Understanding Snellen Chart OD/OS Notation
OD stands for "Oculus Dextrus" (right eye) and OS stands for "Oculus Sinister" (left eye) - these are standard Latin abbreviations used in ophthalmology to document which eye's visual acuity is being measured on the Snellen chart. 1
Standard Ophthalmic Abbreviations
- OD (Oculus Dextrus): Refers to the right eye measurement 1
- OS (Oculus Sinister): Refers to the left eye measurement 1
- OU (Oculus Uterque): Both eyes together (though not specifically mentioned in your question) 1
Clinical Context of Snellen Chart Testing
Visual acuity testing should be performed monocularly (one eye at a time) with the fellow eye properly occluded using an adhesive patch or tape to prevent peeking. 2
Important Testing Considerations
- The Snellen chart assesses both binocular and monocular visual acuity by having patients read progressively smaller letters at a standardized distance 1
- The Snellen chart is actually less desirable than modern alternatives because individual letters are not of equal legibility and spacing does not meet World Health Organization standards 2
- For older children and adults, Sloan letters with logMAR size progression (as used in ETDRS charts) are preferred over traditional Snellen charts 2
Common Clinical Pitfall
Snellen measurements typically yield worse visual acuity readings compared to ETDRS charts, particularly in patients with poor vision - for example, 20/200 on Snellen may actually be 20/95 on ETDRS (more than 3 lines difference). 3 This discrepancy is important when comparing clinical practice measurements (typically Snellen) with clinical trial outcomes (typically ETDRS). 3
Proper Documentation Format
When recording visual acuity, the format is typically written as:
- VA OD: 20/20 (right eye vision)
- VA OS: 20/40 (left eye vision)
This standardized notation ensures clear communication among healthcare providers about which eye has which level of visual function. 1