Liquid-based cytology (LBC) has become the standard for cervical cancer screening, largely replacing the conventional Pap smear. This modern method involves collecting cervical cells and placing them directly into a vial of liquid preservative, rather than smearing them onto a glass slide. LBC improves specimen quality by reducing air-drying artifacts and removing obscuring elements like blood and mucus. SurePath and ThinPrep are the two proprietary systems dominating the market, each using a distinct approach to prepare the final slide for microscopic evaluation.
Differences in Sample Preparation
The primary distinction between the two systems lies in how collected cells are separated from the preservative fluid and transferred to the glass slide. The SurePath system utilizes a multi-step process centered on density gradient sedimentation and centrifugation. The sample is first strained to break up mucus and large cell clumps, then subjected to centrifugation in a sucrose density gradient to separate diagnostic cells from debris. The resulting cell pellet is resuspended and allowed to settle onto the slide by gravity, with the collection device retained in the vial to ensure a higher cell yield.
The ThinPrep system relies on membrane filtration and vacuum pressure. The sample is homogenized, and a vacuum draws the fluid through a polycarbonate filter with controlled pore sizes. Cells are captured on the filter’s surface, and the filter is pressed against a glass slide to transfer the cells in a thin, uniform layer. ThinPrep uses a methanol-based preservative, while SurePath uses an ethanol-based preservative, a difference impacting subsequent molecular compatibility.
Slide Visualization Characteristics
The final appearance of the prepared slide varies between the two methods due to their distinct processing mechanisms. ThinPrep’s filtration process creates a thin, uniform layer of cells within a defined 20mm diameter circle. This results in a near-monolayer appearance, where well-separated cells improve the clarity of individual cell structures. However, in samples with excessive mucus, the filtration membrane can become obstructed, potentially leading to diminished retrieval of epithelial cells.
The SurePath method, using centrifugation and gravity sedimentation, often results in a smaller cell deposition area, typically 12.5mm in diameter. While density gradient centrifugation effectively removes non-diagnostic material like background blood and mucus, the cell arrangement can appear slightly thicker with some overlapping or three-dimensional cell clusters. This slight multilayering may require the cytologist to adjust the microscope’s focus more frequently during screening.
Compatibility with Molecular Testing
Both LBC platforms offer an advantage over conventional Pap smears by preserving the residual specimen for ancillary molecular diagnostics, such as human papillomavirus (HPV) testing. The difference in the preservative fluid base—ThinPrep’s methanol versus SurePath’s ethanol—affects the viability and protocol for these secondary tests. While both media support successful DNA extraction for HPV testing, the specific FDA-approved protocols and required instrumentation for downstream testing may differ.
Studies have shown that the ThinPrep method can result in a higher rate of insufficient residual volume for certain molecular assays, such as the Hybrid Capture 2 (HC2) HPV test. Conversely, the SurePath system is associated with a lower rate of insufficient residual volume for this type of testing. Additionally, the ThinPrep residual specimen has a longer shelf life of up to three months at room temperature, compared to approximately three weeks for the SurePath specimen.
Comparative Detection Rates
Clinical studies comparing the two LBC methods have yielded complex results, though a consensus on their performance relative to conventional cytology has emerged. While some studies suggest the two platforms are largely equivalent in their ability to detect cervical lesions, large-scale data sets often show minor differences. One large analysis found the use of the SurePath system was associated with an 8% increase in the detection rate of high-grade lesions (CIN II+) compared to conventional cytology.
The same analysis indicated that implementing the ThinPrep system did not result in a significant alteration in the detection rates of high-grade lesions compared to conventional cytology. Differences in preparation, particularly SurePath’s ability to retain the collection device and effectively clear obscuring material, are hypothesized to contribute to its marginally higher detection rate for high-grade lesions. The rate of unsatisfactory slides, where the sample cannot be analyzed, is consistently lower for SurePath compared to ThinPrep, indicating a technical advantage in sample preparation.