ETDRS Chart: Visual Acuity, Scoring, Snellen Conversion & Instructions

ETDRS Chart:

• What is ETDRS Chart?
• Visual Acuity
• Scoring
• Snellen Conversion
• Instructions

What is ETDRS Chart?

The ETDRS chart (Early Treatment Diabetic Retinopathy Study chart) is a standardized vision testing tool designed to measure visual acuity with high reliability and repeatability. Unlike traditional eye charts that vary in letter spacing or line progression, the ETDRS format follows strict design rules: equal numbers of optotypes (letters) per line, logarithmic (base-10) size progression of 0.1 logMAR per line, and uniform crowding/spacing between letters and lines. These features reduce testing bias and make results comparable across clinics, studies, and time. The chart usually contains ten Sloan letters (C, D, H, K, N, O, R, S, V, Z) chosen for equal legibility, arranged to avoid easy guessing patterns. ETDRS testing is widely used in clinical research, low-vision assessment, cataract and refractive surgery follow-up, and retina clinics where precise change detection matters.

Historically, clinicians relied on Snellen charts, which are helpful for screening but less precise for tracking small changes. ETDRS introduced a logMAR (logarithm of the minimum angle of resolution) framework that aligns letter size changes with a constant step size. This lets clinicians detect subtle improvements or declines, even just a few letters’ difference, which can be meaningful in disease monitoring or therapy trials. ETDRS charts are typically back-illuminated or displayed on calibrated electronic systems at standardized testing distances (commonly 4 m or 3 m), with room illumination controlled. Because the method is anchored in standardized geometry and scoring rules, an ETDRS result is not just “can read line 6”—it becomes a precise numeric outcome that supports evidence-based decisions, inter-visit comparisons, and research-grade documentation.

Visual Acuity

Visual acuity on ETDRS charts is expressed in logMAR, where 0.0 logMAR corresponds to 20/20 Snellen-equivalent vision at the standard distance. Each line changes by 0.1 logMAR, and each letter is worth 0.02 logMAR (because there are five letters per line). Lower logMAR values indicate better acuity; for example, −0.1 logMAR is better than 0.0 and roughly corresponds to 20/16. ETDRS testing aims to minimize non-visual factors that can artificially inflate or deflate performance. That is why letter spacing, line spacing, and luminance are controlled, and why optotypes with similar legibility are used. With ETDRS, two people tested in different clinics can be compared fairly if both clinics follow the same protocol, which is critical for multi-center clinical trials and for patients who move between providers.

Clinically, ETDRS visual acuity provides a sensitive metric for monitoring change over time. In macular disease, for instance, a five-letter loss may prompt closer follow-up, whereas a 10- to 15-letter gain after treatment can signify meaningful functional improvement. Because letters are scored individually, partial-line performance still yields a precise value; you are not constrained to “whole line” thresholds. Furthermore, ETDRS testing can be adapted for low-vision ranges using shorter testing distances or low-vision variants of the chart, maintaining the logMAR step size. This continuity across acuity ranges makes ETDRS particularly useful in comprehensive eye care where patients may present with anything from better-than-normal vision to severe impairment, yet you still need one consistent yardstick to quantify function.

Scoring

ETDRS scoring uses a letter-by-letter method known as the “total letter score.” The patient starts at an appropriate line (often near expected acuity, determined by screening or prior results) and reads as many letters as possible, line by line, usually from top to bottom. Each correctly identified letter earns one point. Because there are five letters per line, each line contributes up to five points. The raw total letter score is then converted to a logMAR value using standardized formulas that account for starting line and testing distance. A common approach is to begin at a line the patient can confidently read and continue until they can no longer identify most letters; missed letters at the beginning lines are subtracted, and correct letters thereafter are added to derive a total.

The strength of letter scoring is its precision and sensitivity to small changes. For example, a three-letter gain (≈0.06 logMAR) may be clinically relevant in research contexts, while five to ten letters often denote meaningful change in routine care. To standardize, clinicians should ensure consistent instructions (“guess if unsure”), enforce a time limit per letter to avoid excessive prompting, and record both the total letters and the final logMAR. If the patient makes consistent substitution errors (e.g., confusing C and O), note this pattern as it may relate to contrast sensitivity or refractive blur. When retesting, try to use different chart versions or randomized displays to reduce memorization. By applying these scoring rules, you gain a repeatable metric that supports accurate follow-up and evidence-based treatment adjustments.

Snellen Conversion

Although ETDRS/logMAR is preferred for precision, many clinicians and patients still think in Snellen terms (e.g., 20/40). Converting between systems is straightforward conceptually: each 0.1 change in logMAR corresponds to one ETDRS line and roughly a doubling/halving of the minimum angle of resolution. Approximate anchors help: 0.0 logMAR ≈ 20/20, 0.1 ≈ 20/25, 0.2 ≈ 20/32, 0.3 ≈ 20/40, 0.5 ≈ 20/63, 1.0 ≈ 20/200. Because ETDRS uses equal steps, you can interpolate between lines using letters (0.02 logMAR per letter). For instance, 0.24 logMAR is about two letters worse than 0.2, which places it slightly poorer than 20/32 but better than 20/40. Many electronic medical record systems automate this conversion, but understanding the logic helps you interpret borderline changes without overreacting to minor fluctuations.

When reporting acuity in publications or referrals, consider providing both values: the precise logMAR (or letter score) for scientific comparability and the Snellen equivalent for general readability. Note that Snellen fractions vary with test distance and chart calibration, which is one reason ETDRS was adopted for studies. Also, Snellen line steps are not uniform; the jump from 20/25 to 20/20 is not the same as from 20/40 to 20/32. In contrast, the ETDRS/logMAR scale maintains constant steps, making statistical analyses more robust. If you must convert manually, use standard tables or a reliable calculator and round sensibly; do not overinterpret a one-letter (0.02 logMAR) shift as a clinically meaningful change unless corroborated by consistent testing and clinical context such as symptom report, refractive status, or ocular disease activity.

Instructions

To run an ETDRS test correctly, standardization is key. Calibrate test distance (commonly 4 m or 3 m) and ensure chart luminance meets specifications (with backlit cabinets or validated electronic displays). Seat the patient comfortably, position the occluder to test monocularly (right eye then left eye), and confirm they are wearing appropriate correction for the testing distance (trial frame or phoropter with distance Rx). Provide uniform instructions: “Please read the letters from left to right, top to bottom; if you are unsure, please guess.” Start near anticipated acuity to save time, but record all letters correctly identified, including partial lines. Enforce a steady pace—typically a second or two per letter—without coaching on mistakes. Track fixation and blinking; pause if tearing, dryness, or glare interferes.

Maintain consistency between visits: same room setup, similar lighting, equivalent chart version, and identical testing distance. Use alternate ETDRS charts or randomized electronic sequences to avoid memorization. If vision is very reduced, reduce testing distance proportionally (e.g., from 4 m to 1 m) and adjust scoring per protocol to keep the same logMAR steps. Document test conditions (distance, illumination method, chart version, refraction worn) alongside the letter score and logMAR/Snellen equivalent. For pediatric or low-vision patients, allow a brief practice line, use encouragement without cueing letters, and consider crowding bars or low-vision variants. Finally, interpret results in context: correlate with refraction, pinhole response, contrast sensitivity, and ocular findings. A small letter change may reflect measurement noise, while consistent multi-line shifts often indicate true clinical change requiring action.