In Reply.—
We agree with Drs Fusaro and Johnson that a positive phototest (ie, increased tolerance to xenon arc light) as performed in our reported study indicates only that the patient so tested has increased tolerance to the light we used and does not disclose the magnitude of increased tolerance to natural sunlight. Certainly an increase in tolerance to light is indicative of the therapeutic efficacy of beta carotene in EPP; a quantitative assessment of the amount of increase is not inherent in the definition of efficacy. Implicit in their present letter and also in their earlier communication published in the Archives (1975;111:394-395) was the suggestion that we wished to show a quantitative correlation between the results of phototestig and the patients' subjective assessments of tolerance to natural sunlight exposure. We have never claimed or implied such a correlation. In fact, we noted in our first report of phototesting that