A 55-year-old man comes to the office to establish care after relocating to the area. The patient has a 6-year history of type 2 diabetes mellitus for which he takes metformin monotherapy. He feels well and has no other medical conditions. The patient does not use tobacco or alcohol. He consumes a balanced diet and jogs for 40 minutes every morning. Review of his blood glucose journal over the last few months shows fasting glucose levels of 150-180 mg/dL. Laboratory analysis shows a hemoglobin A1c level of 4%. Further evaluation with hemoglobin electrophoresis shows a hemoglobin A2 level of 7.5% (normal: 1.5%-3.5%). Which of the following is the most likely explanation for this patient's lower-than-expected hemoglobin A1c level?
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