| Summary: | ABSTRACT: Objective: Earlier cases have documented a discrepancy between glycated hemoglobin A1c (HbA1C) levels and glycemic control in patients with diabetes and previously diagnosed hereditary spherocytosis (HS). However, this has not been reported in patients with normal hemoglobin (Hb) and red blood cell (RBC) indices.Methods: A 22-year-old Caucasian woman with type 1 diabetes since age 11 was referred to Endocrinology for persistent hyperglycemia.Results: The initial HbA1C was 5.1% with normal Hb, hematocrit, mean cell volume, mean corpuscular Hb concentration, and RBC distribution. Given self-reported hyperglycemia, fructosamine was checked and was elevated (358 μmol/L, normal [nl] 200–285 μmol/L). Peripheral smear was normal. Professional retrospective continuous glucose monitoring (CGM) demonstrated marked hyperglycemic excursions, particularly in the evenings, with mean sensor glucose of 195 mg/dL ± 48, corresponding HbA1C of 5.5%, and fructosamine of 370 μmol/L. She later reported new diagnosis of HS in her mother, prompting workup for HS. A reticulocyte count was elevated at 5.4% (nl 0.5–1.5%), and an osmotic fragility test was supportive of HS with increased erythrocyte osmotic fragility.Conclusion: This case demonstrates a marked distortion of HbA1C in a patient with previously undiagnosed HS with normal Hb and other RBC indices. Given the relatively high frequency of mild HS, early testing should be considered in patients with an apparent discrepancy in HbA1C and meter readings, even in the absence of anemia. Alternate glycemic markers are necessary to monitor glucose control in affected patients.Abbreviations: CBC complete blood count CGM continuous glucose monitoring CSII continuous subcutaneous insulin infusion Hb hemoglobin HbA1C glycated hemoglobin A1c HS hereditary spherocytosis MDI multiple dose injection nl normal RBC red blood cell
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