From the Mallory Institute of Pathology and the Department of Obstetrics and Gynecology, Boston University Medical Center, Boston, Massachusetts, U.S.A. Dr. O’Brien is Professor, Mallory Institute of Pathology. Dr. de las Morenas is Director of Cytology and Associate Clinical Professor, Mallory Institute of Pathology. Dr. Soto-Wright is Associate Director of Gynecologic Oncology, Department of Obstetrics and Gynecology. Dr. Mulligan is Resident in Anatomic Pathology. Address reprint requests to: Antonio de las Morenas, M.D., M.I.A.C., Mallory Institute of Pathology, 784 Massachusetts Avenue, Boston, Massachusetts 02118. Received for publication June 24, 1996. Accepted for publication August 25, 1997. 928 Acta Cytologica 0001-5547/98/4204-0928/$06.00/0 © The International Academy of Cytology Acta Cytologica OBJECTIVE: To demonstrate empirically that the effi- ciency of rescreening to discover false negative cytologic diagnoses is greatly en- hanced by prospectively stratifying accessions ac- cording to risk level. STUDY DESIGN: We strat- ified accessions from 11 clin- ical sources and established the rate of diagnoses accord- ing to three categories: (1) “within normal limits”/ “benign cellular changes” (WNL/BCC), (2) “atypical squamous/glandular cells of undetermined significance” (ASCUS/AGCUS) and (3) “squamous intraepithelial le- sion/invasive carcinoma” (SIL/CA). We then prospec- tively rescreened all negative smears from sources with rates of positive diagnoses (ASCUS/AGCUS and SIL/CA) in excess of 20% and 5% of negative smears from sources with rates of positive diagnoses < 20%. We compared the detection rates of false negatives on re- screening target groups with random rescreening of 10% of all negative smears. RESULTS: The rates of SIL/CA, ASCUS/AGCUS and WNL/BCC varied from 0 to 43%, 4% to 14% and 46% to 94%, respectively. Rescreening 10% of all negative smears revealed a false negative fraction of 3%; rescreening target groups revealed a false negative fraction of 5.9%. CONCLUSION: The yield of prospectively detected false negative diagnoses was sig- nificantly increased by tar- geting high-risk accession groups. When cytology labo- ratories serve diverse popula- tions, stratifying accessions by risk to permit increased sampling from the propor- tionately higher risk categor- ies is a simple and effective device to maximize the yield and benefit from rescreen- ing. (Acta Cytol 1998;42:928–932) Keywords: quality assurance, health care; mass screening; cervix neoplasms. Cervical cytology has assumed a major role in the prevention and early diagnosis of carcinoma of the cervix over the last 30 years. The ability of this test to reliably detect precancerous lesions of the cervix is central to its success. There is no doubt that the decline in mortality due to cervical carcinoma over the last 20 years can be attributed, for the most part, Percentages of Cervical Cytologic Diagnoses as a Quality Assurance Method Niall J. Mulligan, M.D., Antonio de las Morenas, M.D., M.I.A.C., Valena Soto-Wright, M.D., and Michael J. O’Brien, M.D. We demonstrated that there is a wide variation in the percentages of SIL/CA, ASCUS/AGCUS and WNL/BCC between different clinical sources within our patient population.