Page 18 - Screening for Cervical Cancer: Systematic Evidence Review

Chapter I. Introduction

1996 and June 1999, respectively. Specimens are collected in the same fashion as those for

conventional cytology; however, rather than smearing the sample onto slide(s), the sample is

suspended in the fixative by stirring the specimen collection spatula and brush in the fixative

solution. The container is sealed and the specimen sent to the cytology laboratory in solution

rather than on slides; this theoretically improves the probability of transferring a representative

sample of cells to the slide. In the laboratory, technicians disperse the sample in the fixative and

then collect the cells on a filter and transfer them to a microscope slide in a monolayer.

Immediate fixation and uniform spread of the cells are designed to reduce detection errors by

assuring that cells are well preserved, not obscured, and more easily assessed by cytology

technicians.

Both conventionally prepared and thin layer specimens read by cytotechnologists are

subject to random manual rescreening of slides that were interpreted as normal at a minimum

rescreening rate of 10%, as required by the Clinical Laboratories Improvement Amendments

(CLIA) of 1988 (Final Rule, Federal Register, 1992).

Computerized rescreening is designed to automate rescreening of Pap smears initially

read as negative by a cytotechnologist. PapNet (TriPath Imaging, Inc.) uses neural-network

technology to interpret computerized images of the Pap slide. The system, approved by the FDA

for rescreening use in December 1995, identifies cells or other material on negative Pap slides

that require review and creates a summary display of up to 128 images that may contain

abnormalities. A cytotechnologist then reviews the summary images and can also return to the

original slide using light microscopy. Several countries allow use of PapNet as a primary

screening as well as a rescreening technology, and the literature contains studies of both forms of

use.

8