Making protocols available with the article improved evaluation of selective outcome reporting

To compare primary outcomes reported in publications, protocols and registries and to evaluate the contribution of available protocols to assess selective outcome reporting (SOR) as compared with registration alone. We included all randomized controlled trials (RCTs) published in 2015 and 2016 in the five leading general medical journals. For each RCT, we evaluated whether the protocol was available and searched for registration. We extracted all primary outcomes reported in publications, registries, and protocols. We evaluated whether SOR was suspected (i.e., at least one discrepancy in primary outcomes), unclear, or not suspected based on comparisons of publications and (1) trial registration alone or (2) protocols in addition to registration. Selective outcome reporting was suspected for 77/274 (28.1%), unclear for 30 (10.9%), and not suspected for 167 (60.9%) when comparing publications and trial registration alone. With protocols available, the classification changed for 38 RCTs (13.9%): 11 not suspected of SOR based on registration became suspected of SOR with protocols available, and 27 with unclear assessment based on registration became suspected of SOR (n = 7) and not suspected of SOR (n = 20) with protocols available. Compared to registration alone, making protocols available allows for a more precise evaluation of SOR. •In this methodological review based on 274 trials published in high impact factor journals with a protocol available, we found that selective outcome reporting (SOR) could not be evaluated for 30 RCTs (10.9%) because of insufficient description of the primary outcomes in trial registries.•With protocols available, there were only three RCTs (1.1%) for which the risk of SOR could not be assessed, and we suspected additional cases of SOR that were not identified with trial registries alone.•Making protocols available along with the article may improve evaluation of SOR.