Background
Literature screening and study selection is a one of the most resource consuming tasks during the conduct of a systematic review [Reference Nussbaumer-Streit, Ellen and Klerings1]. During comprehensive literature searches for a systematic review, reviewers usually screen hundreds or thousands of records obtained from various databases, such as PubMed, in random order to identify the (often few) relevant hits to include in the systematic review. Machine learning algorithms have been developed to reduce the workload of manual screening by reordering and reranking the obtained records from the database searches based on their relevance [Reference Cierco Jimenez, Lee and Rosillo2,Reference van de schoot3].
ASReview is one such machine learning assisted screening tool developed at the University of Utrecht, the Netherlands, in 2019 [4,Reference Schoot, Bruin and Schram5]. It is open-source, free to use for noncommercial use, and it has an extensive GitHub community [6]. ASReview employs a supervised machine learning algorithm, which is updated after each decision made by the reviewer, to continuously reorder the records [7]. The reviewer may choose to screen all or only a proportion of the full sample assuming that most, or all, relevant records are shown among the first records. A quick summary of how to use ASReview can be found in Appendix 1.
ASReview’s developers reported that one had to screen 8% to 33% of all records to identify 95% of relevant studies (“95% recall”) based on four simulation studies [Reference Schoot, Bruin and Schram5]. A case study in health economics [Reference Wolcherink, Pouwels, van Dijk, Doggen and Koffijberg8] reported to screen 8% of their sample to obtain 100% recall. Based on these case studies, one may save up to 92% of time and resources at the cost of missing 5% of relevant hits. A collection of scientific articles related to ASReview can be found elsewhere [9]. We tested ASReview to reduce the workload related to literature screening for our continuously updated database of immunotherapy trials, the Cancer Immunotherapy Evidence Living (CIEL) Library [10,Reference Boesen, Hirt and Düblin11].
Materials and methods
Our case study’s objectives were to test ASReview’s feasibility and to identify advantages and limitations compared to a traditional literature screening setup with two human reviewers screening all retrieved records in random order. We did not formally compare sensitivity/specificity and used resources for an ASReview assisted single-screener versus a traditional double-screener setup.
Information source and search strategy
We systematically searched PubMed as of May 2, 2023 (Appendix 2). We included interventional trials of tumor-infiltrating lymphocytes as the treatment for any type of cancer [10], regardless of the trial design (e.g., single-arm or parallel group), blinding, randomization, and type of control (e.g., placebo, another cancer drug, usual care, or no treatment).
ASReview configuration
We used ASReview (version 1.2, Python 3.8.16, published April 12, 2023) and ASReview’s default configuration (naïve Bayes classifier; term-frequency- inverse document frequency; certainty-based sampling; and dynamic resampling). One reviewer (KB) screened title/abstract and full-texts, and ambiguous cases were resolved with another reviewer (PJ). Records were screened based on title/abstract. When this was not enough to decide, the full text was obtained to make a final decision about inclusion or exclusion using the digital object identifier (DOI). If the DOI was not available, we searched PubMed or Google using the verbatim title.
Two-phase screening workflow
We split our screening procedure into two phases: In phase 1 (enriched training phase), we created an enriched “training sample” by using the PubMed filter “Clinical study.” We anticipated that many relevant records would be tagged with this filter. One reviewer (KB) screened all retrieved records and labeled them as “relevant” or “irrelevant.” In phase 2 (screening phase), we then merged the fully labeled phase 1 dataset with the remaining unscreened pool of records. This way we trained the algorithm using the training sample from phase 1 (all records now labeled as “relevant” or “irrelevant”). We applied an arbitrary stopping rule of 100 consecutive irrelevant records, which has been used in another case study [Reference Van Dijk, Brusse-Keizer and Bucsán12].
Evaluation
We narratively evaluated the tool’s feasibility regarding setup and interface. We noted whether there were important advantages and limitations compared to traditional screening setups of two human reviewers screening all records.
Results
Search results and training phase (phase 1)
The PubMed search returned 14.004 records. In phase 1 (enriched training phase), 604 records were tagged with the “Clinical Study” filter and imported into ASReview (Figure 1). We selected 5 known irrelevant and 20 known relevant records as “prior knowledge,” that is, ineligible and eligible tumor-infiltrating lymphocyte clinical trial publications that we knew already. See recall curve for phase 1 in Figure 2.
