Analysis of error in calculating the false-negative rate in the interpretation of cervicovaginal smears: the need to review abnormal cases.

Abstract

Determining the false-negative rate (FNR) of cervicovaginal smear interpretation is a necessary step for any quality assessment and improvement program. All tests estimate the FNR, but the accuracy of these estimates varies from test to test. Two methods for determining the FNR have been proposed, specifically "seeding" of the initial screening population with smears from patients with a known diagnosis and rescreening a random sample of negative smears. However, the accuracy of neither method is known. A review of the literature, an analysis of the sources of error, and an estimate of their magnitude was performed for each method. Seeding has a large sampling error, and more important, the FNR that this test measures does not reflect the FNR of the laboratory as a whole. Random rescreening underestimates the FNR of primary screening by the FNR of rescreening. Currently, the FNR of rescreening is not known, not measured, and may be high. Nevertheless, the FNR of rescreening and the false-positive rate (FPR) of initial screening both can be measured by rescreening abnormal cases. Knowledge of both the FNR and the FPR of initial screening allows the efficiency of cervicovaginal smear interpretation to be measured, which may be a better measure of overall accuracy than the FNR alone. Random, blinded rescreening of normal and abnormal smears can more accurately measure the FNR of screening than rescreening of normal smears alone.