On the statistical reliability of letter-chart visual acuity measurements

Area of Science:

• Ophthalmology
• Optometry
• Vision Science

Background:

• Accurate visual acuity measurement is crucial for clinical decision-making.
• The Lighthouse/ETDRS chart is widely used, but its reliability and sensitivity to change require rigorous evaluation.
• Understanding the statistical accuracy of optotype chart testing is essential for interpreting results.

Purpose of the Study:

• To establish boundary values for test-retest reliability and sensitivity to acuity change for the Lighthouse/ETDRS visual acuity chart.
• To evaluate three different scoring methods for the chart.
• To discuss methodological issues in optotype chart testing and link reliability measures to sensitivity to change.

Main Methods:

• Utilized a computer-controlled acuity testing system to minimize measurement error.
• Employed five highly practiced subjects who underwent 156 chart readings.
• Generated 78 test-retest comparisons to assess reliability and sensitivity.

Main Results:

• With 95% confidence, visual acuity can be determined within +/- 0.1 log units under optimal conditions using the recommended letter-by-letter scoring.
• Detecting a significant change in visual acuity requires approximately +/- 0.14 log units for the same confidence level.
• These findings represent near the upper limit of the chart's reliability.

Conclusions:

• The established reliability values approach the maximum achievable with this letter chart.
• Actual clinical reliability is likely lower than the determined boundary values.
• These results highlight the importance of considering measurement error and variability in clinical practice.