BACKGROUND Glycosylated hemoglobin (HbA1c) is commonly used to assess long-term blood glucose control in patients with diabetes mellitus. Numerous conditions including hemoglobinopathies can alter HbA1c measurements and cause misleading results. OBJECTIVE To report a 13-year-old male with Type 1 diabetes mellitus who had low HbA1c measurements, despite persistent hyperglycemia. METHODS HbA1c was initially measured by immunoassay. Hb electrophoresis was then employed to assess potential Hb variants. Electrospray ionization (ESI) tandem mass spectrometry of isolated Hb and gene sequencing of the Hbβ gene were used to specifically identify the Hb variant. RESULTS HbA1c measurement by immunoassay revealed an unusually low HbA1c of 3.9%. Hb electrophoresis revealed an aberrant Hb. The ESI mass spectrum of the intact Hb sample revealed a variant β-chain of 15,881 Da, 14 Da heavier than the mass of the normal Hb β-chain (15,867 Da). Sequence analysis of the 965.45 Da peptide suggested a substitution of valine (Val) to acetylated alanine (Ala). The DNA sequence of the patient's Hbβ gene revealed a single-base heterozygous mutation (GTG to GCG) at Base 2 of the codon of the first amino acid, producing a Val→Ala substitution, previously termed Hb-Raleigh. Because the acetylated N-terminal amino acid of the Hb-Raleigh β chain cannot be glycated, the HbA1c immunoassay will result in falsely low HbA1c levels. CONCLUSIONS In managing diabetic patients, knowledge of hemoglobinopathies influencing HbA1c determination methods is essential because hemoglobin variants may cause mismanagement of diabetes. Unusual results should prompt further analysis for a hemoglobinopathy as the potential cause of aberrant results.