A team of researchers based at the School of Optometry & Vision Science at the University of Waterloo, Canada, have introduced and validated theWaterloo Differential Acuity Test (WatDAT) – a tool for measuring recognition visual acuity (VA) in children as young as 18 months. The study offers data on testability and early normative values, potentially reshaping how clinicians detect amblyopia and other early visual deficits.
“In young children under three years old… it is not [currently] possible to measure VA in a way that is equivalent to letter visual acuity, as measured in adults,” explains Susan J. Leat, professor emerita at the School of Optometry and Vision Science and lead author of the study. “Since this type of visual acuity (called recognition visual acuity) is accurate and sensitive for detecting and monitoring eye disorders, we wanted to develop a test which would enable us to measure recognition VA as young as possible.”
“In young children under three years old… it is not [currently] possible to measure VA in a way that is equivalent to letter visual acuity, as measured in adults,” explains Susan J. Leat, professor emerita at the School of Optometry and Vision Science and lead author of the study. “Since this type of visual acuity (called recognition visual acuity) is accurate and sensitive for detecting and monitoring eye disorders, we wanted to develop a test which would enable us to measure recognition VA as young as possible.”
Bridging a diagnostic gap
Traditional recognition acuity tests — such as Kay Pictures or Patti Pics — typically require naming or matching, skills that develop around age three. Before then, clinicians rely on preferential-looking tests (e.g., Cardiff Acuity cards), which measure resolution visual acuity rather than recognition visual acuity and tend to overestimate visual function. WatDAT was designed to fill this gap by using an “odd-one-out” differential task that even toddlers of 18 months old and over could perform.
Children are shown four images (three identical distractors and one different target — a house among circles or a face among non-faces) and asked to identify the odd one out. This enables assessment of recognition VA without requiring the necessary language or matching skills of children to articulate their choice to the examiner.
Superior testability in younger children
The Waterloo team tested 57 children aged between 18 and 48 months, dividing them into “typical” and “atypical” developmental groups. They compared WatDAT’s performance with Patti Pics and Kay Pictures, two widely used commercial tests.
The results showed:
96% of typical children successfully completed binocular testing with the WatDAT House/Circle (HC) version, compared with 75% for Patti Pics and 80% for Kay Pictures.
Even monocular testing was feasible in more than half of participants — 56% for WatDAT HC and 54% for WatDAT Faces.
82% of children with ocular or systemic conditions (in the “atypical” group) were able to complete WatDAT binocularly.
These findings suggest WatDAT can reliably capture recognition VA at younger ages than current tools allow, helping clinicians to spot developmental or disease-related visual deficits earlier.
WatDAT produced VA values of around 0.21–0.27 logMAR in 18-48 month olds, which is expected as childrens’ VA is not fully developed at this age. In earlier adult validation of the WatDAT against ETDRS, WatDAT gave similar values to ETDRS VA and the differences between WatDAT, Patti Pics, and Kay were consistent in both studies. Also, WatDAT maintained excellent reliability, with over 90% of sessions in children rated as dependable.
Average testing time, including training, was roughly four minutes, comparable to existing methods. To keep the child engaged during this time, WatDAT’s digital version includes a number of animated rewards throughout the process. “This helps to engage and motivate them, and it makes the whole test into more of a game,” says Leat.
WatDAT produced VA values of around 0.21–0.27 logMAR in 18-48 month olds, which is expected as childrens’ VA is not fully developed at this age. In earlier adult validation of the WatDAT against ETDRS, WatDAT gave similar values to ETDRS VA and the differences between WatDAT, Patti Pics, and Kay were consistent in both studies. Also, WatDAT maintained excellent reliability, with over 90% of sessions in children rated as dependable.
Average testing time, including training, was roughly four minutes, comparable to existing methods. To keep the child engaged during this time, WatDAT’s digital version includes a number of animated rewards throughout the process. “This helps to engage and motivate them, and it makes the whole test into more of a game,” says Leat.
Clinical and developmental implications
The study authors emphasize that recognition acuity is a more sensitive indicator of visual deficits than resolution-based methods, and so should be used when possible to measure VA in young children. The WatDAT’s ability to measure it in children under three years old represents a significant advance in pediatric screening.
The tool may be especially useful in screening for amblyopia, congenital cataracts, or high refractive errors — conditions where early detection is key to preventing long-term visual impairment.
In terms of commercialization, WatDAT prototypes are now being evaluated in various practices and clinics across Canada, as well as in the US and UK. “[We] are evaluating the test regarding how it works in an everyday clinic setting, and so far we have had good feedback,” Leat says. “If and when it becomes commercialized, optometrists and ophthalmologists who are involved in pediatric eye care could purchase it and use it for measuring VA in children between 18-36 months and upwards. This would aid clinicians in detecting possible eye deficits, deciding when to start correction/treatment of those deficits and monitoring that treatment.”
As pediatric ophthalmology moves toward earlier, behaviorally adaptable testing, the WatDAT could transform vision assessment in toddlers — bringing accurate, developmentally appropriate acuity measurement into everyday practice. Leat adds: “We hope that since the WatDAT will be an additional tool for pediatric eye care providers, it will improve the quality of eye care for young children and encourage more optometrists and ophthalmologists to provide eye care for this population.”
