Optometric glaucoma referrals--measures of effectiveness and implications for screening strategy

The effectiveness of disease screening is conventionally evaluated using the epidemiological indices of sensitivity and specificity, which measure the association between screening test results and final diagnoses of all the patients screened. The effectiveness of optometric glaucoma referrals cannot be measured using such indices because diagnoses are obtained only on patients who are referred, while the true disease status of those not referred remains unknown. Instead, glaucoma referral effectiveness has been evaluated using measures of 'detection rate'--the proportion of those screened who are correctly referred, and 'referral accuracy'--the proportion of those referred who are correctly referred occurrence. Examination of these operational measures shows that their obtainable values and, hence, their interpretation are influenced by the total proportions of diseased and referred patients, one or both of which will generally be unavailable in evaluating samples of referrals. On the other hand, if valid estimates of these proportions can be obtained from other sources, it is possible to rescale detection rate and referral accuracy to take account of them. This rescaling produces a pair of weighted kappa coefficients, chance-corrected measures of association between referral and diagnosis, which provide a better indication of true referral effectiveness than other measures. An important consequence of this approach is that it provides a clear quantitative illustration of the need for a dual strategy to improve the overall quality of optometric glaucoma screening; widespread adoption of more comprehensive modes of screening to improve accuracy, together with a significant increase in the total numbers of patients screened to improve detection. In order for detection rates to reach desirable levels, the total number of referrals in any sub-population of patients must match or exceed the number of patients with disease. This analysis confirms quantitatively that which is intuitively obvious; not only that glaucoma awareness and uptake of screening opportunities must be encouraged in all patients over 40 years of age, but also that the older and/or more at risk patients are, the greater is their need to take advantage of glaucoma screening.