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Lynn McIntyre, MD; G. Robert LaRoche, MD, FRCSC
Halifax, Nova Scotia

Arch Ophthalmol. 1988;106(8):1031.

	Since this article does not have an abstract, we have provided the first 150 words of the full text PDF and any section headings.

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.²

If 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 . . . [Full Text PDF of this Article]

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