Liquid-Based Cytology in Cervical Cancer Screening: A Systematic Review and Meta-Analysis

Authors: Gerd Böhmer, Claudia Stolte, Tobias Vogelmann, Tino Schubert

Abstract

Introduction

While the conventional Papanicolaou (Pap) smear has long been the cornerstone of cervical cancer (CC) screening, it presents well-documented limitations in sample collection, processing, and interpretation. Liquid-based cytology (LBC) was introduced to address these drawbacks by offering more uniform cell samples and enabling standardized evaluation. However, due to ongoing debate and conflicting study results regarding their comparative diagnostic accuracy, a rigorous evidence-based assessment is needed. The objective of this systematic review and meta-analysis was to systematically assess the diagnostic accuracy of LBC compared to conventional Pap cytology in the detection of cervical precancerous and cancerous lesions.

Methods

PubMed and Cochrane Library were systematically searched. Comparative studies of LBC and conventional Pap cytology in primary CC screening were included. Outcomes included unsatisfactory sample rate, abnormal histology-confirmed cytology detection rate (ADR) CIN2+ lesions (cervical intraepithelial neoplasia), squamous cell carcinoma (SCC), and glandular abnormalities. Random-effects models were used to estimate risk ratios (RRs). A two-sided p value of 0.05 was used to determine statistical significance.

Results

Ninety-seven studies were included. LBC had a lower unsatisfactory sample rate than conventional Pap cytology (RR: 0.63, 95% CI: 0.47–0.85, p = 0.003). The detection rate of SCC was similar (RR: 1.77, 95% CI: 0.96–3.26, p = 0.07). The glandular abnormality detection rate showed no overall difference (RR: 1.23, 95% CI: 0.75–2.02, p = 0.42), but a subgroup analysis (FDA-approved tests) showed superiority of LBC (RR: 1.52, 95% CI: 1.18–1.96, p = 0.001). LBC had a 35% higher abnormal cytology detection rate confirmed by histology (RR: 1.35, 95% CI: 1.20–1.52, p < 0.0001) and a 27% higher detection of confirmed CIN2+ abnormalities (RR: 1.27, 95% CI: 1.11–1.45, p = 0.0006).

Conclusion

LBC improves CIN2+ and histology-confirmed abnormal detection rates while reducing unsatisfactory samples, supporting its use in CC screening. SCC detection rates were similar. Findings suggest that LBC, particularly when using FDA-approved tests, enhances glandular abnormality detection. Further research should evaluate its cost-effectiveness in low-resource settings.

Introduction

Cervical cancer (CC) remains among the leading causes of cancer-related burden in women, particularly in low- and middle-income countries where access to effective screening programs is limited [1]. Invasive CC begins with a precursor lesion called squamous intraepithelial lesion (SIL) which can develop years before development of invasive carcinoma [2]. In 1988, at a meeting in Bethesda (Maryland, US), SIL were classified into main Atypical squamous cells of undermined significance (ASC-US, ASC-H).Low-grade squamous intraepithelial lesion (LSIL) which corresponds to the histopathology grade mild dysplasia cervical intraepithelial neoplasia (CIN I).High-grade squamous intraepithelial lesion (HSIL) which corresponds to histopathology moderate and severe dysplasia, CIN II and CIN III [3].Glandular lesions (atypical glandular cells, AIS).

Early detection of cervical precancerous lesions and timely intervention are crucial for reducing morbidity and mortality and has been shown to minimize CC incidence [4, 5]. Over the past decades, cytological screening has played a pivotal role in CC prevention [6]. The Papanicolaou (Pap) smear, introduced in the mid-20th century, has been the cornerstone of CC screening programs, leading to a marked decline in CC incidence and mortality in countries with well-established screening protocols [6].

