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January 2002
Feature Story

Liquid-based Pap testing benefits becoming clear

R. Marshall Austin, MD, PhD

Several recent articles report on the accumulating data on intended-use direct-to-vial Pap testing by liquid-based cytology. They provide additional evidence that new liquid-based Pap testing methods offer significant sampling advantages over the conventional method.

The largest accumulated experience is with Cytyc’s ThinPrep Pap test, FDA-approved since 1996. A meta-analysis of available data was reported in the August issue of the American Journal of Obstetrics and Gynecology (Bernstein SJ, Sanchez-Ramos L, Ndubishi B. Liquid-based cervical cytologic smear study and conventional Papanicolaou smears: a metaanalysis of prospective studies comparing cytologic diagnosis and sample adequacy. Am J Obstet Gynecol. 2001;185:308-317). The most significant observation of the study was the report that direct-to-vial peer-reviewed publications on ThinPrep reported a 0.71 percent incidence of HSIL lesions in 154,380 patients, more than double the 0.30 percent rate of HSIL lesions reported in the same studies from 311,175 conventional Pap smears. The studies also reported a 3.94 percent incidence of ASCUS with ThinPrep, compared with a 3.28 percent incidence of ASCUS with conventional smears. Also, 87 percent of 127,537 direct-to-vial ThinPreps were satisfactory for evaluation versus 76 percent of 273,398 reported conventional smears.

Two additional studies report the first available direct-to-vial intended-use data on TriPath Imaging's AutoCyte Prep, an alternative liquid-based method approved by the FDA for Pap testing in 1999. Bill Tench, of Escondido, Calif., reported the first intended-use AutoCyte Prep data in the Journal of Reproductive Medicine (Preliminary assessment of the AutoCyte Prep direct-to-vial performance. J Reprod Med. 2000;45:912- 916). In 2,731 year 2000 AutoCyte Prep cases, Tench reported 0.67 percent HSIL interpretations, compared with 0.46 percent HSIL interpretations in 10,367 1999 conventional Pap smear cases. AutoCyte Prep cases had a 5.5 percent 2000 ASCUS rate compared with a 3.75 percent ASCUS rate for 1999 conventional Pap smear historical controls, and the AutoCyte Prep unsatisfactory rate declined to 0.40 percent from 2.94 percent for conventional smears.

The second intended-use AutoCyte Prep report appeared in the same journal not long after the Tench study (Merino JF, Fremont-Smith M. Direct-to-vial experience with AutoCyte Prep in a small New England regional cytology practice. J Reprod Med. 2001;46:353-358). A total of 15,534 year 2000 AutoCyte Prep cases had a 0.82 percent HSIL rate, compared with a 0.53 percent rate for 35,496 1999 historical control conventional Pap smear cases. AutoCyte Prep ASCUS cases declined to 1.33 percent compared with 1.62 percent for 1999 conventional Pap smear cases, and unsatisfactory year 2000 AutoCyte Prep cases declined to 0.10 percent from 0.26 percent for 1999 smear cases.

Studies with biopsy confirmation of the Pap interpretations are starting to emerge also and suggest the increased detection of clinically significant HSIL lesions is real. Users will have to assess in their individual practice settings such issues as efficacy for detecting clinically significant target lesions (HSIL+), cost of equipment and supplies and ease of use in the laboratory, relative liquid-based Pap reimbursement, opportunities for adjunctive HPV and other microbial testing, and name recognition and co-marketing arrangements.

Dr. Austin is a member of the CAP Cytopathology Committee and medical director and director of cytopathology and gynecologic pathology services for Coastal Pathology Laboratories, Charleston, SC.