There are two different approaches to involve participants in consecutive rounds of a Delphi survey: (1) invitation to every round independent of response to the previous round (“all-rounds”) and (2) invitation only when responded to the previous round (“respondents-only”). This study aimed to investigate the effect of invitation approach on the response rate and final outcome of a Delphi survey. Both experts (N = 188) and patients (N = 188) took part in a Delphi survey to update the core outcome set (COS) for axial spondyloarthritis. A study with 1:1 allocation to two experimental groups (ie, “all-rounds” N = 187 and “respondents-only” N = 189) was built-in. The overall response rate was lower in the “respondents-only group” (46%) compared to the “all-rounds group” (61%). All domains that were selected for inclusion in the COS by the “respondents-only group” were also selected by the “all-rounds group.” Additionally, the four most important domains were identical between groups after the final round, with only minor differences in the other domains. Inviting panel members who missed a round to a subsequent round will lead to a better representation of opinions of the originally invited panel and reduces the chance of false consensus, while it does not influence the final outcome of the Delphi.