From the Research
The next step in evaluating a patient with glucosuria and normal fingerstick glucose readings who is only on metformin is to check for renal glucosuria by ordering a comprehensive metabolic panel (CMP) and hemoglobin A1c. This discrepancy between urine glucose and normal blood glucose suggests either a low renal threshold for glucose or a measurement error. The CMP will assess kidney function through creatinine and BUN levels, while also checking electrolytes and confirming serum glucose levels. The A1c will provide information about longer-term glycemic control over the past 2-3 months. If these tests are normal, consider ordering a 24-hour urine collection to quantify glucose excretion and potentially genetic testing for familial renal glucosuria, a benign condition caused by mutations in the SGLT2 transporter gene 1. This condition reduces the kidney's ability to reabsorb glucose, causing glucosuria despite normal blood glucose levels. Metformin does not typically cause glucosuria on its own, so this finding warrants investigation to rule out early diabetic nephropathy or other renal tubular disorders that could affect glucose handling. Some studies suggest that metformin can be used in patients with diabetes and advanced chronic kidney disease 2, but this does not directly relate to the patient's current presentation of glucosuria with normal blood glucose levels. Additionally, the effectiveness of metformin in combination with SGLT2 inhibitors for glycemic control has been studied 3, but this is not relevant to the current diagnostic dilemma. Recent studies have also discussed the use of metformin in prediabetes and its potential effects on progression to diabetes 4, as well as the impact of lifestyle and metformin interventions on the risk of progression to diabetes and regression to normal glucose regulation 5, but these do not directly address the issue of glucosuria with normal fingerstick glucose readings. Therefore, the focus should remain on investigating the cause of glucosuria in this patient, with an emphasis on renal function and potential genetic causes.