam in IJCAHPO’s 2025 content areas. Record each eye’s IOP separately, in mmHg. Readings run falsely high when the patient holds their breath, when the globe is pressed, or when the fluorescein mires are too wide; mires that are too thin read low. Pressure varies over the day, and calibration follows the manufacturer.
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Exam weight
Tonometry is 7% of the COA exam (IJCAHPO content areas, 8/1/2025); the technique itself is on tonometry for the COA.
What to record
- Eye: OD and OS on separate entries.
- Value and unit: IOP is given in millimeters of mercury, mmHg.
- Anything unusual: breath-holding or a difficult measurement.
Try the simulator
The applanation tonometer simulator shows how mire alignment and width change the reading. Use it with the error list below.
Technique errors
Readings that come out falsely high:
- The patient holds their breath.
- Pressure on the globe, often from fingers holding the lids. Brace the lids on the bony orbit instead.
- Fluorescein rings that are too wide.
Readings that come out falsely low:
- Fluorescein rings that are too thin.
The cornea itself can also shift a reading, so note anything unusual about it for the doctor.
Calibration and care
Check calibration as the manufacturer recommends, using the bar supplied with the instrument. A tonometer that fails the check is sent back to the manufacturer, not adjusted in the office. Clean and disinfect the tip between patients according to your clinic’s protocol and the manufacturer’s instructions.
Practice: chart notes
Original scenarios.
You write “IOP 18/21” with nothing else. What is missing?
Show answer
Which number is which eye, and the unit. A clearer entry: OD 18 mmHg, OS 21 mmHg, plus whatever your clinic’s chart format asks for.
The patient held their breath during the left-eye reading. Which way is that reading likely off?
Show answer
Falsely high. Note it, let the patient relax and breathe normally, and remeasure.
While holding the lids open, your fingers pressed on the eye. Effect?
Show answer
Falsely high. Brace the lids on the bony orbit, away from the globe.
The fluorescein rings look very thin. Effect on the reading?
Show answer
Falsely low. Thick rings give the opposite error.
The calibration check reads off at one of its positions. What do you do?
Show answer
Take the tonometer out of use, tell the person responsible, and send it back to the manufacturer per its recommendations.
Standard text used for this page: Ledford JK. Certified Ophthalmic Assistant Exam Review Manual, 3rd ed. SLACK, tonometry chapter answers (units, calibration, mire width, breath-holding, daily variation). The Journal of Current Glaucoma Practice and Review of Optometry articles are linked under sources.
COA® is a registered mark of IJCAHPO.
Frequently asked questions
How should IOP be recorded in the chart?
One reading per eye, labeled OD and OS, in millimeters of mercury (mmHg). Note anything that could have affected the reading, such as a patient who held their breath.
Sources: Ledford, Certified Ophthalmic Assistant Exam Review Manual, 3rd ed. (textbook), ch. 17 Tonometry answers, printed pp. 282–284 (opens in a new tab) ; Journal of Current Glaucoma Practice (PMC4741114) — Tonometry and Care of Tonometers (opens in a new tab)
Why does the time of an IOP reading matter?
Eye pressure varies through the day. A reading taken in the morning and one taken late in the afternoon are not directly comparable.
Sources: Ledford, Certified Ophthalmic Assistant Exam Review Manual, 3rd ed. (textbook), ch. 17 Tonometry answers, printed pp. 282–284 (opens in a new tab) ; Journal of Current Glaucoma Practice (PMC4741114) — Tonometry and Care of Tonometers (opens in a new tab)
How often should a Goldmann tonometer be calibrated?
Follow the instrument manufacturer’s recommendations. The check uses the bar supplied with the tonometer at set positions. If it reads outside the expected range, stop using it and send it back to the manufacturer rather than adjusting it yourself.
Sources: IJCAHPO — Core Certification Criteria Handbook (PDF) (opens in a new tab) ; Journal of Current Glaucoma Practice (PMC4741114) — Tonometry and Care of Tonometers (opens in a new tab)
Should an assistant tell a patient their IOP is normal?
No. Interpreting the pressure is the doctor’s call. A single number does not diagnose or rule out glaucoma: it is read alongside the optic nerve, visual field and other findings. You can tell the patient the doctor will go over the results.
Official sources
- IJCAHPO — COA Examination Content Areas, effective 8/1/2025 (PDF) (opens in a new tab)
- IJCAHPO — Core Certification Criteria Handbook (PDF) (opens in a new tab)
- Journal of Current Glaucoma Practice (PMC4741114) — Tonometry and Care of Tonometers (opens in a new tab)
- Review of Optometry — The do’s and don’ts of measuring IOP (opens in a new tab)
Last reviewed . Requirements, fees, and dates change — always confirm with the official source before you register or apply. ABO-NCLE and IJCAHPO are the only authorities for their own exam facts, so those facts rest on a single source.