Liquid-based cytology (LBC) has largely replaced conventional smear preparation for cervical cytology and is increasingly adopted for non-gynecologic specimens. By collecting cells into a liquid preservative, LBC enables consistent slide quality, reduced obscuring material, and — critically — reflex molecular testing from the residual sample.
LBC Platforms
ThinPrep (Hologic) — the collection device is rinsed into PreservCyt solution (methanol-based). The processor disperses cells, collects them on a polycarbonate filter, and transfers them to a slide in a 20 mm diameter circle. The standardized monolayer reduces cell overlap and obscuring blood or inflammation.
SurePath (BD) — the collection device is rinsed into CytoRich solution. The processor uses density gradient centrifugation to separate diagnostic cells from blood, mucus, and debris before depositing cells in a 13 mm circle. SurePath produces cleaner slides for bloody specimens.
Advantages of LBC
Reduced unsatisfactory rate — LBC decreases unsatisfactory Pap tests from 5-15% to 1-3% by filtering obscuring blood, inflammation, and thick cell groups. Improved sensitivity — meta-analyses show LBC has equivalent or slightly higher sensitivity for HSIL+ compared to conventional smears. Residual sample — the remaining PreservCyt or CytoRich fluid (typically 10-20 mL) is available for HPV testing, Chlamydia/Gonorrhea testing, and molecular biomarker analysis. Multiple slides — additional slides can be prepared from the same vial for special stains or IHC.
LBC in Non-Gynecologic Cytology
LBC is increasingly used for body fluids, urine, FNA specimens, and respiratory samples. Advantages include concentrated cell preparation, removal of obscuring blood, and consistent background. Disadvantages include loss of architectural clues (three-dimensional clusters may be dispersed) and changes in cellular morphology (cells appear smaller, chromatin less crisp than conventional smears). Laboratories must validate LBC against conventional preparations for each non-gynecologic specimen type.
HPV Testing from LBC
Cervical LBC samples are the primary specimen for HR-HPV testing. Signal amplification methods (Hybrid Capture 2, Cervista) detect HPV DNA from a pool of 13-14 high-risk types without genotyping. Target amplification methods (PCR-based: Cobas 4800, Aptima) detect and genotype HPV 16 and 18 separately from other HR types. Cobas 4800 is FDA-approved for primary HPV screening and co-testing. Aptima detects E6/E7 mRNA, indicating transcriptionally active infection rather than transient HPV DNA. HPV testing from LBC has equivalent sensitivity and specificity to testing from separately collected cervical specimens.
Molecular Testing from Cytology Samples
FISH (fluorescence in situ hybridization) on cytology specimens detects chromosomal abnormalities. UroVysion FISH (chromosomes 3, 7, 17 and 9p21) increases sensitivity for urothelial carcinoma in urine cytology. ALK break-apart FISH on lung FNA cell blocks identifies ALK-rearranged adenocarcinoma.
PCR-based testing on cytology samples detects mutations (EGFR, KRAS, BRAF), viral DNA (HPV, EBV, CMV), and clonality (IgH and TCR gene rearrangements for lymphoma). DNA from cytology slides or cell blocks is adequate for most PCR applications. Sensitivity is comparable to histology samples.
Next-generation sequencing (NGS) — cell blocks from FNA and effusion specimens provide sufficient DNA for targeted NGS panels (50-500 genes). Comprehensive genomic profiling from cytology samples guides targeted therapy selection. Success rates (adequate DNA for NGS) exceed 90% when cell blocks contain at least 20% tumor cellularity.
Quality and Validation
Molecular testing on cytology specimens requires validation against histology or known mutation status. Pre-analytic variables include fixative type (methanol-based PreservCyt vs. ethanol), storage time and temperature, and cell concentration. Laboratories must establish minimal cellularity requirements and document sample adequacy for molecular testing. Quality assurance programs include proficiency testing for molecular assays on cytology material.
Future Directions
Digital cytology — whole slide imaging of LBC slides enables remote review, computer-assisted screening, and artificial intelligence-based detection of abnormal cells. Multiplex biomarker panels — protein (IHC) and nucleic acid (FISH, mutation) analysis from the same cell block provides integrated diagnosis and therapy selection. Circulating tumor cells (CTCs) and liquid biopsy (cell-free DNA from blood) are extending cytology beyond solid tissue sampling to non-invasive cancer detection and monitoring. These technologies complement rather than replace traditional cytomorphology, and their interpretation requires integration with clinical and imaging context.