Figure 1. The two-phase screening process. Abbreviations: n = number.
Figure 2. Recall curves.
Note: The yellow lines show the identified relevant records, and the blue lines show the theoretical recall curve if the records were screened and identified randomly.
The screening phase (phase 2)
Using ASReview’s DataTools package [13], we merged our sample of labelled records from phase 1 with the remaining unscreened sample of PubMed records (i.e., those records not filtered with “Clinical Study”) for a final sample of 13.994 records (10 records were deduplicated). This partly labelled dataset was then screened. We screened 872 further records before reaching our stopping rule of 100 consecutive irrelevant records. We included 59 further eligible records, see recall curve in Figure 2.
Screening summary
In total, we screened 1476 records (11%) of the total sample; 136 (0.97%) were relevant and 1340 were irrelevant. We stopped screening after 100 consecutive irrelevant records, at which there were 12.518 (89%) unscreened records. As we did not screen the full sample, we do not know the true number of relevant records and cannot estimate the 50%, 95%, and 100% recall rates.
Evaluation
We identified several advantages and limitations compared to traditional screening tools, summarized below and highlighted in Table 1.
Table 1. Advantages and limitations of ASReview
Abbreviation: DOI = Digital object identifier; PMID = PubMed ID.
Setup
Compared to other commonly used screening software (e.g., Rayyan or Covidence), ASReview does not provide a ‘“Software as a Service’” product, where the user logs into a website and uses the software without having to install it locally. Therefore, set up is required on a local computer (‘“local installation’”) or on a server/cloud provider (‘“ASReview Lab Server’”). The installation requires no knowledge of Python, other than the ability to install Python on one’’s own computer. Some features to prepare and modify datasets are not part of the ASReview main code base, but are offered by ASReview as separate Python packages, for example, “ASReview Datatools” [13]. The functionalities of these Python packages are accessible via the command line and requires basic knowledge about working with Unix shells.
Mandatory decision-making
During the screening of records in ASReview, the user must always make a final decision whether to include or exclude a record before proceeding to the next record. It is not possible to ‘“skip’” a record while further information is retrieved. In conventional literature screening tools, like Covidence or Rayyan, one can skip a record until there is sufficient information to make a final decision. Often immediate decisions cannot be made due to incomplete information, for example, if the article is behind paywall; the DOI is missing and one has to manually search for the title; the record is hidden in a journal supplement, or the study authors must be contacted to retrieve additional information [Reference Mullan, Flynn and Carlberg14,Reference Meursinge Reynders, Ladu and Di Girolamo15]. In these scenarios it is a limitation that one cannot postpone the decision for a certain record, while working in parallel to solve the uncertainty. One potential solution is to in- or exclude the record tentatively and add a note (e.g., “awaiting full text access”). These records can then later be retrieved (using the “find notes” filter) and the decisions be reverted, if necessary. Each decision influences the algorithm and it may influence ASReview’’s performance if irrelevant records are tentatively included. The obligatory decision-making will lead to scenarios that require prespecified rules to ensure a standardized and reproducible workflow. In scenarios when there are no abstracts and/or the decisive information comes from the full text (which ASReview has no access to) or from other external information, such as author correspondence, it would be desirable with a feature to include records without training the algorithm. Such inclusions may potentially obscure the algorithm’’s performance.
Single-screener setup
The default single-reviewer setup may worry traditional systematic reviewers who are used to a two-reviewer setup. One may argue that a single reviewer assisted with ASReview as a secondary reviewer can substitute a traditional two-reviewer setup. Empirical testing and comparison of ASReview assisted single-screener versus double-screener setups are needed to assess the specificity, sensitivity, and used resources of each setup. It is possible to set up a double reviewer workflow having two reviewers screen the same dataset (using the same algorithm configuration and training it on the same records); the reviewers can screen independently, and once a target sample is reached (x percentage of total sample or y number of consecutive irrelevant records), the datasets can be exported (e.g., as excel files). The files can then be compared and uncertainties can be resolved like in regular systematic reviews. It should also be highlighted that it is possible to work collaboratively multiple screeners on the same dataset (using ASReview Lab Server). This may be desirable when screening very large datasets or when records need to be screen within a certain timeframe. Multiple screener setups may likely introduce interrater variability, which should be acknowledged. The interface links to the full text using the DOI only. In phase 1 (training phase), the DOI was missing for 10% of the records. The developers may consider including additional full text identifiers, such as the PubMed Identifier (PMID), which is available from imported PubMed records.
