Worldwide, cervical cancer is the second most common cause of death amongst women with cancer. Fortunately, the incidence of cervical cancer has decreased by more than 50% in the past 30 years, largely due to the increasing use of cervical cancer screening with cervical cytology.
Although worldwide cervical cancer rates have decreased dramatically with the increase in screening efforts, incidence and prevalence in developing countries remains high due to lack of screening programs.
Cervical cancer screening methods
The conventional pap smear (CPS)
The mainstay of cervical cancer screening for the last 60+ years has been the Papanicolaou test. The Papanicolaou test, also known as the Pap smear, was developed in the 1940s by Georgios Papanikolaou. It involves exfoliating cells from the transformation zone of the cervix to enable examination of these cells microscopically for detection of cancerous or precancerous lesions. The cervical cells obtained is directly transfer onto a microscope slide for evaluation.
The conventional papanicolaou (Pap) test is considered suboptimal due to false-negative and false-positive test results. This is caused by the poor quality of sampling and preparation (obscuration by blood or inflammation, bad cell fixation, and inhomogeneous distribution of cells) and by errors in detection and interpretation. All these contributed to an unsatisfactory smear and need for repeat test albeit a small incidence of 1%.
Liquid based cytology (LBC)
Liquid-based cytology was developed as an alternative to conventional pap smear. This method involves the collection of cervical cells using a traditional sampling device and rinsed into a vial with preservation solution rather than being smeared on a slide. It is centrifuged to filter out cervical cells from other contaminants and the product is then analyzed in the cytology lab. Because only a representative portion of the sample is used, the residual material in the vial may be used for ancillary testing such as reflex human papillomavirus (HPV) testing and other tests.
Is the liquid based cytology superior to CPS?
Siebers et al (JAMA 2008) had conducted a large prospective trial with 90000 women, comparing the screening performance of the two screening methods. The results showed no differences were found in the cytological test positivity rates between methods. He concluded that liquid-based cytology was not superior to conventional Pap test regarding detection rates of histologically confirmed outcomes (gold standard). A reduced unsatisfactory rate was found when using liquid-based cytology even though the added value was limited because the unsatisfactory rates were already low. On the other hand, liquid-based cytology tests cost more money than a conventional Pap test.
The results of a systematic review and meta-analysis by Arbyn (2008), utilizing strict criteria had similarly found that liquid-based cervical cytology is neither more sensitive nor more specific for detection of high-grade cervical intraepithelial neoplasia (abnormal premalignant cervical cells) compared with the conventional Pap test.
Despite the above findings, Schiffman and Solomon (2009) observed that liquid-based cytology often results in fewer unsatisfactory specimens and allows for HPV testing on the same sample. They also observed that liquid-based cytology has virtually replaced conventional Pap smears in the United States. Although liquid-based cytology is more expensive, its ease of use allows laboratories to process slides more quickly and efficiently makes it the preferred method at the expense of a limited public program funding.
The bigger question, however, is how liquid based cytology screening itself will fit into the broader picture of cervical cancer prevention in low resource countries, where cervical cancer is more prevalent couple with lack of adequate screening. Saraiya (2010) argued that findings of Siebers et al validated the practices of clinicians and laboratories that have continued to use conventional cytology.
Both conventional cytology and liquid-based cytology have been shown to have similar sensitivity and specificity for cervical dysplasia (premalignant cervical cells). In addition, both types of cytological screening are considered acceptable by the American College of Obstetricians and Gynecologists.
Arbyn M et al., Liquid compared with conventional cervical cytology. Obstet Gynecol (2008);111(1): 167-177
Schiffmans et al., Screening and prevention methods in cervical cancer. JAMA (2009);302(16):1809-1810
Siebers AG et al., Cytological detection of cervical abnormalities using liquid-based compared with conventional cytology . Obstet Gynecol (2008);112(6): 1327-1334.
Suraiya et al., To the Editor: Liquid based cytology versus conventional cytology in detecting cervical cancer. JAMA(2009);303(11):1034