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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.
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.
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.
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.
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.
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 range
Average is approximately 15-16 mmHg. Most patients fall in this range.
Elevated -- needs evaluation
May indicate ocular hypertension or early glaucoma. Doctor will assess further.
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.
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.
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.
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 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.
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.
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.
Understanding the disease that IOP measurement helps screen for.
Another key instrument in the anterior segment examination workflow.
Calibration and cleaning protocols for the NCT and other instruments.
Browse all CPO and CPOA study topics organized by category.
Normal intraocular pressure (IOP) is generally considered to be between 10 and 21 mmHg, with an average of about 15-16 mmHg. However, "normal" IOP does not guarantee the absence of glaucoma (normal-tension glaucoma exists), and IOP above 21 does not automatically mean a patient has glaucoma. IOP is one risk factor among several that the doctor evaluates together with optic nerve appearance, visual fields, and other tests.
Central corneal thickness significantly affects NCT accuracy. A thicker-than-average cornea (above ~555 microns) requires more force to flatten, causing the instrument to overestimate IOP -- the reading will be artificially high. A thinner cornea (below ~535 microns) flattens more easily, causing an artificially low reading. This is clinically important because thin corneas are themselves a risk factor for glaucoma, and the IOP reading may appear reassuringly low when the actual pressure is higher. Pachymetry measures CCT and allows the doctor to interpret the IOP reading in context.
The brief pulse of air can startle patients, especially on the first attempt. The surprise is the main issue -- the puff itself is not painful. Instruct patients beforehand that they will feel a brief puff of air, which helps reduce the startle response. Some patients may squeeze their eyelids, pull back, or blink reflexively. In these cases, reassure them, allow a moment to relax, and try again. If a patient consistently cannot tolerate the NCT, the doctor may use Goldmann applanation tonometry instead.
NCT has three main advantages: (1) it does not require topical anesthesia (no numbing drops), making it faster and more comfortable for screening; (2) there is no direct contact with the cornea, eliminating the risk of corneal abrasion or infection transmission between patients; and (3) it can be performed by paraoptometrics as part of pre-testing, saving doctor chair time. The main disadvantage is that NCT is less accurate than Goldmann, particularly at higher IOPs, and is more affected by corneal thickness and properties.
Ideally, yes. IOP follows a diurnal (daily) pattern, typically being highest in the early morning and lowest in the late afternoon or evening, with variations of 2-6 mmHg throughout the day. When monitoring a patient over time (such as a glaucoma suspect), measuring at a consistent time of day provides more comparable readings. However, some doctors intentionally measure at different times to capture the patient's IOP at various points in the diurnal cycle.