SummaryMastocytosis is a heterogeneous group of diseases characterized by abnormal proliferation of mast cells. Systemic mastocytosis (SM), in which abnormal mast cells are present in tissues beyond the skin, is divided into seven subcategories with varying degrees of severity and prognosis. Very little is known about the epidemiology of SM and its subcategories. This retrospective cohort study of 548 adults with SM diagnosed 1997-2010 was constructed using linked Danish national health registries. The most common subtype of mastocytosis was indolent SM (including urticaria pigmentosa) (n = 450; 82%), followed by SM with subtype unknown (n = 61; 11%), SM with associated clonal haematological non-mast cell lineage disease (n = 24; 4%), aggressive SM (n = 8; 2%), and mast cell leukaemia (n = 5; 1%). The incidence rate for SM (all subtypes including urticaria pigmentosa) was 0Á89 per 100 000 per year. Cumulative incidence was 12Á46 per 100 000, and the 14-year limited-duration prevalence as of 1 January, 2011 was 9Á59 per 100 000. This nationwide cohort from Denmark is the first population-based epidemiological study of mastocytosis. In this cohort of patients aged 15 years and older, SM was found to be overall relatively rare with notable variation by subtype for patient characteristics, survival and epidemiological measures.

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ObjectivesTo estimate prevalence of renal impairment, rate of decline in kidney function and changes in metformin use after decline in kidney function, in metformin initiators.Design, setting and participantsWe conducted this 2-country cohort study using routine data from northern Denmark and the UK during 2000–2011. We included metformin initiators among patients aged ≥30 years with medically treated diabetes.Main outcome measuresWe described patients’ demographics, comorbidity, co-medications and their estimated glomerular filtration rates (eGFR). Furthermore, we described the patients’ characteristics according to eGFR level. Finally, we examined the rate of any decline in eGFR and changes in metformin use within 90 days after first decline in eGFR during follow-up.ResultsWe included 124 720 metformin initiators in the 2 countries. Prevalence of eGFR <60 mL/min/1.73 m2 among metformin initiators was 9.0% in Denmark and 25.2% in the UK. In contrast, prevalence of eGFR values <30 mL/min/1.73 m2 among metformin initiators was 0.3% in Denmark and 0.4% in the UK. Patients with renal impairment were older and more likely to have received cardiovascular drugs. Incidence rate of decline in renal function was 4.92 per 100 person-years (95% CI 4.76 to 5.09) in Denmark and 7.48 per 100 person-years (95% CI 7.39 to 7.57) in the UK. The proportion of patients continuing metformin use, even after a first decline brought the eGFR below 30 mL/min/1.73 m2, was 44% in Denmark and 62% in the UK. There was no clinically significant dose reduction with decreasing baseline eGFR level discernible from the data.ConclusionsMild to moderate renal impairment was common among metformin initiators, while severe renal impairment was uncommon. Patients with severe renal impairment frequently continued receiving/redeeming metformin prescriptions even 90 days after eGFR decline.