Discussion
A well-planned use of ASReview may be beneficial in many evidence synthesis projects requiring manual screening of large numbers of records. We cannot make generalizable estimations on the time saving potential based on our single methodological case study. The potential time and resource savings must be weighed against the risk and impact of missing eligible studies if one decides to not screen all records. For topics with a high risk of missing relevant hits (e.g., a topic new to the reviewers, or a topic with heterogenous terminology), it may be difficult to prespecify a minimum of records to screen, for example, 5%, 10%, or 50% of the total sample. Stopping rules on how to safely use active learning systems like ASReview have been published by the ASReview developers [Reference Boetje and van de Schoot16]. The authors propose to screen a certain proportion (e.g., 10%) of the total sample and that a certain number of consecutive irrelevant records must be passed (50 records is mentioned). These recommendations seem to be arbitrarily selected and are not based on empirical testing. Another strategy is to use ASReview exclusively as a second reviewer and have the human reviewer screen all retrieved records. This method (theoretically) reduces the human screening hours with 50% in comparison with a standard double-screener setup. The generation of “evidence corpuses,” that is, labeled datasets adhering to specific population-intervention-comparison-outcome (PICO) inclusion criteria could be used and shared across different research projects. For instance, our CIEL dataset of tumor-infiltrating lymphocyte trial publications may be used to train algorithms in other systematic reviews on the same, or similar, topics. Importantly, in our use case of building a continuously updated database of clinical trials, the value of using ASReview may likely increase for each update of the PubMed search as the algorithm is refined.
Limitations of our study
In phase 1 (enriched training phase), we knew many (20 of 77; 25%) of the relevant records, which may have inflated the algorithm’’s performance in comparison to a smaller training sample. The arbitrary stopping rule of 100 consecutive records must also be questioned. It is important to empirically assess the “sweet spot” between searching the lowest number of records with the highest recall rate. This will likely have to be decided on a case-by-case situation and it likely depends on the sample size (larger sample, smaller threshold?), field and consistency of terminology across publications, and the reviewers’ prior knowledge of the literature and thus the size of the training sample. We screened 11% of the sample and we may have missed entire clusters of eligible records if these appeared further down the ranking. We did not systematically note which records were full-text assessed, so we cannot ascertain whether there were systematic differences (e.g., year of publication or journal) between records assessed based on title/abstract or full-text. Finally, our case study pertained to one clinical topic, thus the generalizability is limited.
Conclusions
Machine learning assisted software like ASReview has the potential to revolutionize the study selection in systematic reviews. Users must be aware of several caveats while using such supervised machine learning algorithms, which are constantly updated and where every decision affects the performance of the software. If a thorough ‘“scenario-based’” protocol is prespecified, the benefits may exceed the harms. Systematic reviewers are encouraged to test such machine learning tools and to publish their results to help establish an empirical foundation to guide best practice of transparent use of artificial assisted screening software in evidence synthesis.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/cts.2025.10173.
Data availability
There are no data associated with this article. The original ASReview project file can be obtained from the authors by request.
Author contributions
Kim Boesen: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing; Pascal Dueblin: Software, Writing – review and editing; Lars G. Hemkens: Funding acquisition, Writing – review and editing; Perrine Janiaud: Funding acquisition, Validation, Writing – review and editing; Julian Hirt: Conceptualization, Data curation, Methodology, Supervision, Validation, Visualization, Writing – review and editing.
Funding statement
The CIEL-Library project was funded by the Krebsliga beider Basel (grant KLbB-5577-02-2022). The funder did not have any influence on the design and results of this work.
Competing interests
RC2NB (Research Center for Clinical Neuroimmunology and Neuroscience Basel) is supported by Foundation Clinical Neuroimmunology and Neuroscience Basel, unrelated to this work. RC2NB has a contract with Roche for a steering committee participation of LGH, unrelated to this work.
Ethics approval
Not applicable.
Consent for publication
Not applicable.
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