A combined analysis of 56 studies found no evidence that Pap tests that rely on liquid-based cytology produce better results than Pap tests that rely on conventional cytology. These results were published in Lancet.
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. The cells are examined by a laboratory and results then are classified into five categories: normal; atypical squamous cells of undetermined significance (ASCUS); low-grade squamous intraepithelial lesions (LSIL); high-grade squamous intraepithelial lesions (HSIL); or cancer.
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 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 summarize the available evidence about the benefits of liquid-based cytology, researchers in Australia conducted a combined analysis of published studies. The researchers identified 56 relevant articles and classified their quality as “high,” “medium,” or “low.”
- Overall, 0.75% (less than 1%) of the liquid-based cytology slides were classified as “unsatisfactory.” The rate in conventional cytology slides was similar-0.81%.
- In high-quality studies, conventional cytology classified more slides as HSIL than liquid-based cytology.
The researchers conclude, “This review does not lend support to claims of better performance by liquid-based cytology.” The researchers recommend that the two methods (liquid-based cytology and conventional cytology) be directly compared in large randomized trials.
Reference:Davey E, Barratt A, Irwig L et al. Effect of Study Design and Quality on Unsatisfactory Rates, Cytology Classifications, and Accuracy in Liquid-Based Versus Conventional Cervical Cytology: A Systematic Review. Lancet. 2006;367:122-32.