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.
How Non-Contact Tonometry Works
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.
Air Pulse Duration
~3 ms
Too fast to cause discomfort
Readings Per Eye
3+
Average all valid readings
Confirm if Above
21 mmHg
Verify with GAT if elevated
NCT vs. GAT: The Fundamental Difference
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.
NCT Technique: Step-by-Step
Prepare the patient
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.
Position at the instrument
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.
Align precisely and trigger
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.
Take 3 readings and average
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.
Document and flag elevated readings
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).
NCT Advantages and Limitations
| 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 |
When to Confirm NCT Readings with GAT
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.
Practice NCT and IOP Assessment Questions
Opterio's COA question bank includes NCT technique, limitations, and clinical decision-making questions with AI-powered explanations tailored to the COA exam content.
Documenting NCT Results
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.