Management Priority: Repeat Blood Test to Confirm Diagnosis
The first priority is to repeat the fasting blood glucose test to confirm the diagnosis of diabetes mellitus before initiating pharmacologic therapy. 1
Rationale for Diagnostic Confirmation
A single fasting blood glucose of 7.5 mmol/L (135 mg/dL) meets the threshold for diabetes (≥7.0 mmol/L or 126 mg/dL), but requires confirmation on a separate day unless the patient presents with unequivocal hyperglycemia and acute metabolic decompensation. 1
This patient has symptoms (polydipsia and fatigue) but does not have a hyperglycemic crisis or random glucose ≥11.1 mmol/L (200 mg/dL), so confirmation testing is required. 1
The American Diabetes Association explicitly states that diagnostic test results should be confirmed by repeat testing on a different day to rule out laboratory error. 1
It is preferable that the same test (fasting glucose) be repeated for confirmation, though two different tests above diagnostic thresholds (such as FBG and HbA1c) also confirm the diagnosis. 1
Why Not Start Metformin Immediately
Starting metformin before confirming the diagnosis would be premature and potentially inappropriate if the patient has impaired fasting glucose (IFG) rather than diabetes. 2
IFG is defined as fasting glucose 5.6-6.9 mmol/L (100-125 mg/dL), while diabetes requires ≥7.0 mmol/L (≥126 mg/dL). 1, 2
A single borderline elevated value at 7.5 mmol/L could represent laboratory variability, particularly given the preanalytic and analytic variability of glucose testing. 1
While metformin can delay diabetes onset in patients with IGT/IFG, lifestyle modifications are more effective than pharmacologic therapy for prediabetes. 2, 3
Optimal Confirmation Strategy
Repeat the fasting plasma glucose on a separate day, and consider adding HbA1c measurement to provide additional diagnostic information and establish baseline glycemic control. 1, 4
If the repeat FBG is ≥7.0 mmol/L (≥126 mg/dL), diabetes is confirmed. 1
HbA1c ≥6.5% (48 mmol/mol) would also confirm diabetes if the repeat FBG is elevated. 1
If both tests are above diagnostic thresholds, the diagnosis is definitively established. 1
Management After Confirmation
Once diabetes is confirmed, metformin becomes the appropriate first-line pharmacologic therapy alongside lifestyle modifications (nutrition counseling and physical activity). 1, 5, 2
Metformin is indicated for metabolically stable patients (A1C <8.5% and asymptomatic) with normal kidney function. 1
Lifestyle intervention targeting 5-7% weight loss and at least 150 minutes of moderate physical activity weekly should be initiated concurrently. 2
The patient should receive comprehensive diabetes self-management education. 1
Common Pitfalls to Avoid
Do not skip confirmation testing unless the patient has unequivocal hyperglycemia with acute symptoms or random glucose ≥11.1 mmol/L. 1
Do not assume diabetes based on family history and mild symptoms alone—up to 40% of people who develop diabetes have normal glucose tolerance 5 years before diagnosis. 3
Do not delay confirmation testing for weeks—repeat within days to establish the diagnosis promptly and initiate appropriate management. 1