Pauline MJ Steylen,1 Frank MMA van der Heijden,1 Witte JG Hoogendijk,2 Willem MA Verhoeven1,21Vincent van Gogh Institute for Psychiatry, Center of Excellence for Neuropsychiatry, Venray, the Netherlands; 2Erasmus University Medical Center, Department of Psychiatry, Rotterdam, the NetherlandsPurpose: To assess the point prevalence of undetected prediabetes (preDM) and diabetes mellitus (DM) in patients treated with antipsychotics and to compare metabolic parameters between patients with normoglycemia (NG), preDM, and DM. Furthermore, conversion rates for preDM and DM were determined in a 1-year follow-up.Patients and methods: In a naturalistic cohort of 169 patients, fasting glucose (FG) and hemoglobin A1c (HbA1c) criteria were applied at baseline and at follow-up after 1 year. A distinction was made between baseline patients diagnosed according to FG (B-FG) and those diagnosed according to HbA1c (B-HbA1c). Conversion rates in the 1-year follow-up were compared between B-FG and B-HbA1c.Results: At baseline, preDM and DM were present in 39% and 8%, respectively. As compared to patients with NG, metabolic syndrome was significantly more prevalent in patients with preDM (62% vs 31%). Although the majority of patients were identified by the FG criterion, HbA1c contributed significantly, especially to the number of patients diagnosed with preDM (32%). Regarding the patients with preDM, conversion rates to NG were much higher in the B-FG group than in the B-HbA1c group (72% vs 18%). In patients diagnosed with DM, conversion rates were found for B-FG only.Conclusion: PreDM and DM are highly prevalent in psychiatric patients treated with antipsychotic drugs. HbA1c was shown to be a more stable parameter in identifying psychiatric patients with (an increased risk for) DM, and it should therefore be included in future screening instruments.Keywords: severe mental illness, prediabetes, diabetes, fasting glucose, HbA1c