Review of Penner's criteria for proficiency testing in cytopathology: implications for the redesign of the federally mandated proficiency testing in cytopathology.

Thirty-three years ago Penner advocated six criteria for the performance of proficiency testing in cytopathology (PTC). Since that time, several further requirements have been added by other authors. The present article critically evaluates and modifies the original criteria and adds two more principles, validity and reliability, that we recognize as crucially important in the performance of PTC. The revised criteria should be taken into consideration in the planned technical redesign of the nationwide PTC.