Editor,—Referral accuracy is an important measure of primary care effectiveness. It is defined as the proportion of patients referred for a particular condition who are subsequently diagnosed as having that condition (that is, the true positive proportion). Statistically, it estimates the probability that a patient who is referred will have the disease (positive predictive value) and, as with all statistical estimates, the value calculated in any sample will be subject to error, the magnitude of which decreases as the sample size increases. In recent years there has been an increase in the number of publications on the accuracy of referrals by optometrists to ophthalmology clinics. This letter has been prompted by reading some of those concerned with referrals for suspected glaucoma,1-12 but the …