A 36-year-old man with diabetes mellitus presents to the nurse practitioner for routine lab work. His disease has been well controlled on 1,000 mg of metformin for 1 year. Today his glycosylated hemoglobin (Hgb A1C) level is 8.2, but his fasting blood glucose was only 99. Which chronic condition falsely elevates Hgb A1C levels?