Liquid-based preparations (LBP) are increasingly being used for both gynecologic and non-gynecologic (non-gyn) cytology including fine needle aspiration (FNA). Although the diagnostic criteria used for conventional preparations can also be applied to LBP, an accurate interpretation of liquid-based slides requires familiarity with the background, architectural, and cytological alterations to avoid misdiagnosis.

The types of LBP include SurePath [(SP), formally known as AutoCyte PREP, BD Diagnostics TriPath, Burlington, NC, USA] and ThinPrep [(TP), Hologic, Marlborough, MA, USA] and Millipore Filter (Bedford, MA, USA) and Liqui-PREP (LGM International, Fort Lauderdale, FL, USA). Currently, only SP and TP have FDA approval since they are the more commonly used LBP. The TP and SP preparations were approved for cervicovaginal (Pap test) cytology in 1996 and 1999, respectively, and both have since also been used for non-gynecological cytology including fine needle aspiration (FNA). Both TP and SP are also FDA-approved for high-risk human papillomavirus (HPV) testing of Pap tests. HPV test can also be performed on SP specimens after laboratories have performed internal validation. The published results have been shown comparable HPV performance of SP to TP. All illustrations depicted in this book are either SP or TP. Details of these commonly used LBP have been extensively published in the literature and also appear in the subsequent section of this chapter and in organ-specific chapters.

In conventional smears, the collected patient sample is smeared on glass slides, and the collection device, containing some of the diagnostic cell sample, is discarded. Moreover, the collected sample is nonuniformly distributed on the glass slide and may span the entire surface area of the slide. In LBP, unlike the conventional smears, the acquired patient sample, from various anatomic sites, is rinsed in specially designated collection preservative media ensuring collection of potentially 100 % of the cell sample. The collection media for TP is CytoLyt (Hologic, Marlborough, MA, USA) and that for SP is CytoRich Red (BD Diagnostics TriPath, Burlington, NC, USA).

There are several advantages to LBP compared to conventional smears. Both LBP techniques reduce debris and cell clumps and also homogenize the specimen, increase specimen adequacy, result in uniform cell distribution in a smaller screening area (the screening area of a conventional smear is 2.5 × 1.5 cm, the size of the glass slide), show cleaner background, reduce slide screening time, remove air-drying artifact due to immediate liquid fixation, enhance cellular and nuclear details for improved diagnostic accuracy, and increase productivity. Moreover, residual material from LBP can be successfully used to process cell blocks, which provides additional diagnostic information, including architectural pattern. Both LBP and cell block material can also be utilized for ancillary studies such as immunocytochemistry, special stains, and molecular tests. If LBP are used for ancillary studies, additional slides can be prepared from the specimen vial. Advantages and disadvantages of various cytological preparations, technical details for the two LBP, and their general and specific cytological characteristics are provided in Figs. 1.1 and 1.2 and Tables 1.1, 1.2, 1.3, and 1.4. Step-by-step preparatory techniques for TP and SP also appear in this chapter.