Validation of Hemoglobin A1c in Patients With Inflammatory Arthritis Treated With Sulfasalazine

The purpose of this study is to examine whether the blood test Hemoglobin A1c (HbA1c) gives an accurate picture of blood glucose levels in patients with inflammatory arthritis who are treated with sulfasalazine. HbA1c is widely used to diagnose and monitor diabetes, but sulfasalazine can shorten red blood cell lifespan and thereby lower HbA1c values independently of actual glucose levels. This may lead to underdiagnosis of diabetes in patients who develop diabetes during sulfasalazine treatment, and to undertreatment in patients with known diabetes due to falsely reassuring HbA1c values. The study aims to answer two main questions: 1. How many patients treated with sulfasalazine have undiagnosed diabetes despite having HbA1c values below the diagnostic threshold? 2. Does HbA1c underestimate actual glucose levels when compared with continuous glucose monitoring (CGM) in patients with sulfasalazine-treated inflammatory arthritis, both in those with known diabetes and those that are not diagnosed with diabetes but have borderline HbA1c values (≥ 38 mmol/mol)?

Inclusion Criteria: * Age ≥18 years * Treatment with sulfasalazine for at least 2 months prior to inclusion * Inflammatory arthritis diagnosis (Reumatoid Arthritis, Reaktive Arthritis, Axial spa, Psoriatic spondylitis, and Juvenil artrit) * HbA1c ≥38 mmol/mol obtained at least 2 months after sulfasalazine initiation OR a diabetes mellitus diagnosis (Type 1 diabetes mellitus, Type 2 diabetes mellitus, Malnutrition-related diabetes mellitus, Other specified diabetes mellitus (andre specificerede former for diabetes), and Unspecified diabetes mellitus (uspecificeret diabetes)) * Can communicate in Danish * Informed consent including permission to upload glucose data and study ID to the Libreview Platform. Exclusion Criteria: * Systemic treatment or local injections with glucocorticoids within the previous 2 months or planned within the following 4 weeks * Clinical conditions interfering with the interpretation of HbA1c expect for sulfasalazine alterations in red cell lifespan (etc. Dapson treatment) * Allergy towards the adhesive used in the CGM * Considered ineligible for participating (e.g. patients without decision-making capacity, , malignancy, terminal illness, ect.)