The liquid-based cytology test ThinPrep now represents 70 percent of the U.S. cervical cancer screening market, but a new report finds it no more effective than the conventional Pap smear at detecting precancerous cells.

In the study, 89,784 Dutch women were randomized to be screened either with ThinPrep or a conventional Pap smear. After evaluating the data, the authors concluded that "liquid-based cytology does not perform better than conventional Pap tests in terms of relative sensitivity and [positive predictive value] for detection of cervical cancer precursors."

Pap smear testing has been in use since the 1940s, while ThinPrep received U.S. approval in 1996. The main difference between the two tests is how the cells are prepared for evaluation. The Food and Drug Administration has allowed ThinPrep's maker, Hologic, to promote the test as being more effective at revealing early and more advanced abnormalities in the cervix.

ThinPrep, while more expensive, "is preferred by most laboratories because the specimen is easier and quicker to scan under the microscope," said an accompanying editorial by Mark Schiffman and Diane Solomon.

Many doctors were forced to switch to the test when labs stopped reading Pap smears, said George Sawaya, an obstetrician/gynecologist at the University of California-San Francisco. The results of the new study "should serve as a cautionary tale that just because something is new doesn't mean it's better," he said.

Lead author Albertus G. Siebers, MSc, said other factors to consider include the ability to use ThinPrep samples to test for human papillomavirus -- the STD that causes nearly all cases of cervical cancer -- and the fact that only one in 300 ThinPrep tests had to be redone, versus one in 100 Pap tests.

The report, "Comparison of Liquid-Based Cytology with Conventional Cytology for Detection of Cervical Cancer Precursors," and the editorial, "Screening and Prevention Methods for Cervical Cancer," were published in the Journal of the American Medical Association (2009;302(16):1757-1764,1809-1810).