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
McIntyre L, LaRoche GR. Clinical Evaluation of a Photorefractor. Arch Ophthalmol. 1988;106(8):1031. doi:10.1001/archopht.1988.01060140183007