In an adult with a strong family history of diabetes, occasional thirst and fatigue, and a fasting plasma glucose of 7.5 mmol/L, should the first step be to start metformin or to repeat the test to confirm diabetes?

Repeat Testing to Confirm Diagnosis Before Starting Treatment

The first priority is to repeat the fasting blood glucose test to confirm whether this patient has diabetes or prediabetes (Option B), because a single FBG of 7.5 mmol/L—while above the diagnostic threshold of 7.0 mmol/L—requires confirmation before initiating pharmacologic therapy. 1

Why Confirmation Testing Is Mandatory

• All abnormal diagnostic glucose results must be confirmed by repeat testing in the absence of unequivocal hyperglycemia (hyperglycemic crisis or classic symptoms with random glucose ≥11.1 mmol/L). 1
• This patient has only occasional thirst and fatigue—not the classic triad of polyuria, polydipsia, and weight loss—so these mild, non-specific symptoms do not replace the need for confirmatory testing. 1
• The 2025 ADA Standards of Care explicitly state that the same test should preferably be repeated to maximize concordance between measurements. 2, 1

The Diagnostic Algorithm

Step 1: Immediate Repeat FBG

• Repeat the fasting plasma glucose within days to weeks using the same certified laboratory method. 1
• Ensure a true 8-hour fast with no caloric intake before the draw. 1
• Use a certified laboratory assay, not point-of-care testing. 1

Step 2: Interpret the Repeat Result

Repeat FBG	Diagnosis	Action
≥7.0 mmol/L	Diabetes confirmed	Start metformin and comprehensive diabetes management [1]
5.6–6.9 mmol/L	Prediabetes	Lifestyle intervention (diet, exercise); annual monitoring; do not start metformin [1,3]
<5.6 mmol/L	Normal	Rescreen in 3 years [2]

Step 3: Consider Adding HbA1c for Clarification

• Simultaneous HbA1c measurement can resolve diagnostic uncertainty. 1
• When both FBG ≥7.0 mmol/L and HbA1c ≥6.5% are present, diabetes is definitively confirmed even if obtained on the same day. 1, 4
• If the two tests are discordant (one elevated, one normal), repeat the elevated test. 1, 4

Why Not Start Metformin Immediately (Option A Is Wrong)

• Initiating metformin without diagnostic confirmation violates ADA standards and may expose the patient to unnecessary medication risks if the initial result was laboratory error or represents prediabetes on repeat testing. 1
• FBG exhibits moderate test-retest variability, so a repeat measurement may fall below the diagnostic threshold into the prediabetes range (5.6–6.9 mmol/L). 1
• If repeat testing confirms prediabetes rather than diabetes, lifestyle modification—not metformin—is the first-line intervention. 1, 3
• Approximately two-thirds of people with prediabetes never develop diabetes, and one-third return to normal glucose regulation, so pharmacologic treatment of prediabetes is not routinely indicated. 3

Critical Pitfalls to Avoid

• Never diagnose diabetes on a single laboratory value unless a hyperglycemic crisis or classic symptoms with random glucose ≥11.1 mmol/L are present. 1, 4
• Do not delay confirmation testing for 3–6 months; such an interval is only appropriate when results are borderline after proper confirmation attempts. 1
• Ensure proper fasting conditions (≥8 hours, no caloric intake) for repeat FBG to avoid false results. 1
• Point-of-care glucose testing should not be used for diagnosis; only certified laboratory methods are acceptable. 1

Addressing the Family History and Symptoms

• A strong family history of diabetes is a major risk factor and appropriately prompted screening. 2
• However, risk factors and mild symptoms do not eliminate the requirement for confirmatory testing before labeling someone with a chronic disease and starting lifelong medication. 1
• The 2025 ADA guidelines recommend screening adults with a first-degree relative with diabetes, but screening and diagnosis are separate processes. 2

Evidence Quality and Consensus

• These recommendations are derived from consistent, high-quality ADA guidelines (2025 Standards of Care) and Diabetes Canada guidelines, representing expert consensus on diabetes diagnostic standards. 2, 1, 4
• The requirement for confirmation is based on recognition of laboratory measurement error and biological variability in glucose testing. 1