Liquid-based cytology

Liquid-based cytology is a method of preparing samples for examination in cytopathology. The sample is collected, normally by a small brush, in the same way as for a conventional smear test, but rather than the smear being transferred directly to a microscope slide, the sample is deposited into a small bottle of preservative liquid. At the laboratory the liquid is treated to remove other elements such as mucus before a layer of cells is placed on a slide. The technique allows more accurate results. The UK screening programmes changed their cervical screening method from the Pap test to liquid-based cytology in 2008.

For many years, efforts have been made to develop methods that would enhance the sensitivity and specificity of the Papanicolaou smear (also called Pap smear). Emphasis has been placed on creating automated screening machines whose success depends on a representative sampling of cells on standardized slides containing a monolayer of well-stained, well-preserved cells.

From this research and development, liquid-based gynecologic specimen collection has evolved. Its proponents argue that liquid-based preparations outperform conventional smears because of improved fixation, decreased obscuring factors, and standardization of cell transfer. Proponents point out that, in direct smears, the cells are not transferred in a representative fashion and that up to 90% of the material scraped from the cervix may be discarded with the sampling device. With liquid-based collection, the sampling will be representative and operator-dependent variation will not occur since processing is controlled by the laboratory.

SurePath (TriPath Imaging, Inc., Burlington, NC) and ThinPrep 2000 System (Cytyc Corp, Marlborough, MA) are two such systems currently approved by the U.S. Food and Drug Administration (FDA) for cervicovaginal testing. With both methods, the sample is collected in the conventional manner with one of the brush instruments but, instead of being spread onto a glass slide, it is transferred to a vial of fixative.

In the SurePath method, the sample is vortexed, strained, layered onto a density gradient, and centrifuged. Instruments required are a computer-controlled robotic pipette and a centrifuge. The cells form a circle 12.5 mm in diameter. The ThinPrep method requires an instrument and special polycarbonate filters. After the instrument immerses the filter into the vial, the filter is rotated to homogenize the sample. Cells are collected on the surface of the filter when a vacuum is applied. The filter is then pressed against a slide to transfer the cells into a 20 mm diameter circle.

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