Decimalization of The Oxford Clinical Cataract Classification and Grading System.
BACKGROUND: A secure methodology for the classification of cataracts into subtypes and for their separate quantification forms a fundamental underpinning of cataract research. The Oxford Clinical Cataract Classification and Grading System provides for this need across a wide range of cataract subtypes. Consideration of the advantages of finer scale intervals in terms of both increased precision and increased sensitivity to change (responsiveness) has stimulated the development of a decimal version of the Oxford system. AIM: To describe rules for the decimalization of the Oxford system and to document the performance following decimalization. METHOD: Theoretical considerations followed by iterative piloting were used to define a set of rules for the decimalization of grading for 10 cataract features. The performance of the decimal version was then formally tested by means of inter- and intra-observer comparisons of repeated measurements. 217 paired observations were pooled to produce a statement relevant to the 'multi-user' environment typical of many clinical research programmes. RESULTS: Repeatability indices were good to excellent for most features. The use of finer scale intervals improved the system's ability to detect change (reduced 95% tolerance limits for change) by a factor of around 2 for most features. CONCLUSION: The finer scale intervals provided by decimalization of the Oxford system have produced substantial improvements in reliability as evidenced by high levels of repeatability and scale sensitivity. These improvements provide practical advantages in clinical cataract research.