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The concern about false-negative Pap smears may soon be eased as new screening technology moves into routine medical practice.
One such product, the ThinPrep Pap Test developed by Cytyc Corp. of Boxborough, MA, actually improves the quality of the test by changing the way the slide is prepared and processed.
While the specimen is taken in the usual manner with a collection device, the device is rinsed into a vial of proprietary preserving solution, rather than smeared and fixed on a slide. The vial is then sent to the laboratory, where a special processor separates cells from obscuring blood, mucus, and inflammation. The processor then applies a thin, uniform layer of cells on a slide for microscopic review.
"The primary advantage of the ThinPrep [test] is that it improves the quality of the preparation of the Pap smear," says Thomas V. Sedlacek, MD, chairman of gynecology and oncology at The Graduate Hospital in Philadelphia. "If you look carefully at the reasons that Pap smears are misdiagnosed, for lack of a better term, they are either because the material is poorly obtained from the patient, or it is poorly presented to the cytopathologist, or it is poorly interpreted. There’s no way that you can improve the collection, but by using the ThinPrep, what we do is that we improve the quality of the material so that more cells are available to the cytotechnician for proper evaluation."
Cytyc received approval from the Food and Drug Administration (FDA) in May 1996 for ThinPrep’s use in cervical cancer screening. It then gained FDA clearance in November to present ThinPrep as significantly more effective than the conventional Pap smear.
A clinical study of 786 women bears out this claim about the method’s sensitivity.noncThe study was performed by taking cervical scrapes from each patient, preparing a conventional Pap smear, then a ThinPrep test sample from the material left on the collecting device. Colposcopic biopsies then were taken from 445 patients. Using the histologic diagnosis as the definitive diagnosis, the two cytologic diagnoses were statistically compared. (See the related charts, pp. 36-37.)
"The two cytologic diagnoses agreed in 89.8% of the cases," researchers noted. "Statistical analysis of the discordant cases showed that the ThinPrep method increased the number of cervical abnormalities diagnosed. When the histologic diagnosis was used to classify the discordant cases into two groups, then the correlation between the two cytologic diagnoses again showed that the ThinPrep method was significantly more sensitive for the diagnosis of lesions than the conventional smear."
"It picks up better than conventional [Pap smears]," says the study’s lead researcher, Ellen E. Sheets, MD, director of the Pap smear evaluation center at Brigham and Women’s Hospital and assistant professor at Harvard Medical School., both in Boston. "It also decreases the unsatisfactory samples, so both of those make it more effective in decreasing what is called false-negative Pap smears."
One of the biggest challenges for clinicians has been to secure an adequate sample and to transfer it to the laboratory for evaluation, says Mary Rubin, RNC, PhD, CRNP, director of clinical education for Education Programs Associates, a nonprofit health, education, and human services organization in Campbell, CA. By placing the specimen in a liquid medium, the ThinPrep method captures virtually all of the collected material.
"All of us know from clinical experience and documented studies that a lot of the sample, whether we secure it with a cyto brush, broom, or spatula goes into the trash, and not to the laboratory to be assessed," she notes. "It’s difficult enough targeting the transformation zone where the activity on the cervix needs to be sampled, but then the type of equipment that you use plays a great role in how much of that sample actually gets to the laboratory."
The ThinPrep method also alleviates the air-drying artifacts that are part of the conventional Pap smear.
"The traditional Pap smear, if you don’t fix it quickly enough, it can dry out and [make] it hard to read," says Michael S. Policar, MD, MPH, associate clinical professor of OB/GYN at the University of California-San Francisco School of Medicine. "That’s not a problem with ThinPrep."
Those who have worked with the ThinPrep method say the increase in cost is outweighed by its benefits. Company representatives offer a comparison estimate of $40 for a ThinPrep Pap test vs. $20 for a conventional Pap smear.
"The increase in cost will probably will come down with time and should be more than paid for in the savings of lives and improved diagnosis," says Sedlacek. "There’s no question that the Cytyc system picks up more low-grade and high-grade lesions."
Any increase in cost, though, can be significant for underserved women, who could well benefit from the method’s enhanced detection rate.
"[These women] need for their Pap smear reading to be as accurate as it can be because they may not come back for three years," notes Rubin. "And so when considering the cost, no matter which technology we’re trying to introduce whether it’s the automated screening of the Pap or whether it’s the technique for obtaining the Pap the companies that produce these things need to keep in mind that the women who need it most are the underserved women who can least afford it."
1. Sheets EE, Constantine NM, Dinisco S, et al. Colposcopically directed biopsies provide a basis for comparing the accuracy of ThinPrep and Papanicolaou smears. J Gynecol Tech 1995; 1:27-34.