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The recent advent of a purportedly more accurate, liquid-based Pap smear led the American Cancer Society to recommend that women go for the test once every two years, as opposed to the annual test recommended for the older version of the Pap smear.
But Australian researchers are now calling that recommendation into question. Their study suggests that the newer cytology (cell-based) screen may be no more effective than the older version.
"The evidence presented here does not lend support to a conclusion that liquid-based cytology is better than conventional cytology," the study authors wrote in the Jan. 14 issue of The Lancet.
For more than 30 years, the conventional Pap smear has been used to screen for cervical cancer, despite its limited accuracy. A newer Pap test, called liquid-based cytology, was developed as an alternative and was believed to yield more accurate smear tests, reducing the number of unusable samples.
Both techniques start with cell samples taken from the cervix.
The difference lies in what happens next. In conventional Pap tests, the cell sample is smeared directly onto a microscope slide.
However, in liquid-based cytology the head of the spatula -- where the cells are -- is broken off into a small glass vial that contains preservative fluid. The cells are then rinsed directly into the preservative fluid.
The sample is sent to the laboratory where it is treated to remove material, such as mucus or pus. Next a thin layer of the cells is put on a slide. Then, as with conventional Pap tests, the slide is examined under a microscope by a cytologist.
In their study, a team led by Elizabeth Davey, from the Screening and Test Evaluation Program at the School of Public Health at the University of Sydney, reviewed 56 studies comparing the accuracy of the two techniques. These studies included a total 1.25 million slides.
Davey's group found no evidence that liquid-based-cytology reduced the proportion of unsatisfactory slides or detected more cancers than the conventional Pap tests.
"Liquid-based cytology did not reduce the percentage of unsatisfactory slides compared with conventional cytology," they concluded. "There are very few studies with which to estimate the relative performance of the two methods validly, and there is no evidence that liquid-based cytology is more accurate than conventional cytology at detecting high-grade disease in high-quality studies."
One expert thinks that, despite the study findings, liquid-based cytology is here to stay.
"This study does not dismiss liquid-based cytology as a means of reading Paps," says Dr. Stephanie V. Blank, a member of the New York University Cancer Institute's Division of Gynecologic Oncology.
Blank notes that many hospitals and laboratories have adopted liquid-based cytology at great expense. "They're not going to switch," she says. "In addition, cytologists find the liquid-based cytology easier to read and the samples are easier to prepare, and you can also do human papilloma virus testing very easily on liquid-based Pap smears." HPV infection is thought to be the leading trigger for cervical cancer.
Blank adds that human factors, rather than the type of test used, may be most important when it comes to accurate Pap test results.
"The key for patients it to make sure that, regardless of which method is used, the Pap test is being analyzed by good cytology labs," Blank says. "A patient needs to know who is reading the Pap smear. That's more important than whether it is liquid-based or conventional."
Another expert, Dr. Herman Kattlove, a medical oncologist and spokesman for the American Cancer Society, doesn't think the Australian study will have any impact on Pap testing.
"This is not a terribly important study," says Kattlove. "No one trumpeted this (liquid-based) test as better than the conventional Pap test. But since it is as good as the conventional Pap test, you are not losing anything."
The reason liquid-based cytology is used is that it can also test for HPV, he says.
"If HPV infection is present, any abnormality found on the test may be serious, and the woman would need further testing," Kattlove says. "The reason everyone is turning to liquid-based cytology is because you can do the HPV test, so a woman doesn't have to come back for another test. So it's much more convenient."
Kattlove also says the results of the new study won't change the cancer society's recommendations on when women should get Pap tests. "It won't change anything," he says.
(The HealthDay Web site is at http://www.HealthDay.com.)
c.2006 HealthDay News
