COA Certified Ophthalmic Assistant common mistakes
COA Certified Ophthalmic Assistant · Understand the traps and how to avoid them.
When converting a Snellen acuity line to logMAR or to a decimal value, the candidate reads the line number as the acuity (e.g., calling the 20/40 line "line 4") instead of converting from the Snellen fraction itself.
Candidates memorize a few anchor values (20/20 = 1.0 = 0.0 logMAR) but never learn the underlying relationship, so any line that is not an anchor gets guessed. The 20/40 line is line 5 on a standard chart, not line 4, and the arithmetic breaks down under time pressure.
What to do instead
Learn the formula: logMAR = log10(denominator / 20), so 20/40 gives log10(2) = 0.30 and decimal acuity = 20/40 = 0.50. Drill a table for 20/15, 20/20, 20/25, 20/30, 20/40, 20/50, 20/60, 20/70, 20/80, 20/100, 20/200, and practice converting in both directions until the values are automatic.
The candidate records a visual acuity without recording the method: pinhole, correction status, chart type, and distance are omitted, so the result cannot be interpreted or compared to a prior visit.
Candidates treat acuity as a single number to write down rather than a measurement with conditions. They also copy prior values forward without re-testing, which hides a real change.
What to do instead
Use a fixed recording template for every acuity: OD/OS/OU, chart (Snellen, ETDRS), distance (20 ft or 6 m), whether the patient wore correction, and pinhole result if performed. If the patient could not read the top line, record the alternative (count fingers, hand motion, light perception, no light perception) with the distance tested. Never transcribe a prior value as a current one.
The candidate performs Goldmann applanation tonometry without checking that the mires are aligned and properly sized, or without adjusting for corneal thickness and astigmatism, and reports the dial reading as the IOP.
The procedure looks simple, so candidates skip the calibration and alignment steps they were taught. They also do not know that the prism must be rotated for corneas with more than 3 diopters of astigmatism, or that a thin cornea underestimates and a thick cornea overestimates true pressure.
What to do instead
Before each measurement, confirm the tonometer is calibrated, set the dial to 0.5 to 1.5 g, and verify the mires form equal semicircles occupying about half the prism width. Rotate the prism axis to 43 degrees for corneas with more than 3 D of astigmatism. Record the reading and note central corneal thickness when available, because the measured value is not the same as the corrected pressure.
The candidate dilates a patient without first checking the anterior chamber depth, iris anatomy, or a documented history of angle closure, and without asking about symptoms of angle-closure glaucoma.
Dilation is routine in most clinics, so candidates treat it as automatic. They do not connect the pharmacology of mydriatics and cycloplegics to the risk of precipitating acute angle closure in a susceptible eye.
What to do instead
Before instilling a dilating agent, assess anterior chamber depth (van Herick or penlight), ask about prior attacks, and review the record for narrow angles or a documented shallow chamber. Know that phenylephrine and tropicamide can precipitate an attack in a predisposed eye. If the angle is narrow, do not dilate without the physician's direction, and know the signs of an attack: severe pain, halos, nausea, and a mid-dilated fixed pupil.
The candidate calculates or verifies a spectacle prescription without accounting for the vertex distance change when converting between glasses and contact lenses, or transposes a cylinder incorrectly.
Candidates learn the flat transposition rule (axis plus or minus 90, cylinder sign flipped) but forget that sphere power changes with vertex distance above about 4 diopters. They also apply the transposition to the wrong component.
What to do instead
For transposition, keep the sphere and cylinder magnitudes, add the two for the new sphere, flip the cylinder sign, and change the axis by 90 degrees (adding or subtracting to stay within 1 to 180). For vertex distance, use the formula Fc = F / (1 - dF) with d in meters, and remember the change matters mainly above 4 D. Verify with a lensometer rather than trusting a calculation alone.
The candidate describes the layers of the cornea or retina in the wrong order, or confuses the functions of the aqueous and vitreous, when answering anatomy questions or explaining a procedure to a patient.
Anatomy is studied as lists rather than as structures with a purpose. The candidate memorizes names but not sequence, so the endothelium and epithelium get swapped, or the choroid and ciliary body are merged.
What to do instead
Build each structure from the outside in and attach a function to every layer. Cornea: epithelium, Bowman's layer, stroma, Descemet's membrane, endothelium, with the endothelium as the pump that keeps the cornea clear. Retina: photoreceptors, outer nuclear layer, outer plexiform, inner nuclear, inner plexiform, ganglion cell layer, nerve fiber layer. Say the order aloud until it is automatic.
The candidate performs or interprets a visual field test without confirming fixation, reliability indices, or the patient's understanding of the task, and treats a single abnormal field as diagnostic.
Automated perimetry produces a printout that looks authoritative. Candidates read the pattern deviation plot without checking fixation losses, false positives, false negatives, or whether the patient understood the test.
What to do instead
Before interpreting, check reliability: fixation losses, false positive and false negative rates, and whether the test was the patient's first. Explain the task fully, provide a trial run, and repeat an unreliable field rather than acting on it. Correlate any defect with the optic nerve appearance and the clinical picture, and compare with prior fields to establish progression.
The candidate takes a patient history that stops at the chief complaint and omits medications, allergies, family history, and the ocular and systemic conditions that change management.
Candidates treat history as paperwork to complete quickly. They do not see how a medication list (e.g., anticoagulants, antimalarials, alpha blockers) or a family history of glaucoma changes the exam plan.
What to do instead
Use a structured history: chief complaint with onset and duration, ocular history, systemic history, medications including over-the-counter and herbal, allergies with the reaction type, family history, and social history. Ask specifically about conditions that affect the eye, such as diabetes, hypertension, autoimmune disease, and prior ocular surgery or trauma.
The candidate handles, instills, or labels a topical medication without confirming the patient identity, the correct eye, the drug concentration, or the expiry date, and does not separate the bottle tips from the ocular surface.
Instillation becomes reflexive in a busy clinic, and candidates assume the bottle in hand is the one ordered. They also do not know that a bottle tip touching the eye or lashes contaminates the solution.
What to do instead
Follow the rights: right patient (two identifiers), right drug, right concentration, right eye, right time, right route. Check the label against the order and the expiry date before instilling. Do not let the bottle tip touch the eye or lashes. After instilling a dilating or anesthetic drop, document the drug, concentration, time, and eye.
The candidate answers a safety or infection-control question by reciting handwashing in general terms but cannot state the specific steps for cleaning a tonometer prism, disinfecting a trial lens set, or handling a chemical splash in the eye.
Safety content is treated as common sense and skimmed. Candidates do not learn the specific disinfection contact times, the difference between cleaning and disinfection, or the irrigation protocol for a chemical injury.
What to do instead
Learn the concrete protocols: wash hands before and after patient contact; clean a tonometer prism with an alcohol wipe and let it dry before reuse; follow the manufacturer's contact time for disinfecting trial lenses and contact lenses; for a chemical splash, irrigate immediately with saline or water for at least 15 to 30 minutes and check the pH before and after. Know where the eyewash station and SDS sheets are kept.