Visual acuity tests are one of the most fundamental components of ophthalmic clinical practice for disease diagnosis and observation of therapeutic effects. The visual acuity test is conducive for early oculopathy detection and timely treatment, reducing social and medical burdens. The recent increase in use of smartphone platforms has facilitated the popularization of vision screening and refractive error identifying in children [10, 11]. The VA self-test can be theoretically carried out anywhere and is faster, safer, and more efficient than conventional tests conducted in hospitals, thus making it especially helpful during the COVID-19 pandemic. However, there are currently few verified VA self-testing programs and even fewer programs that can be used for both distance and near VA testing.
This study found good correlation and agreement among four VA tests between the mobile applet-based VA self-test and the conventional method, with LoAs ranging from − 0.26 to 0.18 (Fig. 4). A study testing UDVA with an iPhone SE at 1.2 m and the Snellen chart or ETDRS chart showed differences of 0.04 LogMAR (LoAs: -0.06, 0.13 LogMAR) and 0.02 LogMAR (LoAs: -0.06, 0.096) [12], while the differences were 0.100 to -0.010 LogMAR (LoAs: -0.400, 0.273 LogMAR) in another study comparing a different program run on iPhone 7 (or 7 plus) and the ETDRS chart [4]. A different study using an Android tablet reported the mean difference was less than 0.1 LogMAR (LoAs: ± 0.3 LogMAR) [13]. Our findings were consistent with those of previous studies, suggesting the feasibility and practical value of mobile devices for performing different kinds of VA tests.
This study provides evidence of good agreement between the VA self-test and the conventional method in different age groups for participants aged 3–40 years. In contrast, a study [13] showed a significant difference between smartphone programs and conventional methods for UNVA tests in adults. Another study [12] also showed a significant difference in adult CDVA testing between the testing App and the Snellen charts, with a mean difference of 0.04 ± 0.05 LogMAR. However, no significant difference was found in the four VA tests in the adult group in the current study, with a maximum mean difference of -0.057 ± 0.037 LogMAR. Visual acuity results of the two methods were relatively close in the adult group, indicating a similarly good performance of the mobile VA testing platform. A study in adolescents [4] reported mean differences of -0.010 LogMAR (LoAs: -0.052, 0.032 LogMAR) and 0.092 LogMAR (LoAs: -0.133, -0.051 LogMAR) for UDVA and UNVA, respectively, between automatic testing programs and conventional methods. Meanwhile, the current study found no significant differences in UDVA or UNVA between the methods in the adolescent group. A previous study on children’s UDVA testing [14–16] reported a difference of -0.018 to 0.04 LogMAR, which was in close agreement with our findings. Collectively, the results of the previous and current studies indicate that the mobile applet-based VA self-test has high potential for application in the population aged 3–40 years.
In the adolescent group, the NVA with FS was significantly higher in the conventional VA test than in the VA self-test. Previous studies have suggested deviations in NVA testing utilizing smart devices [17, 18], and this could be attributed to the differences in device parameters, including brightness, contrast ratio, or resolution [4]. Satgunam et al. [13] suggested that a viewing angle of approximately 1.4’ for one pixel at a distance of 40 cm was larger than the minimum resolution angle of the human eye (1’) and may lead to image distortion, thereby underestimating VA in the younger population with relatively good vision. Similarly, Phung et al. [19] suggested that the difference in NVA between various VA testing methods was greater in subjects with good VA.
Another explanation may be the visual crowding phenomenon [20] because of the different ways of optotype presentation: the smartphone-based testing program presented fewer or only one optotype at a time [17]. In the current study, the pixel density of the conventional VA testing light box was lower than that of the smartphone (92 vs. 376 ppi), and thus, the display of optotypes’ edges may be more delicate in the smartphone. In addition, the highest brightness of the smartphone screen was greater than that of the VA light box (425 cd/m2 vs. 250 cd/m2), and it was easier to distinguish optotypes. Some studies have shown a slight underestimation of NVA with FS [4, 13, 19] or no significant difference [21]. In addition, the discrepancy in the initial size of the optotypes and settings of the testing endpoints may also be a contributing factor to the difference in results. Further investigations on the initial size of optotypes are warranted to clarify their impact on the testing outcomes. However, the overall mean difference was relatively small (0.045 LogMAR), indicating that the VA self-test is a feasible, efficient, and reliable VA testing tool [21].
Multiple devices were used in previous studies, such as a mobile phone [22]; computer or tablet[15]; and a computer screen to display and a mobile phone to input [14, 23]. With respect to the testing distance, a flexible tape and marker pen were required to determine the distance in this study. Birch et al. [15] provided a 3-meter-long string to determine the test distance. Thirunavukarasu et al. [23] introduced a novel automated distance calibration system using the camera of a second device to determine the measurement distance, which could automatically adjust the size of the optotypes within a certain range (1.5–2.5 m) according to the actual distance. This study used only a mobile phone as the display and input device. For home-based testing, a one-time mark is adequate for long-term measurements. The future development of automatic distance measurements is of high practical value for applications of the VA self-test under different circumstances.
The VA self-test utilizes the WeChat mini-program (also called applet) platform; therefore, patients only need to search for the program in WeChat to load and use it immediately, without additional installment. WeChat is an instant messaging service worldwide, with more than 1 billion users, and the number of daily active users of WeChat mini-programs reached 450 million in 2021 [24]. The COVID-19 pandemic has overwhelmed the healthcare systems [25], and ophthalmic health issues associated with home confinement or online learning have increased [26, 27]. The mobile VA self-test is easy to operate, with no need for professional guidance, in favor of its rapid promotion and application.
This study had some limitations. First, this study did not include different devices for the VA self-test, but differences in the brightness, contrast ratio, and resolution of screens may have an impact on the test results, especially the NVA test [13]. Future research comparing different devices for the VA self-test may help to further distinguish the different testing outcomes. Second, subjects were required to maintain a correct measuring distance; therefore, a home-based or fixed setting was recommended to ensure the best accuracy.
In conclusion, there is good agreement between the mobile applet-based VA self-test and the conventional VA tests. The VA self-test has good practical value and important social significance in the COVID-19 era owing to its usefulness for self-screening of visual acuity, myopia control, and remote management for visual impairment in oculopathy.