Liquid-based cytology (LBC) was introduced in the mid-1990s as a way to improve the performance of the test. Rather than having the clinician prepare the cytological specimen at the bedside by spreading the exfoliated cells onto a glass slide, the cells are transferred to a liquid preservative solution that is transported to the laboratory, where the slide is prepared. A number of different LBC techniques are in use worldwide.(Figure 29).
These include ThinPrep®, SurePath™, Cytoscreen™, Cyteasy®, Labonord Easy Prep, Cytoslide, SpinThin and PapSpin. The first two of these are approved for use in the
The epithelial cells collected on the membrane filter are then transferred onto a glass slide and stained. This produces a relatively thin, monolayer-type preparation. The ThinPrep-2000 processor allows one specimen to be processed at a time, whereas the newer ThinPrep-3000 processor is more fully automated and allows up to 80 samples to be processed at a time. In contrast, with the SurePath method, clumps of cells and mucus are broken up by aspiration through a syringe. The cell suspension is then layered on top of a density gradient and the red blood cells and inflammatory cells are separated from the epithelial cells by density gradient centrifugation. The resulting cell pellet containing predominantly epithelial cells is then inserted into a robotic workstation, where it is resuspended and transferred to a glass microscope slide.
The SurePath method allows up to 48 samples to be processed at a time. LBC is purported to have a number of advantages over conventional cervical cytology. These include a more representative transfer of cells from the collection device to the glass slide, a reduction in the number of unsatisfactory cytology specimens, the availability of residual cellular material for subsequent molecular testing or for making additional glass slides, and possibly increased detection of HSIL.
Performance of liquid-based cytology methods
Numerous studies have evaluated the comparative performance of the two most commonly used LBC methods (ThinPrep and SurePath) and conventional cytology with respect to test positivity, their sensitivity and specificity for identification of CIN, the time required for evaluation of the specimens, and specimen adequacy. Although there is reasonable agreement that LBC improves specimen adequacy and reduces screening time compared to conventional cytology, there is considerable controversy surrounding the relative sensitivity and specificity of the two approaches, largely due to a lack of well designed comparative studies. Most comparative studies have utilized one of two types of study design: split-sample studies and historical control studies. Split-sample studies collect cells from the cervix using a single collection device and a conventional cervical cytology specimen is prepared first. Residual cells remaining on the device are then transferred to a liquid based cytology preservative.
Therefore each woman acts as her own control and detection rates in conventional and LBC specimens are compared. The other widely used study design, known as ‘direct to vial’, compares the performance of LBC collected in the routine manner (direct transfer to the preservative solution) during a given time period with historic control data obtained using conventional cytology. Both study designs have significant limitations. With split-sample studies, it is difficult to ensure that the two cytology specimens are comparable. Since the conventional cytology slide is prepared first and the LBC specimen is prepared second, this design would seem to lead inherently to bias against LBC. Therefore it has been argued that split-sample studies do not demonstrate the full benefit that could be obtained when LBC is utilized in routine clinical practice. Studies utilizing historical controls avoid the need to prepare several cytology specimens from a single woman, but introduce other potential biases, including the comparability of the populations being compared.
Other significant limitations found in many of the studies evaluating LBC include failure to compare test performance with a reference standard of ‘blinded’ colposcopy/biopsy and a study population of women followed up for a prior cytological abnormality rather than women undergoing routine screening. A review of new cervical cytology methods conducted in 2001 for the US Preventive Services Task Force and the Agency for Healthcare Research and Quality found that out of 962 potentially relevant studies, not one met their predefined inclusion criteria (Hartmann et al., 2001). This was commonly due to lack of an adequate reference standard, but most studies were excluded for more than one reason.