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Non-contact tonometry (NCT), commonly called the "air puff test," is the most widely used method for IOP screening in optometry and ophthalmology settings. Unlike Goldmann applanation tonometry, NCT requires no topical anesthetic, no fluorescein, and no probe-to-eye contact — making it fast, accessible, and well-tolerated by most patients. For COA candidates, understanding both its technique and its limitations is essential.
NCT appears on the COA exam as part of the Assessments domain, which accounts for 42% of the exam. Questions focus on the measurement principle, proper technique, sources of error, clinical limitations, and the decision rule for when to confirm elevated readings with GAT. This guide covers all of these areas.
The NCT uses a calibrated air pulse directed at the central cornea from a fixed working distance. As the air pulse is delivered, it momentarily flattens (applanates) the corneal surface. An infrared emitter and photodetector monitor the reflectance of the cornea throughout the pulse. The cornea acts as a mirror — when it is perfectly flat, reflected light reaches the detector at maximum intensity. The instrument records the air pressure at the exact moment of maximum reflectance and converts this to an IOP estimate in mmHg.
The key insight is that the cornea can only be flattened to that maximum-reflectance shape when the air pressure equals the IOP from inside the eye. Higher internal pressure requires more air pressure to flatten the cornea; lower internal pressure requires less.
~3 ms
Too fast to cause discomfort
3+
Average all valid readings
21 mmHg
Verify with GAT if elevated
GAT applanates the cornea to a precise 3.14 mm diameter and measures the mechanical force required. NCT monitors the optical change in corneal reflectance and uses the air pressure at maximum reflectance as a proxy for IOP. Both methods rely on the Imbert-Fick principle but implement it differently. GAT remains more accurate because the optical approach of NCT is more susceptible to corneal irregularity and patient movement.
Explain the procedure: "This device will blow a small puff of air at your eye. It happens very quickly and should not cause pain. Try to keep your eye open and look at the fixation light." Soft contact lenses do not need to be removed for NCT (unlike GAT where fluorescein would stain them). However, note that contact lenses on the eye may slightly affect NCT readings.
Have the patient place their chin in the chin rest and forehead against the headrest strap. The instrument tip should be approximately 11–14 mm from the corneal apex (exact distance depends on the specific NCT model). Most instruments have a built-in alignment system — a target ring that the patient looks at and an indicator on the operator screen that confirms correct working distance and centration.
Use the joystick to center the corneal reflex in the target ring and confirm the working distance indicator is green (or within the acceptable range). Many modern NCTs auto-trigger when alignment is confirmed. If triggering manually, wait for the patient to be stable (not mid-blink) before firing. If the patient blinked during the measurement, discard that reading and repeat.
Obtain at least 3 measurements per eye. Most NCT instruments automatically average and display results. If any individual reading differs from the others by more than 2–3 mmHg, discard it (it is likely an outlier caused by a blink, eye movement, or poor alignment) and take an additional reading to replace it. Always test right eye (OD) first.
Record the averaged IOP for OD and OS, the time of measurement, and the instrument type (NCT). If either reading exceeds 21 mmHg, flag it for confirmation with GAT by the examining provider. Note any factors that may have affected accuracy (patient anxiety, multiple blinks, contact lenses worn).
| Advantages | Limitations |
|---|---|
| No topical anesthetic required | Less accurate than GAT, especially at higher pressures |
| No fluorescein needed | Cannot be used with irregular corneas (keratoconus, scars) |
| No corneal contact — no infection risk from probe | Patient blink reflex causes frequent measurement failures |
| Fast — 3 readings take under 1 minute | Air puff startles anxious or pediatric patients |
| Can be performed by staff without medication authorization | Still affected by corneal thickness (same as GAT) |
| Excellent for high-volume screening | Elevated readings always require GAT confirmation |
| Contact lenses can remain in place | Not reliable in edematous or post-surgical corneas |
NCT is a screening tool, not a diagnostic instrument. Understanding when to escalate to GAT is a key clinical competency tested on the COA exam.
Any reading above 21 mmHg
The most important rule. NCT readings exceeding 21 mmHg should always be confirmed with GAT before the clinician makes treatment decisions. NCT frequently overestimates IOP in anxious patients or those with reflex blink responses.
High inter-reading variability
If three NCT readings on the same eye span more than 4–5 mmHg (e.g., 14, 18, 22 mmHg), the measurements are unreliable. GAT with careful technique provides a more consistent result.
