Repeat the Fasting Blood Sugar Test
The first next step is to repeat the fasting blood sugar test on a different day to confirm the diagnosis, as diabetes diagnostic criteria require confirmation of abnormal glucose values in the absence of unequivocal hyperglycemia. 1
Diagnostic Rationale
This patient's FBS of 7.2 mmol/L (approximately 130 mg/dL) exceeds the diagnostic threshold for diabetes of 7.0 mmol/L (126 mg/dL), but the symptoms described (occasional fatigue and thirst) are not classic unequivocal hyperglycemia. 1
Key diagnostic principles:
- Diabetes can be diagnosed with FPG ≥126 mg/dL (7.0 mmol/L) on two separate occasions. 1
- In the absence of unequivocal hyperglycemia (random glucose ≥200 mg/dL with classic symptoms), abnormal test results must be confirmed by repeat testing on a different day. 1
- It is preferable that the same test be repeated for confirmation, as there will be greater likelihood of concurrence. 1
- Day-to-day variance in fasting blood glucose values ranges from 12-15%, making single measurements unreliable for diagnosis. 1
Why Not Start Treatment Immediately
Starting metformin (Option C) before confirming the diagnosis would be premature because:
- The diagnosis has not been confirmed with a second test as required by diagnostic criteria. 1
- This patient's symptoms are mild ("occasional" fatigue and thirst) and do not constitute the unequivocal hyperglycemia that would allow diagnosis on a single test. 1
- Classic symptoms requiring only one test include polyuria, polydipsia, unexplained weight loss, or random glucose ≥200 mg/dL (11.1 mmol/L). 1
Clinical Context and Family History
While this patient has a significant risk factor (first-degree family history of diabetes increases diabetes risk with HR 1.19-1.21), 2 this does not eliminate the need for diagnostic confirmation. 1
Family history implications:
- First-degree family history increases type 2 diabetes risk even after controlling for other factors. 2
- Family history is a useful screening tool but does not replace formal diagnostic criteria. 3
Confirmation Testing Strategy
If the repeat FBS is ≥126 mg/dL (7.0 mmol/L), the diagnosis of diabetes is confirmed. 1
Possible outcomes of repeat testing:
- If both tests show FBS ≥126 mg/dL: Diabetes is confirmed, and treatment should be initiated. 1
- If repeat FBS is 100-125 mg/dL (5.6-6.9 mmol/L): Impaired fasting glucose (prediabetes) is diagnosed. 1
- If repeat FBS is <100 mg/dL: Consider alternative causes for symptoms or measure HbA1c for additional assessment. 1
Common Pitfall to Avoid
Do not dismiss the need for confirmatory testing based on family history or mild symptoms. The diagnostic criteria exist specifically because of the significant day-to-day variability in glucose measurements and the lifelong implications of a diabetes diagnosis. 1 Premature diagnosis and treatment without confirmation could lead to unnecessary medication burden, while premature reassurance without confirmation could delay appropriate intervention. 1