However, despite its widespread use, the conventional Pap test has several limitations that have spurred the development of alternative screening These limitations of the conventional Pap smear are sample collection, processing, and interpretation [7]. A major drawback is that the sample is manually transferred onto a glass slide, which can lead to uneven cell distribution with thick layers of cells and a poor fixation of cells. Additionally, interobserver variability and the subjective nature of cytological interpretation contribute to inconsistent diagnostic accuracy. To address these issues, liquid-based cytology (LBC) was introduced in the 1990s as an alternative to conventional Pap testing to allow for computer-assisted screening. Unlike the conventional smear technique, where cells are directly transferred onto a slide, LBC involves suspending the collected cells in a liquid fixating medium before processing. This approach offers several potential it reduces sample degeneration, provides a more uniform and representative sample, and allows for automated or semiautomated standardized cytological evaluation, potentially improving detection rates [8]. Moreover, LBC enables the simultaneous use of adjunct tests, such as human papillomavirus (HPV) testing, which has become increasingly relevant in modern CC screening strategies [9].

Despite these technological improvements, there is ongoing debate regarding the comparative diagnostic accuracy of LBC versus conventional Pap cytology. Some studies suggest that LBC may be superior in detecting cervical precancerous and cancerous lesions [10–12], while others report comparable or even lower performance than the traditional method [13–15]. Furthermore, differences in study design, population characteristics, and cytological interpretation criteria have contributed to variability in reported outcomes. These inconsistencies highlight the need for a rigorous, evidence-based evaluation of the relative diagnostic accuracy of LBC and Pap testing. The objective of this systematic review and meta-analysis was to systematically assess the diagnostic accuracy of LBC compared to conventional Pap cytology in the detection of cervical precancerous and cancerous lesions.

Methods

A systematic literature search was conducted in PubMed and the Cochrane Library to identify studies comparing LBC and conventional cytology (Pap smear) in CC screening. The search strategy was based on the PICOS framework, considering relevant terms for population, intervention, comparison, outcomes, and study design. The full search strategy, including all keywords and Boolean operators, is provided in the online supplementary Material (for all online suppl. material, see https://doi.org/10.1159/000551747).

Studies published until July 25, 2024, were included if they evaluated women undergoing primary CC screening and compared LBC with conventional cytology. No restrictions were placed on age, country, or manufacturer of the LBC test. Only comparative studies, including randomized and non-randomized controlled trials as well as prospective and retrospective observational studies, were considered. Studies were excluded if they focused on secondary screening, feasibility assessments, or described only technical features without diagnostic performance data. Non-original studies, such as reviews and meta-analyses, were also excluded.

After the removal of duplicates, two independent reviewers screened the titles and abstracts for relevance. Full-text articles were retrieved for studies that met the inclusion criteria or lacked sufficient information for exclusion at the abstract level. Discrepancies were resolved by consensus or, if necessary, by consulting a third reviewer. A standardized data extraction form was used to collect information on study characteristics, population details, diagnostic methods, outcome measures, and key results. The extracted data were cross-checked for accuracy and consistency.

To ensure comparability in the calculation of abnormality histology-confirmed cytology detection rates (ADRs), studies were included only if they reported at least the following four cytological ASC-US, ASC-H, LSIL, HSIL, and carcinoma. If the category “carcinoma” was not reported, the ADR was not extracted from the study. Additionally, if glandular lesions were reported, they were included in the ADR calculations. The abnormalities detected on cytological smears were required to be confirmed by the reference method specified in each respective study.

The Bethesda system provided a cytological framework for classifying abnormal Pap test results. Smears for the CIN2+ outcome were considered for analysis if they were classified as LSIL+, i.e., LSIL, HSIL, or carcinoma.

Cervical intraepithelial neoplasia (CIN), a histopathology classification system, was used to describe the precancerous changes in cervical epithelial cells. CIN is divided into three CIN 1 (mild dysplasia), CIN 2 (moderate dysplasia), and CIN 3 (severe dysplasia and carcinoma in situ), based on the extent of epithelial involvement by abnormal cells.

In addition to the ADR and CIN2+ detection rates, the following key outcome measures were First, unsatisfactory sample rate, defined as the proportion of cytology samples deemed inadequate for interpretation due to insufficient fixation and poor sample quality, or the presence of obscuring factors such as blood, mucus, and thick cell layers. Second, squamous cell carcinoma (SCC) detection the proportion of cases where SCC was identified through cytological and subsequent histopathological assessment. Third, the glandular abnormality detection rate (GDR), defined as the rate at which glandular lesions, including atypical glandular cells and AIS, were detected through cytology and confirmed by histopathology.

