Liquid Cytology: What is it and how does it compare to conventional cytology.

Major advancements in cervical cancer screening have dramatically lowered the mortality rate worldwide. Cervical cancer is a potentially preventable disease, with the correct screening programme, along with reliable and accurate laboratory techniques. The first major advancement in cervical cancer screening was the Pap test, followed by the use of liquid-based cytology. Since the introduction of liquid-based cytology, there have been debates surrounding its reliability and accuracy in terms of sensitivity in comparison to the use of the conventional Pap test when detecting precancerous lesions. Many trials and studies have been undertaken to compare both techniques with many questions still unanswered, but it is certain that in the future there will be a need for the introduction of molecular aspects when analysing cervical cytology.

Globally cervical cancer is the second most common form of cancer in women. (Bosch, et al., 1995). It is preventable when the precursor lesions are detected and treated early. Cervical cytology has been implemented for over 50 years, and results have shown that this is the main weapon of defence against this disease. (Koss, 1989). Due to the success in the advancement of screening within cervical cytology, the number of incidences of cervical cancer has decreased by more than 50% in the past 30 years (Karjane, 2018). One of the major changes in the preparation method for screening cervical cancer cells has come in the form of liquid-based cytology.  Cells are no longer being smeared onto a glass slide but are now washed into a vial of liquid and filtered, which results in a random sample being presented in a thin layer on a glass slide. (Denton, 2007). Although some countries have changed to liquid-based cytology for cervical screening, controversy remains over the accuracy and reliability of liquid-based cytology in comparison to conventional cytology.

Major advancements in screening techniques have dramatically lowered the death rate from cervical cancer worldwide. The first major advancement in cervical cancer screening was the Papanicolaou (Pap) test in 1928 (Tan & Tatsumura, 2015). Pap testing was responsible for reducing the incidence of cervical cancer between 1955 and the mid-1980s (Institute, 2007). While the second major advancement in cervical cancer screening came in the form of liquid-based cytology (LBC), and is now implemented on over 90% of cervical cytology samples today. Since the introduction of liquid-based cytology, there has been debate surrounding its reliability in comparison to the use of the conventional Pap test when detecting precancerous lesions. (Sawaya, 2010).

Although the conventional Pap test was responsible for the initial advancement in reducing the incidence of cervical cancer, the clinical performance of this technique is not without limitations. When using the Pap test a broad range of sensitivity (30%-87%) has been reported when detecting high-grade lesions (Gynecologists, 2008). It was also found that the conventional Pap test has a false positive rate of about 14% to 33% (Hartmann, 2002). It is known that the Pap test has been successful in detecting and diagnosing squamous cell carcinomas, although it is less useful in the detection of adenocarcinomas which now contribute to 10% of all cervical cancers. There is a possibility that this could be linked to the difficulty in obtaining enough sample to entirely represent the endocervical canal, which is where most glandular lesions occur. When employing the pap technique, the equipment used to obtain a sample during a conventional Pap smear means that only a small portion of the sample taken from the patient is transferred on the slide. Most of the sample is discarded, along with the sampling equipment, which may lead to inaccuracies when testing the slide. The detection of adenocarcinoma is between 45% and 76% due to the varying degree of sensitivity of the Pap smear. The problems faced when using the Pap smear technique results from obscuring by blood or inflammation, bad cell fixation and inhomogeneous distribution of cells on the slides. Worldwide it is accepted that there are major limitations to conventional cytology.

It is the limitations of the conventional Pap test which have encouraged the scientific community to rethink the diagnostic technique used to detect cervical cancer cells. The introduction of liquid-based cytology has decreased the number of inadequate smears resulting in the improved detection of abnormal cells significantly. (Astbury, et al., 2006). In liquid-based cytology, the cervical cells are obtained in an identical manner to the conventional Pap test, but instead, a cervical broom, a spatula and an endocervical brush or an extended spatula are used to collect the cells (Arbryn, et al., 2007). The difference is that the cells are then rinsed into a vial with preservation solution instead of being smeared onto a slide. This technique eliminates blood, mucus, inflammatory cells and mucus which allows for a more representative sample to be used for testing. The residual material in the vial can also be used for further testing such as reflex human papillomavirus (HPV) testing (Siebers, 2009).

In 1996 the ThinPrep Pap test became the first liquid-based cytology technique to be approved by the FDA.  The processing of a ThinPrep Pap test sample involves the rotation of a filter within the sample vial which creates currents in the fluid that separates debris and disperses mucus while preserving the cells morphology. Within the filter, a gentle vacuum is created which collects cells on the exterior surface of the membrane.  Once the cells are collected on the membrane, the filter is inverted which is then pressed against the slide. The slight positive air pressure causes the cells to adhere to the slide, which results in the even distribution of cells, as can be seen in Fig 1 (Gibb & Martens, 2011)