What is the maximum number of incorrect answers a patient can have on a Snellen (Standard Visual Acuity) chart?

Maximum Number of Incorrect Answers on a Snellen Chart

The majority of optotypes (letters/symbols) must be correctly identified to "pass" a line on the Snellen chart, meaning a patient can miss less than half of the optotypes on any given line. 1

Understanding Snellen Chart Testing Standards

The Snellen chart is a standard tool for measuring visual acuity that follows specific protocols:

• Optotypes should be presented in a full line of five whenever possible 1
• To "pass" a line, the majority of optotypes must be correctly identified 1
• For children ages 3-5 years, the referral criteria states "fewer than four of six correct on 20-foot line" constitutes a failure 1

This means:

• On a standard line with 5 optotypes, a patient can miss at most 2 letters (must get at least 3 correct)
• On a line with 6 optotypes (as used in pediatric testing), a patient can miss at most 2 letters (must get at least 4 correct)

Proper Testing Technique

For accurate Snellen chart testing:

• Testing should be performed monocularly with best refractive correction in place 1
• The non-tested eye should be covered with an adhesive occluder patch or held by the examiner 1
• The examiner must ensure the patient cannot peek with the non-tested eye 1
• A full line of optotypes is preferred over single optotypes to account for the "crowding phenomenon" 1
• Individual optotypes should not be covered or masked when testing a line 1

Chart Design Considerations

The design of the Snellen chart affects testing accuracy:

• Standard Snellen charts are less desirable than logMAR-based charts (like ETDRS) because:
  • Individual letters are not of equal legibility 1
  • Letter spacing does not always meet World Health Organization standards 1
  • The chart design is typically not standardized 1

Clinical Implications

Understanding the maximum allowable incorrect answers is important because:

• Visual acuity measurements can vary significantly between Snellen and more standardized ETDRS charts, with differences of up to 3 lines in patients with poor vision 2
• Clinical Snellen VA is typically 1-2 lines worse than ETDRS protocol testing 3
• These differences can affect clinical decision-making, especially in patients with macular degeneration or other vision-threatening conditions 2

Common Pitfalls to Avoid

• Using single optotypes can overestimate visual acuity in patients with amblyopia due to the absence of the crowding phenomenon 1
• Not using proper occlusion techniques can lead to inaccurate results if patients peek with the non-tested eye 1
• Snellen acuity alone is a poor predictor of functional vision tasks such as the ability to read a license plate at the distance required for driving 4

Remember that the maximum number of incorrect answers depends on the total number of optotypes per line, but generally, a patient must correctly identify the majority of optotypes on a line to be considered as having passed that line of visual acuity.