Further validation of comfortable print size as a parameter for clinical low vision assessment

Purpose Comfortable Print Size (CfPS) has been proposed as a clinical alternative to deriving Critical Print Size (CPS) in the assessment of reading function of vision impaired patients. This study aimed to assess the repeatability of CfPS, and to compare assessment duration and values to CPS measures and acuity reserves. Methods Thirty-four adults with vision impairment had their reading function assessed. Two assessments of CfPS were made by asking ‘what is the smallest print size that you would find comfortable using?’ Reading parameters including CPS were determined using the MNREAD card chart and MNREAD app. Results CfPS was quicker to assess (mean 144 (SD 77) sec) than the MNREAD card (mean 231 (SD 177) sec) or app (mean 285 (SD 43) sec). Within-session repeatability of CfPS showed no significant bias or variation across the functional range, and limits of agreement (LoA) of ±0.09logMAR. CfPS values were 0.10logMAR bigger than card CPS values, but no different from app CPS values, with LoA of ±0.43-0.45logMAR. Acuity reserve (comparing CfPS to card reading acuity) was 1.9:1 on average, with a maximum of 5.0:1. Conclusions Comfortable Print Size (CfPS) offers a quick, repeatable and individualised clinical measure of the print size required for sustained reading that reflects CPS values obtained by more traditional measures. Translational relevance CfPS is an appropriate clinical measure of reading function to use in determining the magnification requirements of vision impaired patients for sustained reading tasks.