These outcome measures were analyzed to compare the diagnostic performance of LBC versus conventional Pap cytology in CC screening. The risk of bias (RoB) and applicability was evaluated by two reviewers independently using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS)-2 tool [16]. In case of any disagreement, a third reviewer was asked for assessment, and consensus was reached by discussion. Studies were assessed for RoB regarding the dimensions of (1) patient selection, (2) index test, (3) reference standard, and (4) flow and timing. Applicability was evaluated by dimensions I to III.

Meta-analyses were performed to estimate pooled measures of the outcomes. Heterogeneity across studies was assessed using the I2 statistic, which quantifies the proportion of variability due to true differences rather than chance. An I2 value above 50% was considered indicative of substantial heterogeneity, and values above 75% were classified as high heterogeneity. When substantial heterogeneity was observed, random-effects models were used to calculate pooled effects, otherwise, a fixed-effect model was used. Pooled risk ratios and corresponding 95% confidence intervals (CIs) were calculated using the inverse-variance method within a random-effects model with Hartung-Knapp adjustment. p values <0.05 were defined as an indicator of statistical significance.

Subgroup analyses were conducted to evaluate variations in diagnostic performance based on cytology classification thresholds (e.g., ASC-US+, LSIL+, HSIL+). These subgroup analyses were prespecified in the study protocol, except for additional exploratory analyses performed post hoc to investigate specific discrepancies in results.

Subgroup analyses were conducted specifically for FDA-approved LBC tests, including ThinPrep^®^ (Hologic, Inc.) and BD SurePath™ (Becton, Dickinson and Company), to evaluate potential variations in diagnostic performance. These subgroup analyses focused solely on manufacturer-specific comparisons and were prespecified in the study protocol. All statistical analyses were conducted using Cochrane Review Manager RevMan [17], and results were reported following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [18]. The PRISMA 2020 Checklist is provided as online supplementary Material. The systematic review protocol was registered under 10.37766/inplasy2025.6.0028.

Results

Study Selection

A total of 1,491 records were identified through database searches in PubMed (n = 1,368) and the Cochrane Library (n = 123). After removing 118 duplicates, 1,373 records remained for title and abstract screening. Of these, 1,127 records were excluded based on the PICOS question. An additional 5 studies were identified through manual searching of reference lists and relevant systematic reviews. In total, 246 full-text articles were assessed for eligibility. After applying the predefined inclusion and exclusion criteria, 97 studies were included in at least one meta-analysis assessing unsatisfactory sample rate, abnormal histology-confirmed cytology detection rate (ADR), CIN2+ abnormalities, SCC, and glandular abnormalities, all histology-confirmed (see online suppl. Material for a full list of all included studies and the outcomes used in our meta-analyses). The study selection process is summarized in Figure 1.

Quality Assessment

Figure 2 presents the RoB and applicability assessment for the 97 studies included in at least one meta-analysis. A considerable proportion of studies were classified as having a high RoB in the domain of “flow and timing,” primarily because not all participants received the same reference standard after cytological assessment. A low RoB in this domain would require that all women, including those with negative cytology results, undergo histopathological verification through colposcopy-directed biopsy or excisional procedures.

Studies in which women with negative cytology results were only followed up with imaging or repeat cytology, rather than undergoing a histopathological reference test, were considered to have a high RoB in the “flow and timing” domain. However, given the ethical and clinical considerations in CC screening, where unnecessary invasive procedures are avoided, these studies were assessed with an overall low RoB, despite the limitations in the verification process.

Synthesis of Results

Unsatisfactory Sample Rate

The meta-analysis of unsatisfactory sample rates included 80 studies with a total of 7,401,210 cytological samples assessed using conventional Pap cytology, with 75,532 classified as unsatisfactory. In comparison, 66,381 unsatisfactory slides were identified among 5,979,632 smears evaluated using LBC. The random-effects model yielded a risk ratio of 0.63 (95% CI: 0.47–0.85; p = 0.003), indicating that LBC was significantly less likely to yield unsatisfactory samples compared to conventional cytology.

Abnormality Histology-Confirmed Cytology Detection Rate

The meta-analysis evaluating ADR included 29 studies with 10,398,889 samples assessed using LBC and 3,618,939 samples evaluated with conventional Pap cytology. Abnormal findings were detected in 12,643 cases using LBC and 24,255 cases using Pap cytology, resulting in an overall abnormal histology rate of 1.22% for LBC and 0.67% for Pap cytology. The random-effects model yielded a risk ratio of 1.35 (95% CI: 1.20–1.52, p < 0.0001) (Fig. 3), indicating that LBC was associated with a 35% higher abnormality detection rate compared to conventional Pap cytology with high statistical significance.