Known or suspected glaucoma
Patients with established glaucoma require precise IOP monitoring to assess treatment adequacy. GAT provides the level of precision needed for these management decisions.
Corneal irregularity
Keratoconus, corneal scars, post-LASIK, or post-keratoplasty corneas alter the reflectance signal and make NCT unreliable. Use a contact tonometer (GAT, Tono-Pen, or iCare) instead.
Inability to obtain 3 valid NCT readings
If a patient cannot tolerate the air puff or blinks repeatedly, switch to iCare rebound tonometry (no air puff) or GAT rather than using an insufficient number of readings.
Opterio's COA question bank includes NCT technique, limitations, and clinical decision-making questions with AI-powered explanations tailored to the COA exam content.
Documentation of NCT results follows the same general principles as GAT, with a few additional considerations specific to the non-contact method.
Record the averaged value
Document the averaged IOP, not individual readings, unless your practice requires all three to be recorded. Example: IOP OD 16 mmHg, OS 14 mmHg by NCT.
Specify the method
Always write "by NCT" or "by AT" (air tonometry) to distinguish from GAT readings in the same chart. This allows the provider to interpret the clinical significance appropriately.
Note contact lens status
If soft contact lenses were in place during NCT, document this. Contact lenses can slightly lower the measured IOP by dampening the reflectance signal. Most practices prefer NCT without lenses but this is not always practical.
Flag for GAT confirmation
Add a note such as "GAT requested — NCT OD 24 mmHg" so the provider knows to perform or order confirmatory applanation tonometry during the exam.
The gold standard IOP method: principle, mire alignment, calibration, and sources of error.
Statistical norms, diurnal variation, ocular hypertension thresholds, and clinical significance.
GAT, NCT, iCare, Tonopen, and Perkins compared for accuracy and best use cases.
Exam format, 5 content domains, eligibility requirements, and registration process.
The non-contact tonometer (NCT) emits a precisely controlled air pulse that momentarily flattens the central cornea. An infrared sensor monitors corneal reflectance in real time. The cornea changes its reflectance as it flattens. The instrument records the amount of air pressure required to reach the moment of maximum reflectance (i.e., maximum applanation), which correlates with IOP. The entire measurement takes about 3 milliseconds — far too fast to feel discomfort. The instrument converts the air pressure reading to mmHg using a calibration algorithm.
Position the patient at the instrument with chin in the chin rest and forehead against the strap. Align the instrument so the target ring is centered over the patient's corneal reflex and the instrument is at the correct working distance (confirmed by the alignment indicator or a focus indicator on the display). Instruct the patient to look at the fixation light and try not to blink. Trigger the measurement — most modern NCTs are automated and self-trigger when alignment is confirmed. Take at least 3 readings per eye and average them. Readings that differ by more than 2–3 mmHg from each other should be discarded and retaken.
Any NCT reading above 21 mmHg (the statistical upper limit of normal) should be confirmed with Goldmann applanation tonometry (GAT) before clinical decisions are made. NCT has a larger coefficient of variation than GAT, especially at higher pressures, and can overestimate IOP — particularly in nervous patients who blink or tense up before the air puff. GAT is the gold standard for definitive IOP measurement. In a busy practice, NCT screening efficiently identifies patients who need closer attention, but GAT provides the measurement the ophthalmologist relies on for diagnosis and treatment planning.
No. This is one of the primary advantages of NCT. Because the instrument never physically contacts the eye, no topical anesthetic is needed. This makes NCT faster to perform, eliminates the risk of anesthetic-related corneal toxicity, avoids the need to wait for the anesthetic to take effect, and makes it accessible to staff without the authorization to administer medications. It also reduces cross-infection risk since there is no probe contact with the ocular surface.
Key sources of NCT error include: (1) Patient blinking during the air pulse — the eyelid in the path of the air stream interferes with the optical system. (2) Patient startling before the air pulse fires, causing eye movement that misaligns the measurement zone. (3) Excessive tearing — tear film irregularity distorts the reflectance signal. (4) Corneal irregularity from scars, keratoconus, or post-refractive surgery changes the reflectance pattern. (5) Valsalva maneuver from breath-holding, same as with GAT. (6) High refractive error — very high myopia or hyperopia can affect corneal geometry. NCT is less accurate than GAT across all these situations.