• Dr Peter Chew

How reliable is the pap smear?

Y was perplexed and confused when the biopsy result of her neck of the womb (cervix) showed that she had the precancerous condition known as Cervical squamous Intraepithelial Neoplasia (CIN). “Doc, I can’t understand this,” she said. "I have gone for my pap smears regularly and they have all been normal."

Y, 38, had been married for 10 years and had 3 children. She saw me recently because of vaginal discharge which was mixed with occasional streaks of blood after sexual intercourse. On examination, her cervix looked inflamed and red. It bled slightly when it was gently touched with a brush. Her pap smear done elsewhere a year ago was reported as normal.

Pap smear or pap test has been widely used as a simple, noninvasive, low-cost screening test for early detection of pre-cancers and cancers in the cervix for the past 80 years. In fact, its use is largely responsible for a reduction of cervical cancer in Singapore.

In the early days, the test involved scraping and collecting cells from the cervix by a wooden spatula and smearing them on a glass slide. Abnormal cells detected on the slide may suggest pre-cancer or cancer of the cervix. But like any other screening test, there are false negative and positive results. In the former, the result indicates no abnormal cells while in fact it does. In the latter, the reverse is true and this may result in unnecessary surgery and anxiety to the patient.

Up till the late 1990s, the false negative rate was reported in approximately 20 to 45 percent of patients screened.  This was confirmed by studies that showed that as many as 30% to 50% of women with cervical cancer had had a prior normal pap smear. To reduce the false negative rate, pap tests are now performed using a liquid-based technology. The entire sample collected is placed inside a vial of liquid and no other substances, such as fixatives, are mixed with the sample. This method of collecting samples has allowed abnormal cells to be identified in about 80 to 90 percent of Pap tests thus reducing the false negative results significantly.

Factors causing a false-negative result include:

  • An inadequate collection of cells

  • The number of abnormal cells is too low

  • Blood or inflammatory cells obscuring the abnormal cells

  • Collection of cells at the wrong location

For pap tests with false positive results, they are less common. The number ranges from approximately one to ten percent of all Pap tests screened.

I repeated the pap test for Y. It showed some inflammatory cells with reactive changes. High risk HPV (human papilloma virus) DNA test was negative. As the cervix looked inflamed  with contact bleeding, I advised her to have a colposcopy. In this procedure, her cervix was stained with chemicals and examined under the microscope(colposcope). Abnormal looking tissue was excised and sent to the laboratory for testing(biopsy). She consented straightaway. The abnormal tissues excised showed the precancerous condition CIN3. After discussing with her husband, Y decided on conservative treatment with continuous surveillance and regular follow-up.

The take-home message in Y ‘s case is that, if the cervix looks abnormal, it should be biopsied to obtain a diagnosis. Though the accuracy of Pap test has improved over the years, it is still not diagnostic. Relying on its result may occasionally lead to false reassurance and underestimation of the severity of disease.

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