Reporting Quality of Systematic Review Protocols in Environmental and Occupational Toxicology: A meta-epidemiology study

V2.0: Updated keywords and figure 3 regarding font size according to the EBT Journal and EBTC SAC. NOTE: This record is the complete, final study (i.e. Stage 2 Registered Report). Earlier versions include the protocol. Background: Systematic reviews (SR) are increasingly used in toxicology to inform regulatory decisions and research prioritization. SR protocols (SRP), which outline planned methods in advance, are essential for ensuring transparency and reproducibility. However, little is known about the extent and quality of protocol reporting in environmental and occupational toxicology. Objective: To assess publishing trends, characteristics, and reporting quality of SRP published in peer-reviewed journals within the fields of environmental and occupational toxicology. Methods: We searched four databases (PubMed, Scopus, Web of Science, EMBASE) using predefined search strings to identify peer-reviewed SRP examining the health effects of environmental or occupational exposures. Deduplication was performed in R. Protocols were screened independently by two reviewers using eligibility criteria based on the Population - Concept - Context (PCC) framework. Data extraction included bibliographic details, exposures, outcomes, evidence streams, and use of reporting checklists. Descriptive statistics were used to analyze publication trends and protocol characteristics. Reporting quality was assessed using a 15-item PRISMA-P-based tool, eleven of which were used to assess reporting quality. Results: We identified 3,787 records and included 37 peer-reviewed SRPs. These protocols were published between 2014 and 2024, with a peak in 2021. Most protocols focused on environmental toxicology (65%). Academic institutions served as the primary affiliations (76%), and most protocols targeted human evidence (78%). Air pollutants were the most common exposures, and the most frequently studied outcomes involved the nervous, respiratory, and female reproductive systems. Twenty-nine protocols (78%) mentioned using a reporting guideline, mostly PRISMA-P. All studies reported funding sources, but only 32% described the funder’s role. Twelve protocols met all 11 minimal reporting quality criteria. Protocols most often underreport data management procedures (51%), confidence in cumulative evidence (73%), and data collection/extraction processes (73%). In contrast, more than 90% of protocols reported the review rationale, information sources, and study selection process. Conclusions: A limited number of peer-reviewed protocols have been published in environmental and occupational toxicology. In general, the protocols address the minimum reporting requirements. Further research is needed to evaluate methodological quality in greater detail, how closely systematic reviews adhere to their pre-published protocols, and to expand the assessment to SRP published in the grey literature.