Is patient identification of 'comfortable' print size a useful clinical parameter for low vision reading assessment?

The purpose of this study was to determine what a person with vision loss considers a 'comfortable' print size to read, and examine whether this reflects any of three currently used parameters for identifying print size required for sustained reading tasks: minimum size to achieve maximum reading speed (the critical print size (CPS)); minimum size for functional reading at 80 wpm and/or a size that is double the reading acuity (representing an acuity reserve of 2:1).

Forty-seven participants entering low vision rehabilitation (mean age 77 years, 24 with macular degeneration) were assessed using MNREAD charts to determine reading acuity, maximum reading speed, CPS and the minimum size allowing functional (80 wpm) reading. Comfortable print size was assessed by asking participants to identify 'the smallest print size that you would find comfortable using' on the MNREAD chart.

There was little difference between comfortable print size and CPS (mean difference 0.05 logMAR (SD 0.18); p = 0.08, limits of agreement ±0.35 logMAR), and no trend for the difference between values to differ across the functional range. Size for functional reading could only be assessed for 41 participants, and the difference between this and comfortable print size varied across the functional range. Comfortable print size was consistently smaller than twice the reading acuity size (mean difference 0.11 logMAR (SD 0.17); p < 0.001), with an average acuity reserve of 1.74:1.

Asking people with visual impairment to identify a print size that is comfortable to read provides a print size similar to the CPS. This can be used as a guide in selecting magnification for sustained reading without having to undertake further analyses. Identification of perceived comfortable print size may offer a time-efficient clinical method of estimating magnification requirements, and be relevant for undertaking effective remote consultations.