Non-contact tonometry (NCT) -- commonly known as the "air-puff test" -- is a screening measurement of intraocular pressure (IOP) that paraoptometrics routinely perform during pre-testing. Elevated IOP is a major risk factor for glaucoma, a group of diseases that can cause irreversible vision loss, so measuring it at every comprehensive exam is standard practice.
The CPO and CPOA exams test your knowledge of how the NCT works, what constitutes normal IOP, what factors can make the reading inaccurate, and why IOP is only one piece of the glaucoma puzzle. You do not need to diagnose glaucoma -- that is the doctor's role -- but you need to understand why you are measuring IOP and what the numbers mean.
This article covers the operating principle, step-by-step procedure, normal values, factors that influence accuracy, the clinical significance of IOP, and how to handle common challenges like anxious patients and inconsistent readings.
How Non-Contact Tonometry Works
The NCT instrument directs a brief, precisely calibrated pulse of air at the cornea. This air pulse momentarily flattens (applanates) a small area of the corneal surface. The instrument measures how long it takes, or how much force is required, to achieve a specific degree of corneal flattening. Higher intraocular pressure means the cornea resists flattening more, requiring a stronger pulse or longer duration.
An optical detection system monitors the corneal surface during the measurement. As the cornea flattens, it reaches a point where it reflects light in a specific pattern -- the instrument detects this moment and calculates the IOP based on the air pressure needed to reach that point. The entire measurement takes only a few milliseconds.
Operating Procedure
1. Patient Preparation
Seat the patient comfortably at the instrument. Explain the procedure: "You will feel a brief puff of air on your eye. It is quick and not painful, but it may surprise you the first time. Try to keep your eyes open and look at the light inside the instrument." Setting expectations reduces the startle response. Contact lenses should be removed -- they interfere with the measurement. Glasses are already off from prior testing.
2. Position and Alignment
Have the patient place their chin in the chin rest and press their forehead firmly against the headrest bar. Adjust the table height so the patient is comfortable. Using the joystick, align the instrument with the patient's eye -- most models display a live image on a screen with alignment guides. Center the crosshairs on the corneal apex (the center of the cornea, over the pupil). Proper alignment is essential for accurate readings.
3. Take Multiple Readings
Most protocols call for three readings per eye, with the instrument automatically averaging them. Start with the right eye. Once aligned, the instrument will fire automatically (or you press a trigger, depending on the model). Allow the patient a moment between puffs. If readings are inconsistent -- for example, one is 14 mmHg and the next is 22 mmHg -- discard the outlier and take additional measurements. Move to the left eye and repeat.
4. Record and Review
Record the readings for both eyes (e.g., OD: 16 mmHg, OS: 17 mmHg). Flag any reading above 21 mmHg for the doctor's attention. Also note if readings were difficult to obtain or inconsistent. If one eye is significantly higher than the other (asymmetry of 4+ mmHg), that is also worth flagging even if both readings are within the normal range.
Normal IOP Values and What They Mean
10-21 mmHg
Normal range
Average is approximately 15-16 mmHg. Most patients fall in this range.
22-29 mmHg
Elevated -- needs evaluation
May indicate ocular hypertension or early glaucoma. Doctor will assess further.
30+ mmHg
Significantly elevated
Requires prompt doctor attention. Possible acute glaucoma or significant ocular hypertension.
Important Concept
IOP alone does not diagnose or rule out glaucoma. Normal-tension glaucoma can cause optic nerve damage at pressures within the "normal" range (below 21 mmHg). Conversely, some patients tolerate higher pressures without damage (ocular hypertension). IOP is one risk factor that the doctor considers alongside optic nerve evaluation, visual fields, pachymetry, and family history.
Factors That Affect NCT Accuracy
Central Corneal Thickness (CCT)
This is the single biggest source of NCT inaccuracy. Thick corneas read artificially high; thin corneas read artificially low. Average CCT is about 545 microns. Pachymetry measures CCT so the doctor can adjust the interpretation. This is a high-yield exam topic.
Time of Day
IOP follows a diurnal pattern. It is typically highest in the early morning hours and lowest in the late afternoon. The variation can be 2-6 mmHg or even more. Consistency in appointment timing helps when tracking IOP over multiple visits.
Patient Squeezing or Breath Holding
Squeezing the eyelids or performing a Valsalva maneuver (breath holding, bearing down) temporarily increases IOP. Instruct patients to breathe normally and relax their face. If you notice squeezing, wait for the patient to relax before measuring.
Corneal Surface Conditions
Corneal edema, scarring, or irregularity can affect how the air pulse interacts with the corneal surface. After LASIK or other refractive surgery, the thinner cornea will read artificially low, which is especially important for glaucoma screening in post-surgical patients.
Tight Collar or Necktie
Anything that increases venous pressure in the head can elevate IOP readings. If a patient is wearing a very tight collar, it may be worth loosening it before measurement. This is a less common but testable factor.
Practice tonometry and glaucoma screening questions
Opterio covers IOP measurement, glaucoma risk factors, and all CPO/CPOA special procedures topics.
Advantages and Limitations of NCT
Advantages
- No topical anesthesia needed
- No corneal contact -- minimal infection risk
- Can be performed by paraoptometrics
- Fast -- ideal for screening
- No risk of corneal abrasion
Limitations
- Less accurate than Goldmann applanation
- Significantly affected by corneal thickness
- Less reliable at higher IOP ranges
- Patient startle response can affect readings
- Not the gold standard for diagnosis
Documenting NCT Results
Record the IOP for each eye in mmHg, the instrument used (NCT), and the time of measurement. A typical entry looks like: IOP (NCT) @ 2:30 PM: OD 16, OS 17 mmHg. If readings were inconsistent or difficult to obtain, note that as well. Some practices also record individual readings rather than just the average, especially if there was significant variability.
If a reading is elevated, do not alarm the patient. Simply note it in the chart and inform the doctor before or during the exam. The doctor will decide whether to recheck with Goldmann tonometry and what further evaluation is needed.