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How Accurate is My Pap Smear Result?

By Jane Harrison-Hohner, RN, RNPJune 06, 2011
From the WebMD Archives

Getting an actual Pap smear may be stressful enough, but what about waiting for the results? Many of us have life histories which contain risk factors for “bad Paps” such as multiple sexual partners, non-condom sex, or cigarette smoking which can increase our tension as we await our results. Then, when we get our results, how do we know they are accurate?

The purpose of this post is to examine some of the causes of inaccurate Pap smears and what solutions might decrease your chances of getting one.

The original Pap smear technique began to be used in the 1940′s. A sample of cells from the face of the cervix was scraped off using a wooden spatula. Another sample was taken from just inside the cervical canal with a more pointed end of the spatula. The GYN tried to apply a thin layer of cells to a glass slide which was then sprayed with a fixative. This could even be hairspray! The rest of the cells on the spatula were tossed into the garbage. If the slide was too thick, or had blood or mucus present, the cytologist looking at the slide under the microscope could have a hard time distinguishing normal from abnormal cells.

Accordingly, one could have an inaccurate Pap smear result of one of two types. The most common would be a “false negative” result. That is the woman was told “No problem, we found nothing suspicious,” when actually there were abnormal cells either on the slide or in the part of the sample which was discarded. Depending upon which study one reads the incidence of false negatives can be reported as 20%-45%.

In an attempt to decrease this type of mistakes, many changes have been made. The so called “liquid based” Pap smears (e.g., Thin Prep®) have a spatula or broom shaped collecting device that is immersed in a vial of liquid. In this way all the collected cells are saved. The material is then filtered to remove obscuring blood, dead sperm, and other debris. The filtered cells are automatically plated in a very thin layer on the slide for the cytologist to read.

Given that about 80% of Pap smears in the US are now utilizing a liquid based Pap (LBP) technology (Saint, 2005), how helpful are they to insure accurate results? Numerous studies have found that LBP alone have increased the rates of diagnosis of severe dysplasia and cancer (Schledermann, 2006). In a review of many studies Cox (2004), found the ability to identify abnormal cells from conventional Pap smears was 71% compared to 81% for LBP.

In addition to changes in the way that cells can be collected, the prepared slides can have supplementary interpretations. Automated, computerized readings of the slides (e.g., AutoPAP® or PAP Net®) can be done before, or after, the human cytologist examines the slide.

Another way to decrease “false negative” results happens after the slide is prepared and read. The person evaluating the slide can give it a more abnormal diagnosis which would mandate an additional work up. This means that the woman may be asked to get more frequent PAP smears, have her cervical cells tested for human papilloma virus (HPV) DNA, or even a colposcopy.

The second type of Pap smear error is called a “false positive” result. The woman is told “We have found abnormal cells,” when she is actually fine. This often leads to more invasive, and stress inducing, testing. Fortunately, this is less common. Depending upon which study is read, the incidence of false positives is variously reported to be between 1%-10%.

Paradoxically, increasing PAP smear sampling among low risk women actual increases your chance of getting a “false positive” one day. For example (DeMay, 2000), if you get a yearly PAP between the ages of 18 to 78, and one assumes a 5% incidence of false positives, you would have a 95% chance of getting a false positive report during that time.

As you can ascertain, Pap smear results may not be perfect. Even with the use of the newer liquid based techniques, perhaps 35% of concerning diagnoses (e.g., CIN 3 or even cancer) might be missed (ACOG, 2005). Consequently there has been increased interest and use of HPV testing — often using the same cells in the liquid PAP smear vials.

Next: We’ll discuss the emerging move to substitute HPV testing for Pap smears in women over age 30. And we’ll address how receiving the new HPV vaccine might change the way we approach Pap smear screening.

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