Detection Rate of CIN2+ Abnormalities

The detection rate of CIN2+ histopathology abnormalities included 27 studies with 1,043,536 samples assessed using LBC and 3,620,614 samples evaluated with conventional Pap cytology. CIN2+ was detected and confirmed in 3,007 cases using LBC and 6,439 cases using Pap cytology, resulting in an overall CIN2+ detection rate of 0.29% for LBC and 0.18% for Pap cytology. The random-effects model yielded a risk ratio of 1.27 (95% CI: 1.11–1.45; p < 0.0006), indicating that LBC was associated with a 27% higher detection rate of CIN2+ lesions compared to conventional Pap cytology (Fig. 4).

SCC Detection Rate

The meta-analysis evaluating the detection rate of SCC on histology included 1,797,139 samples analyzed with LBC and 790,229 samples assessed with conventional Pap cytology. SCC was detected in 191 cases using LBC and 48 cases using Pap cytology, resulting in an overall detection rate of 0.01% for both methods. The random-effects model yielded a risk ratio of 1.77 (95% CI: 0.96–3.26; p = 0.07), suggesting no significant difference in the risk of detecting SCC between LBC and Pap cytology (Fig. 5).

Glandular Abnormality Detection Rate (GDR)

The meta-analysis evaluating the detection rate of glandular abnormalities on histology included 328,328 samples analyzed using LBC and 489,089 samples assessed with conventional Pap cytology. Glandular abnormalities were detected in 167 cases using LBC and 153 cases using Pap cytology, resulting in an overall detection rate of 0.05% for LBC and 0.03% for Pap cytology. The random-effects model yielded a risk ratio of 1.23 (95% CI: 0.75–2.02; p = 0.42), indicating no significant difference in the risk of detecting glandular abnormalities between the two cytological techniques (Fig. 6).

The subgroup meta-analysis evaluating the GDR for the FDA-approved LBC tests (SurePath and ThinPrep) included 272,356 samples assessed LBC and 442,761 samples evaluated with conventional Pap cytology. Glandular abnormalities were detected in 143 cases using LBC and 123 cases using Pap cytology, resulting in an overall detection rate of 0.05% for LBC and 0.03% for Pap cytology. The risk ratio of 1.52 (95% CI: 1.18–1.96, p = 0.001) indicates that LBC was associated with a significantly higher detection rate of glandular abnormalities compared to conventional Pap cytology when using only FDA-approved LBC tests (Fig. 7).

Discussion

This meta-analysis compared the diagnostic performance of LBC and conventional Pap cytology in detecting cervical abnormalities, including CIN2+, SCC, and glandular abnormalities. The findings indicate that LBC is associated with higher detection rates for ASC-US+ histology-confirmed cytology results and CIN2+ lesions, with a 35% and 27% increase, respectively, compared to conventional Pap cytology. Detection rates for SCC were similar between both methods, suggesting that LBC may improve sensitivity for precancerous lesions. Given the fact that the detection of precancerous lesions is the main goal in CC screening, not the detection of already invasive cancer, LBC shows significant advantages for CC screening. Regarding unsatisfactory sample rates, LBC demonstrated a significantly lower rate compared to conventional cytology, supporting its role in reducing the need for repeat testing. Additionally, the subgroup analysis for the GDR in FDA-approved tests revealed that LBC had a 52% higher detection rate compared to Pap cytology. These results suggest that LBC offers improved diagnostic accuracy, particularly for high-grade lesions and glandular abnormalities, while also ensuring better sample adequacy and the possibility of further reflex testing (e.g., HPV testing, immunocytology).

In studies conducted within a screening setting, only detection rates can be reported, not sensitivity. Sensitivity assessment would require confirmation of negative cytology results, which is generally not feasible in a screening context. Because women with negative cytology results typically do not undergo colposcopy or histological verification, it is impossible to determine the true-negative rate [19].

