Vistech VCTS 6500 charts--within- and between-session reliability.

The aim of the study was to measure the reliability of the Vistech VCTS 6500 charts, in test score units, in order to allow clinicians to derive estimates of what constitutes a clinically meaningful change in performance over time. The reliability of a more familiar test, Bailey-Lovie high contrast visual acuity, was also measured to provide a comparison. Patients with normal vision and with early or subtle eye disease were recruited so that the results would be representative of the population likely to present for primary vision screening. Patients were tested on all three VCTS charts on two separate occasions at least 3 weeks apart to give estimates of within- and between-session reliability. Reliability was found to be low in all circumstances; between-session reliability could be improved by using the mean score for the three charts, but the 95% range of difference scores still encompassed at least one-half of the total performance range of the test. It was concluded that Vistech charts are unlikely to be of use for clinical measurements or for research studies.