The relationship between glycated haemoglobin levels and the risk of giant cell arteritis – a case–control study

OBJECTIVES: The EULAR core dataset for observational studies in GCA does not include glycated haemoglobin (HbA(1c)). A multivariable score to stratify the pre-test probability of GCA also does not include HbA(1c). There have been contradictory reports about diabetes mellitus being a risk factor for GCA. We report the first study analysing the relationship of pre-diagnosis HbA(1c) with the risk of GCA. METHODS: This was a single-centre retrospective case–control study conducted in Norfolk, UK. All GCA cases were diagnosed with imaging or biopsy. Each case was assigned two age- and sex-matched controls. The primary outcome measure was the glycaemic status (HbA(1c) categorized into euglycaemia, pre-diabetes or diabetes mellitus) at diagnosis between cases and controls. The HbA(1c) was compared between two groups using the Mann–Whitney U test. The glycaemic categorization was compared using the χ(2) test. RESULTS: One hundred and twelve cases and 224 controls were included. The median (interquartile range) of HbA(1c) of cases and controls was 40 (37, 43) and 41 (39, 47) mmol/mol (P < 0.001), respectively. Ten of 112 cases and 52 of 224 controls had diabetes mellitus. The χ(2) test demonstrated a significant interaction between glycaemic state and GCA (P = 0.006). Individuals with diabetes mellitus had an odds ratio (95% CI) of 0.32 (0.13, 0.74) (P = 0.008) of having GCA compared with euglycaemic individuals. CONCLUSION: HbA(1c) in the diabetic range reduces the probability of GCA. HbA(1c) should be considered in any multivariable score to calculate the risk of GCA, and in future development of diagnostic and classification criteria. There is a need for an epidemiological study looking at the possibility of a protective nature of diabetes mellitus against GCA or whether it is only a mimic.