Limitations of Hemoglobin A1c for the Diagnosis of Posttransplant Diabetes Mellitus

Background. Posttransplant diabetes mellitus (PTDM) is usually detected 2 to 3 months after transplantation by fasting plasma glucose (fPG) >= 7.0 mmol/L (>= 126 mg/dL) and/or 2 hr post-challenge plasma glucose >= 11.1 mmol/L (>= 200 mg/dL) during an oral glucose tolerance test (OGTT). Recently, glycosylated hemoglobin (HbA1c) of 6.5% or higher (>= 47.5 mmol/mol) has been proposed as an alternative diagnostic criterion (the HbA1c criterion). We aimed to assess the sensitivity of applying the HbA1c criterion alone or in combination with a single measurement of fPG of 7.0 mmol/L or higher (>= 126 mg/dL) at 10 weeks after transplantation as screening tests for the diagnosis of PTDM. Methods. From 1999 to 2011, measurements of fPG, HbA1c, and OGTT were performed in 1,619 nondiabetic renal transplant recipients. Results. The HbA1c criterion detected 38.0% of patients with PTDM diagnosed with the standard diagnostic criteria. The specificity was 86.3%. When the HbA1c threshold value was lowered to 6.2% (44.3 mmol/mol), sensitivity increased to 57.8% with a corresponding reduced specificity of 80.4%. A combination of the HbA1c criterion and fPG of 7.0 mmol/L or higher (126 mg/dL) at 10 weeks after transplantation improved diagnostic precision with a sensitivity of 77.7% and a specificity of 96.1%. Conclusion. The proposed diagnostic HbA1c criterion failed to detect most cases of PTDM, and one of four cases of PTDM was detected by OGTT alone. This indicates that the HbA1c threshold value likely needs to be lowered for renal transplant recipients and supports continued use of OGTT as a diagnostic tool for detection of PTDM.