Clinical Evaluation of a Photorefractor

To the Editor.  —Morgan and Johnson's¹ clinical evaluation of a commercial photorefractor in the November 1987 Archives illustrates the problems associated with evaluating a screening instrument in a tertiary care setting. Their subjects were highly selected children who were scheduled to undergo complete eye examinations. In fact, among the 57 patients who had analyzable photographs, 34 (59.6%) were confirmed to have an abnormal ocular status. This prior probability of an abnormal result is more than ten times higher than the prior probability of 5% for abnormal ocular status reported in a general population of children.²If we accept Morgan and Johnson's results for the sensitivity of photorefraction as 91% (4.55 to 5.0) and its specificity as 74% (70.3 to 95.0), values that should be unchanged regardless of the prevalence of disease in the population, then the positive predictive value of the test in the general population (calculated from