A notable and rare exception to this limitation is the landmark 2003 independent study by the French Society of Clinical Cytology [20]. This study remains uniquely robust because a colposcopy and biopsy were performed for every woman included, regardless of their cytological results. As highlighted by this trial, when a reference test is not systematically applied to all patients, the sensitivity of the cytological technique can be artificially inflated due to verification bias. Because most primary research does not employ the universal biopsy approach seen in this study, most available data are inherently limited to reporting detection rates rather than true sensitivity. By focusing our meta-analysis on histology-confirmed abnormality detection rates, rather than attempting to pool inflated sensitivity metrics from variably biased trials, we mitigate the impact of this verification bias.

Several previous meta-analyses have investigated the diagnostic performance of LBC and conventional Pap cytology, but differences in methodology, study populations, and outcome measures make direct comparisons challenging. The meta-analyses by Arbyn et al. [21] and Arybn [22] concluded that LBC is neither more sensitive nor more specific than conventional Pap cytology for detecting high-grade cervical intraepithelial neoplasia (CIN2+) [21, 22]. The evidence report compiled by Arbyn [22] for CC prevention guidelines incorporated findings from multiple sources but did not conduct a direct meta-analysis. Instead, it summarized sensitivity, specificity, and abnormality detection rates based on individual studies, with mixed conclusions regarding the superiority of LBC over conventional cytology. Additionally, previous meta-analyses often included mixed populations, combining primary screening cohorts with referred populations, which may have introduced bias in estimating true screening effectiveness.

Similarly, Abulafia et al. [23] focused on ThinPrep and conventional cytology but did not include unsatisfactory sample rates, SCC detection rates, or other critical performance measures assessed in our study.

More recent analyses, such as Pyo et al. [24], evaluated cytology for SIL and SCC, but did not distinguish between LBC and conventional Pap cytology in their sensitivity estimates. Additionally, Fontaine et al. [25] compared unsatisfactory sample rates between ThinPrep and SurePath but did not assess overall test performance. In contrast, our study provides a more comprehensive assessment, considering multiple diagnostic performance indicators, including unsatisfactory sample rates, SCC detection rates, CIN2+ detection rates, and glandular abnormalities while exclusively analyzing studies that directly compare LBC and conventional cytology [25].

In addition to evaluating these diagnostic parameters, our study provides a more recent and extensive dataset compared to prior meta-analyses. We include studies that consider the latest screening methodologies.

Conclusion

This meta-analysis demonstrates that LBC offers significant advantages over conventional Pap cytology in CC screening. LBC substantially improves the detection of CIN2+ lesions, abnormal histology, and, when using FDA-approved tests, glandular abnormalities, while also reducing unsatisfactory sample rates. Although SCC detection rates were comparable between methods, the enhanced identification of precancerous lesions highlights the superior clinical utility of LBC, given that early detection and treatment are central to effective screening programs.

By synthesizing data from 97 comparative studies, this analysis provides robust and up-to-date evidence supporting the implementation of LBC in routine screening. The improved sample adequacy and potential for reflex testing further strengthen its value in modern screening algorithms. However, cost-effectiveness considerations remain essential, particularly in low-resource settings where conventional cytology is still widely used. Future research should evaluate long-term outcomes, economic impact, and the role of LBC within HPV-based screening strategies to guide optimal integration into global CC prevention efforts.

Statement of Ethics

This manuscript reports on publicly available information only; no ethics approval was necessary.

Conflict of Interest Statement

Gerd Böhmer received honoraria from Hologic and Roche. Claudia Stolte received honoraria from Hologic. Tobias Vogelmann and Tino Schubert are owners and employees of LinkCare, which received honoraria from Hologic and BD.

Funding Sources

This work was supported by Hologic. Hologic paid a grant for study design, data collection, analysis, and manuscript preparation and had no role in decision to submit.

Author Contributions

Gerd Böhmer conceived and designed the study, contributed to data interpretation, and critically revised and finalized the manuscript. Claudia Stolte contributed to the interpretation of the results and critically revised the manuscript for important intellectual content. Tobias Vogelmann conducted the literature review, performed the statistical analysis, and drafted the initial version of the manuscript. Tino Schubert contributed to the literature review and participated in manuscript revision and final approval. All authors contributed to the manuscript, reviewed and approved the final version, and agreed to be accountable for all aspects of the work.