Context: One in four hospitalized patients has diagnosed diabetes. The prevalence of unrecognized, or undiagnosed, diabetes among hospitalized patients is not well established.

Objective: To determine the prevalence of unrecognized probable diabetes in this patient population determined by elevated hemoglobin A1c level.

Design: Prospective observational cohort trial with retrospective follow-up of patients with elevated hemoglobin A1c levels and no diagnosis of diabetes. Hemoglobin A1c (HbA1c) levels were obtained for all patients.

Setting: Acute care general hospital.

Patients: 695 adult, non-obstetric patients admitted on 11 days in 2006.

Main Outcome Measures: Rate of unrecognized probable diabetes, defined as admission HbA1c >6.1% and no diagnosis of diabetes or treatment with anti-diabetic medications prior to or during their admission; and rate of unrecognized diabetes 1 year after discharge.

Results: Eighteen percent of hospitalized patients had elevated HbA1c levels without a diagnosis of diabetes. Random glucose levels poorly predicted elevated HbA1c levels (area under ROC curve, 0.60). Neither diagnosed diabetes nor HbA1c level was associated with length of stay or costs (p >0.1 for all comparisons). Only 15% of patients with elevated HbA1c levels who continued to receive care within the system studied had diabetes diagnosed in the year after the index admission.

Conclusions: Nearly one in five adult patients admitted to a large general hospital had unrecognized probable diabetes, based on elevated HbA1c levels. Random glucose levels during the hospital stay were poorly predictive of this condition. Few hospitalized patients with elevated HbA1c levels were diagnosed within the year following admission.