Accuracy of cervical cytology: comparison of diagnoses of 100 Pap smears read by four pathologists at three hospitals in Norway

BACKGROUND Cervical cancer can be prevented by early detection and treatment for precancerous lesions. Since 1995, there has been a national cervical cancer screening program in Norway, where women aged 25-69 years are recommended to take Pap smears every three years. There are 17 cytology laboratories covering a population of 5 million people. The detection rate of cervical abnormalities varies from laboratory to laboratory. We wanted to investigate the accuracy of cytology diagnoses by four different pathologists at three different hospitals in Norway. METHODS One hundred Pap smears (20 Normal, 20 ASC-US, 20 LSIL, 20 ASC-H and 20 HSIL) screened at UNN in 2015 were evaluated by four pathologists at three hospitals in Norway. All patients were followed up through December 2016. Histologically confirmed high-grade dysplasia (CIN2+) was considered as study endpoint. RESULTS The number of Pap smears evaluated as abnormal (ASC-US+) by the four pathologists varied from 61 to 85. The number of high-grade cytology (ASC-H+) varied from 26 to 50. There was moderate agreement (weighted kappa 0.45-0.58) between the observers. There were 32 women with high-grade histology (CIN2+) in the follow-up, including 19 CIN2, 12 CIN3 and one squamous cell carcinoma (SCC). Using high-grade cytology (ASC-H+) as cut-off, the sensitivity for CIN2+ varied from 68.8% to 93.8% (mean 77.4%) and specificity from 70.6% to 95.6% (mean 81.3%). The pathologist with the highest sensitivity for CIN2+ had the highest false positive rate and the lowest specificity (p<0.05). The accuracy for CIN2+ varied from 74.1% to 83.8% (mean 79.4%). The Pap smear from the woman with cervical cancer was diagnosed as high-grade (ASC-H+) by one of the four pathologists. CONCLUSIONS Cervical cancer screening based on cytology has limited accuracy. The study revealed a moderate agreement between the observers, along with a trade-off between sensitivity and specificity. This might indicate that hospitals with high detection rates of cervical cytology have higher sensitivity for CIN2+ but lower specificity.