COA common mistakes
COA · Understand the traps and how to avoid them.
When converting a spectacle prescription to a contact lens power, the candidate applies the vertex-distance formula in the wrong direction or forgets to apply it at all for powers above about ±4.00 D.
The formula is memorized as a string of symbols without a physical picture of where the lens sits relative to the cornea, so the sign convention and the 'which distance is which' step get reversed under time pressure.
What to do instead
Before calculating, state aloud: the phoropter sits ~12-13 mm in front of the cornea; a minus lens moved closer to the eye becomes less minus, a plus lens becomes more plus. Work three practice problems in each direction and sanity-check that the contact lens power is closer to zero than the spectacle power for both signs.
The candidate reads a prescription written in plus-cylinder form and reports or transcribes the axis 90 degrees off, or changes the sign of the cylinder without transposing the axis.
Transposition is done as a rote rule ('add 90, flip the sign') but the candidate loses track of which meridian the axis refers to, and axes near 0/180 invite arithmetic slips.
What to do instead
Always transpose in three explicit steps written out: new sphere = old sphere + old cylinder; new cylinder = old cylinder with sign flipped; new axis = old axis ± 90 (subtract if >90, add if ≤90). Verify by confirming the two forms describe the same two principal meridians.
When questioned on refractive error or accommodation, the candidate describes the cornea as the main refracting element but then attributes the eye's ability to change focus to corneal curvature change, or misstates which structure produces aqueous humor versus which focuses light.
Basic optics and basic anatomy are studied as separate blocks, so the candidate never links structure to optical function and defaults to a vague 'the front of the eye does it' answer.
What to do instead
Build a one-page structure-to-function map: cornea = fixed ~two-thirds of refractive power; crystalline lens = variable power via ciliary muscle and zonules; aqueous produced by ciliary epithelium, drained at the angle. Recite it before each practice set until the pairing is automatic.
The candidate describes the pupil light reflex pathway or the drainage angle in the wrong order, for example sending the efferent signal from the optic nerve directly to the iris without the pretectal nucleus and Edinger-Westphal relay, or placing Schlemm's canal before the trabecular meshwork.
Pathways are memorized as lists of names rather than as directional flows, so sequence errors survive rehearsal because the candidate never tests the order.
What to do instead
Draw each pathway as arrows from start to finish (light → retina → optic nerve → chiasm → optic tract → pretectal nucleus → bilateral Edinger-Westphal → ciliary ganglion → sphincter pupillae) and trace the aqueous route (ciliary body → posterior chamber → pupil → anterior chamber → trabecular meshwork → Schlemm's canal → collector channels). Redraw from memory twice weekly.
The candidate confuses the intended action or key caution of common ophthalmic drugs, such as mixing up a mydriatic with a cycloplegic, or stating that a topical anesthetic is appropriate for take-home pain relief.
Drugs are learned by name only, without the receptor, the clinical purpose, or the specific warning attached to each class, so similar-sounding agents blur together.
What to do instead
Make a drug card for each agent with four fields: class/mechanism, clinical use, key contraindication or caution, and one distinguishing fact (e.g., topical anesthetics are for procedures only, never dispensed for home use because of corneal toxicity). Test yourself by covering the use column.
When describing instillation of eye drops, the candidate omits or misorders critical steps, such as touching the dropper to the eye, instilling into the central cornea, skipping punctal occlusion, or failing to separate two different drops by several minutes.
The technique is recalled as a general idea ('put in a drop') rather than as a checked sequence, and the infection-control and absorption rationale behind each step was never learned.
What to do instead
Memorize a fixed sequence: hand hygiene, check drug/eye/expiry, pull lower lid to form a pocket, instill into the cul-de-sac without touching the tip to any surface, release the lid, apply punctal occlusion, wait about 5 minutes between different drops. Rehearse the sequence verbally in the exact order.
The candidate records visual acuity without specifying the conditions, for example writing '20/40' with no notation of whether it was with or without correction, which eye, or whether pinhole was used, and converts Snellen to logMAR or decimal incorrectly.
Acuity is treated as a single number to obtain rather than a measurement with required parameters, and the candidate has not practiced the notation conventions.
What to do instead
Adopt a fixed recording template (VA OD/OS, sc/ph, pinhole if used, method and distance) and write it out on every practice patient. Practice converting a handful of Snellen lines to logMAR and decimal until the direction of the scale (smaller logMAR = better) is second nature.
During history taking, the candidate asks leading or closed questions, omits the medication and allergy history, or fails to follow up a red-flag symptom such as sudden vision loss, flashes, or pain with a targeted question.
The candidate focuses on completing a form quickly and has not internalized which symptoms change urgency, so the history becomes a checklist rather than a branching inquiry.
What to do instead
Use open questions first ('Tell me what brought you in'), then branch on red flags. Keep a short mandatory list that is always asked: current medications including drops, allergies, ocular and systemic history, and onset/duration of the chief complaint. Practice two mock histories where a red flag is deliberately planted.
The candidate mishandles the equipment or the patient during preparation and assisting, for example failing to calibrate or clean a tonometer between patients, positioning the slit lamp or chair at the wrong height, or not warning the patient before a bright light or a contact procedure.
Attention is on the procedure's result rather than on setup, and the candidate has not rehearsed the pre-procedure routine, so steps are skipped when nervous.
What to do instead
Learn a pre-procedure checklist that runs in the same order every time: identify and consent the patient, explain what they will feel, adjust chair and instrument to the patient, confirm the device is clean and functioning, then proceed. Rehearse the checklist aloud for tonometry, slit-lamp exam, and dilation.
The candidate demonstrates weak infection control or administrative habits: not performing hand hygiene at the correct moments, reusing a contaminated item, or discussing a patient's information in an open area or leaving records unattended.
These behaviours are seen as background formalities rather than as core competencies, so they are not practiced deliberately and lapse under the pressure of a busy room.
What to do instead
Treat hand hygiene and privacy as observable steps with defined moments (before and after patient contact, after contact with ocular secretions or equipment). Practice stating your hand-hygiene point aloud during mock encounters, and adopt a habit of closing or turning away records before any conversation.