When you want to estimate a patient’s visual acuity, the Snellen chart is the go to option for many clinicians. Like so many other tools in medicine, a number of mobile apps have attempted to replicate this functionality. Until recently, there was little data regarding the validity of these apps.
Researchers in Australia looked at multiple Snellen chart apps on the iPhone 4 to test the validity of their use. There were two aspects to the study: review of the various Snellen apps available and a comparison between an app and an actual Snellen chart.
To evaluate each Snellen app, researchers measured each optotype (i.e. symbol on the chart) and compared that to the correct size based. Calculation of the correct size for each optotype requires two pieces of data:
- the recommended distance between the iPhone and the patient
- the visual acuity that the optotype is supposed to represent
A percent error was calculated between each optotype and its theoretical correct size. The average error was calculated for each app by averaging the percent error of each line on the chart (i.e. each level of visual acuity).
There was considerable variation between averages ranging from 4.4 to 39.9%. Of the eleven apps tested, eight (72%) had greater than 10% average error. Eye Test by Bokan Technologies performed the best at 4.4%.
For the second part of the study, the researchers compared a real 6 meter Snellen chart with the best performing free app: Snellen by Dr. Bloggs Limited. A single clinician tested the visual acuity of 88 patients using both methods. The data showed no statistically significant difference between the two methods. This shows that as an overall tool, the Snellen app was no worse than the real Snellen chart.
When stratified by visual acuity, the difference increasingly diverged as visual acuity decreased. In other words, the worse the vision was, the less accurate the app was when compared to the real chart.
There are 2 main take aways from this article. The first conclusion is that nearly every Snellen app is an inaccurate substitute for the traditional chart.
Secondly, we cannot even be certain of the validity of the apps proven to be accurate by this study. As we discussed here, the apps are old and not tested on newer devices. In the study, the apps were tested on the iPhone 4. Newer phones have different screen sizes and dimensions. Unless you’re using an iPhone 4, you can’t be certain that your Snellen app is a valid tool in your practice.