BackgroundInpatient hyperglycaemia and diabetes mellitus (DM) are common and are associated with an increased risk of complications and mortality. The severity of hyperglycaemia determines the rate of complications in patients treated in the emergency department (ED). Our aim was to examine whether determination of the capillary haemoglobin A1c (HbA1c) is a reliable method for detecting unknown diabetes and poor glycaemic control in the ED.MethodsA prospective observational study was conducted in adult (>18 years) patients treated in a single-centre ED. We compared the results of HbA1c levels measured by Bio-Rad in2it point-of-care device on a capillary blood sample and by the hospital laboratory.ResultsA total of 187 ED patients with an average age of 57.1 ± 19.2 years were studied. The mean HbA1c value was 5.78 ± 1.26 % by capillary POC testing and 6.10 ± 1.12 % by the hospital laboratory (correlation = 0.712, P < 0.001). A total of 17.1 % of cases had a prior diagnosis of DM. The diagnosis of DM (plasma glucose > 126 mg/dL and/or HbA1c > 6.5 %) was made in ten (5.4 %) additional cases (prior undiagnosed DM) for a total prior DM prevalence of 22.5 % (95 % CI 16.4–28.5 %). Capillary HbA1c detected 11 additional cases of unknown DM (5.9 %). A capillary HbA1c value greater than 6 % has a sensitivity of 85.7 % and specificity of 85.3 % for the screening of DM.ConclusionsDetermination of the capillary HbA1c in the ED is a reliable, fast, and simple system for the screening of unknown or uncontrolled DM.