Liquid-based Cytology No More Accurate than Conventional Pap Test
For the detection of cervical abnormalities, liquid-based cytology does not appear to be more accurate than the conventional Pap test. This was the conclusion of a combined analysis published in the journal Obstetrics and Gynecology.
A Pap test is a routine screening test that is used for the early detection of abnormal or cancerous cells in the cervix. During a Pap test, a sample of cells is removed from the cervix using a small wooden spatula or a brush.
Cytology refers to the study of cells. Conventionally, cells that were removed from the cervix during a Pap test were smeared on a glass slide and then viewed under a microscope. Potential limitations of the conventional approach include incomplete transfer of material from the spatula or brush to the slide and drying and distortion of cells. Furthermore, blood or mucous may be transferred to the slide along with cervical cells, potentially making the slide more difficult to interpret.
In response to these limitations, the method of liquid-based cytology was developed. In this approach, cervical material that is removed by the spatula or brush is rinsed in liquid. The liquid is then processed to isolate the cells that need to be analyzed. These cells are spread in a thin layer on a slide and viewed under a microscope.
Some studies have suggested that liquid-based cytology detects more cervical abnormalities than the conventional approach and produces fewer samples that are classified as “unsatisfactory” (unable to be reviewed because of poor sample quality). However, liquid-based cytology is also more expensive than conventional cytology.
To compare liquid-based cytology to the conventional Pap test, researchers combined information from studies that assessed both types of tests, and that were published between 1991 and 2007.
The analysis assessed the sensitivity of the two tests as well as the specificity.
Sensitivity refers to how well the test detected cervical abnormalities (such as among all women with cervical abnormalities, how many received a positive test result?). Specificity refers to how well the test was able to exclude cervical abnormalities (such as among all women without cervical abnormalities, how many received a negative test result?).
Both of these measures are important. If a test has low sensitivity it means that many cases of disease will be missed by the test. And if a test has low specificity there will be many false-positive results.
- The sensitivity of the two tests was similar.
- Depending on how a positive test was defined, the specificity of liquid-based cytology was either similar to or lower than the specificity of the conventional Pap test.
The researchers conclude that liquid-based cytology is neither more sensitive nor more specific than the conventional Pap test.
Reference: Arbyn M, Bergeron C, Klinkhamer P, Martin-Hirsch P, Siebers AG, Bulten J. Liquid compared with conventional cervical cytology. Obstetrics and Gynecology. 2008;111:167-177.