Study: Pap Smear Not Always Reliable

B O S T O N, March 20 -- How do women know that when they get their Pap smear it is being diagnosed correctly?

Well, most of the time pathology technicians and doctors get it right.

But despite the success of the Pap smear to detect cervical cancer, an estimated 4,400 American women will die of cervical cancer this year. Some will die because they were never screened. Others received the test and still died.

Why are women who get screened still dying?

A National Cancer Institute study appearing in this week’s women’s health issue of the Journal of the American Medical Association shows that the Pap smear may not be as accurate as the medical community once thought.

Even the most experienced pathologists have trouble agreeing on what they see under the microscope, the researchers say.

In the study, Dr. Mark Stoler from the University of Virginia, in Charlottesville, and Dr. Mark Schiffman, from the National Cancer Institute, in Bethesda, gathered 11 pathologists to analyze 7,720 cervical tissue slides. Stoler is a professor of pathology and clinical obstetrics and gynecology. Schiffman is the chief of the interdisciplinary section in the NCI's division of cancer epidemiology and genetics.

In a Pap smear, a doctor scrapes some cells from the opening of the cervix onto a microscope slide. The slide is fixed and sent for analysis for abnormal cells.

George Papanicolaou first introduced the Pap smear in 1943 and since then it has helped decrease the number of deaths from cervical cancer. When detected early, cervical cancer is treatable, with a 95 percent five-year survival rate, according to the American Cancer Society.

Disagreement Over Diagnoses

For the study, pathologists examined 4, 948 Pap smears and other diagnostic procedures — colposcopy and loop electrosurgical excision — that provide cervical cells for analysis.

The researchers found that the pathologists only had moderate agreement on several diagnoses. Especially important was the diagnosis of "atypical squamous cells of undetermined significance." Pathologists use this diagnosis for cells that do not look normal, but also do not look like early stage cancer.