BACKGROUND: Observational data on composite scores often comes with missing component information. When a complete-case (CC) analysis of composite scores is unbiased, preferable approaches of dealing with missing component information should also be unbiased and provide a more precise estimate. We assessed the performance of several methods compared to CC analysis in estimating the means of common composite scores used in axial spondyloarthritis research. METHODS: Individual mean imputation (IMI), the modified formula method (MF), overall mean imputation (OMI), and multiple imputation of missing component values (MI) were assessed either analytically or by means of simulations from available data collected across Europe. Their performance in estimating the means of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), the Bath Ankylosing Spondylitis Functional Index (BASFI), and the Ankylosing Spondylitis Disease Activity Score based on C-reactive protein (ASDAS-CRP) in cases where component information was set missing completely at random was compared to the CC approach based on bias, variance, and coverage. RESULTS: Like the MF method, IMI uses a modified formula for observations with missing components resulting in modified composite scores. In the case of an unbiased CC approach, these two methods yielded representative samples of the distribution arising from a mixture of the original and modified composite scores, which, however, could not be considered the same as the distribution of the original score. The IMI and MF method are, thus, intrinsically biased. OMI provided an unbiased mean but displayed a complex dependence structure among observations that, if not accounted for, resulted in severe coverage issues. MI improved precision compared to CC and gave unbiased means and proper coverage as long as the extent of missingness was not too large. CONCLUSIONS: MI of missing component values was the only method found successful in retaining CC's unbiasedness and in providing increased precision for estimating the means of BASDAI, BASFI, and ASDAS-CRP. However, since MI is susceptible to incorrect implementation and its performance may become questionable with increasing missingness, we consider the implementation of an error-free CC approach a valid and valuable option. TRIAL REGISTRATION: Not applicable as study uses data from patient registries.

Aberdeen Centre for Arthritis and Musculoskeletal Health University of Aberdeen Aberdeen UK

Amsterdam UMC Department of Rheumatology and Clinical Immunology and Department of Experimental Immunology Amsterdam Institute for Infection and Immunity University of Amsterdam Amsterdam The Netherlands

Center for Treatment of Rheumatic and Musculoskeletal Diseases Diakonhjemmet Hospital Oslo Norway

Centre for Rheumatology Research Landspitali University Hospital Reykjavik Iceland

Copenhagen Centre for Arthritis Research and the DANBIO Registry Centre for Rheumatology and Spine Diseases Centre for Head and Orthopaedics Rigshospitalet Glostrup Denmark

Copenhagen Centre for Arthritis Research Centre for Rheumatology and Spine Diseases Centre for Head and Orthopaedics Rigshospitalet Glostrup Denmark

Department of Clinical Sciences Lund Rheumatology Skåne University Hospital Lund University Lund Sweden

Department of Medicine Solna Clinical Epidemiology Division Karolinska Institutet Stockholm Sweden

Department of Rheumatology Aarhus University Hospital Aarhus Denmark

Department of Rheumatology Geneva University Hospital Geneva Switzerland

Department of Rheumatology Kuopio University Hospital Kuopio Finland

Department of Rheumatology University Hospital Zurich University of Zurich Zurich Switzerland

Department Rheumatology and Immunology Inselspital University Hospital Bern Bern Switzerland

Division of Rheumatology Department of Internal Medicine School of Medicine Kocaeli University Kocaeli Turkey

Faculty of Medicine University of Iceland Reykjavik Iceland

Faculty of Medicine University of Ljubljana Ljubljana Slovenia

Institute of Rheumatology Prague Czech Republic

Rheumatology Inflammation Center Helsinki University Hospital and University of Helsink Helsinki Finland

Rheumatology Radboud University Medical Centre Nijmegen The Netherlands

Section for Rheumatology Department for Neurology Rheumatology and Physical Medicine Helse Førde Førde Norway

Sociedade Portuguesa de Reumatologia Reuma Pt Lisbon Portugal

Statistics Group Swiss Clinical Quality Management Foundation Aargauerstrasse 250 Zurich 8048 Switzerland

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