Page 172 - Haematologica Vol. 110 - January 2025

ARTICLE - Multiple myeloma and the risk of infections Methods
Patients and controls
All symptomatic MM patients from the Swedish Myeloma Registry (SMR), diagnosed from 2008 to 2021 were included in the study. The SMR contains patient characteristics at diagnosis and first-line treatment with a coverage of >95%, compared to the Swedish Cancer Register.7 The treatment strategies in Sweden in the current study period have been reported elsewhere.8 In short, PI and IMiD were introduced after 2005 and MoAb in 2020. Immunotherapies, such as bispecific antibodies and chimeric antigen receptor (CAR) T-cell therapy, were not reimbursed in the study period, so the number of patients treated with immunotherapy is considered negligible. For each MM patient, four popula- tion-based controls matched by sex, year of birth, and county of residence were chosen randomly from the Swedish Total Population Register (TPR). The control subjects had to be alive and without preceding hematologic malignancy at the date of diagnosis of the corresponding MM patient. From the Swedish Patient Register, which captures information on discharge diagnosis from inpatient and outpatient care with high coverage and accuracy,9 we obtained information on infections using the International Classification of Diseases (ICD-10), from 5 years before the MM diagnosis to the end of follow-up which was December 31, 2022 or time of death/
C.H. Blimark et al.
emigration. Events were defined as the diagnosis of any specific infectious disorder leading to a hospital visit. Each infectious episode had to have occurred at different time points and at least 1 month apart. For severe acute respi- ratory syndrome coronavirus 2 (SARS-Cov-2) infection this interval was set to 3 months, as prolonged viral replication is common in immunosuppressed patients. Furthermore, the Cause of Death Register was used to identify causes of death among patients and controls. As a sensitivity analysis, the prescription of antibiotics from the Swedish Prescribed Drug Register10 was used as proxy for the incidence of infections in patients and controls. Data on comorbidities at time of diagnosis were retrieved from the Swedish Patient Register and the Swedish Cancer Register. In order to construct the Charlson Comorbidity Index from register-based data, the categories and weights proposed by the Royal College of Surgeons were used,11 including diagnoses registered within 5 years preceding myeloma diagnosis. Using the nationwide Cause of Death Register,12 we obtained information on date and cause of death for all subjects who had died up to De- cember 31, 2022. Approval was obtained from the Swedish Ethical Review Board for this study (2020-01729 and 2021- 06236-02).
Statistical analysis
Characteristics of patients and controls are presented as
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Figure 1. Consort diagram of the MMBaSe study (Multiple MyelomaBaSe); a linked database from population-based regis- ters in Sweden.