COT common mistakes
COT · Understand the traps and how to avoid them.
When measuring visual acuity, the candidate shows the whole line of optotypes at once or lets the patient lean forward, squint, or peek through a pinhole while reading, then records the line reached as the acuity.
Candidates memorize the Snellen notation but not the test mechanics; they focus on the chart itself and forget that distance, occlusion, and no-squinting are part of the procedure. Squinting acts like a pinhole and artificially improves the result, so the recorded acuity is not valid.
What to do instead
Practice the full sequence aloud: correct testing distance, occlude the non-tested eye without pressing on the globe, watch the patient's face for squinting, and stop at the line where the majority of optotypes are missed. Re-test any suspicious result with a pinhole to separate refractive from organic loss.
Confusing applanation tonometry calibration and technique details, such as not checking that the prism mires are aligned and centered, or accepting a single reading without repeat measurement.
The skill is taught as 'touch the cornea and read the dial', so candidates skip the alignment check, the fluorescein and anesthetic sequence, and the repeatability check. They also confuse Goldmann applanation with non-contact or indentation tonometry when answering theory items.
What to do instead
Learn the sequence as a checklist: anesthetic, fluorescein, slit-lamp angle, blue filter, mires centered and equal, average of at least two readings. Know which tonometer type each question describes and what each measures (force needed to flatten a fixed area versus time to flatten).
Describing ocular anatomy as isolated structures instead of tracing the pathway or blood supply, for example naming the layers of the cornea but not their function, or identifying the optic nerve head without linking it to the visual pathway and pupil reflex.
Candidates study from lists and diagrams and memorize names in order without building a functional map. Exam items often ask about consequences (where a lesion causes a field defect, which layer limits drug penetration), which list-memorization does not answer.
What to do instead
Redraw each pathway from memory: tear film and corneal layers, aqueous outflow route, visual pathway from retina to occipital cortex with the chiasm and radiation, and the pupillary light reflex arc. For each structure, write one clinical consequence of damage to it.
Treating refraction as a single 'best sphere' guess: the candidate finds the sphere that makes the chart clearest and stops, skipping cylinder refinement, fogging, or duochrome and cross-cylinder checks.
Sphere-only refraction looks convincing to the patient and the candidate, so the missing cylinder or wrong axis is never noticed. Candidates also confuse plus and minus cylinder notation and transpose incorrectly under time pressure.
What to do instead
Follow a fixed routine every time: sphere to best acuity, then Jackson cross-cylinder for axis and power, then refine sphere, then binocular balance. Practice transposition until it is automatic, and always verify the final prescription against the patient's symptoms and previous correction.
When fitting contact lenses, the candidate selects a lens by refractive power alone and does not evaluate fit, movement, centration, or corneal coverage, and does not convert spectacle power to contact lens power when needed.
Candidates treat the contact lens as a small spectacle lens. They also forget vertex distance effects at high powers and the difference between base curve, diameter, and material parameters, so a lens that 'sees well' is judged a good fit.
What to do instead
Assess fit with fluorescein pattern, centration, movement on blink, and corneal coverage before judging vision. Learn when vertex distance conversion matters (roughly beyond plus or minus 4 diopters) and how base curve, diameter, and material oxygen permeability interact. Document each parameter and the observed fit.
Recording history and findings as vague phrases such as 'vision reduced' or 'eye red' without laterality, duration, severity, associated symptoms, or the specific measurements taken, and omitting the medication and allergy history.
Candidates write notes to satisfy the form rather than to communicate with the next clinician, and they are uncomfortable asking follow-up questions. Under time pressure they summarize instead of documenting specifics.
What to do instead
Use a structured history: onset, laterality, duration, character, aggravating and relieving factors, associated symptoms, prior episodes, medications including drops, allergies, and ocular and systemic history. Write findings as numbers with units and the method used, for example acuity with and without pinhole and the tonometer type.
Skipping or rushing infection control steps: not performing hand hygiene at the correct moments, reusing a tonometer tip or trial lens without disinfection, or touching the patient's eye area after handling equipment.
Candidates see these steps as background routine and prioritize speed during a busy clinic. They also do not know which disinfectant is appropriate for which surface or instrument, so they improvise.
What to do instead
Memorize hand hygiene moments and the disinfection requirement for each item class: single-use where indicated, appropriate wipe or soak for tonometer prisms and trial lenses, and clean-to-dirty order. Practice the sequence until it is automatic, and treat it as a scored step of every procedure, not an optional extra.
Misinterpreting test results in isolation, for example reading a visual field printout without checking reliability indices, or interpreting a single IOP reading as diagnostic of glaucoma without considering corneal thickness, time of day, and optic nerve appearance.
Candidates learn to read the numbers on a printout but not the assumptions behind them. Each instrument has reliability and confounding factors that are tested more often than the raw value.
What to do instead
For every test, learn three things: what it measures, its reliability or quality indicators, and its confounders. For fields, check fixation losses, false positives, and false negatives first. For tonometry, note corneal thickness, diurnal variation, and technique. Interpret results together with the clinical picture, never alone.
Confusing the optics of the eye with the optics of lenses: applying the wrong formula for vergence, magnification, or effective power, or ignoring how a lens changes image size and position when placed at different vertex distances.
Candidates memorize formulas without the sign conventions and physical meaning, so they plug numbers in the wrong direction. They also treat the reduced eye as exact rather than as a model with stated assumptions.
What to do instead
Work problems with explicit sign conventions and a sketch showing object, lens, and image positions. Derive vergence and effective power once by hand so the formula becomes meaningful, and check every answer for plausibility (image real or virtual, magnified or minified, myopic or hyperopic correction).
Managing an ocular emergency or medication reaction by continuing the routine examination instead of recognizing the red flags and escalating, for example continuing to check acuity in a patient with sudden painless vision loss or a chemical splash.
Candidates rehearse routine workflows and have little exposure to emergencies, so they default to the familiar sequence. They also underestimate the time-critical nature of chemical injury or acute angle closure.
What to do instead
Build a short red-flag list and rehearse it: sudden vision loss, chemical exposure, penetrating injury, acute pain with halos and a fixed mid-dilated pupil, and new flashes or a curtain across the field. For each, know the immediate action (irrigate, do not press, call the clinician) before any routine testing.