Laboratory reliability of the Papanicolaou smear.

Variation in clinical cytology diagnoses was examined at the microscope and in the cytology laboratory as a unit. Results provide preliminary information about the comparability of cytologic diagnosis under specified conditions. In onsite proficiency testing using "normed" smears with a wide range of findings, 814 individual cytotechnologists and cytopathologists from 306 laboratories in 44 states made the target diagnosis in 63.8% of 8929 cases. False negative diagnoses were made in 7.5% of 4520 smears with moderate dysplasia through frank malignancy, and false positive diagnoses were recorded in 8.9% of 3808 smears with no more than benign atypia. Evaluation of a sample of actual work output from two laboratories showed 3.2 to 3.4% false positive and 4.8 to 11.2% false negative diagnoses on the cases rescreened.