Diagnosis Confirmation: Repeat Fasting Blood Sugar Test
The correct next step is to repeat the fasting blood sugar test (Option A) to confirm whether this patient has diabetes or prediabetes, as a single elevated fasting plasma glucose requires confirmation with repeat testing on a different day before making a definitive diagnosis. 1
Diagnostic Rationale
This 42-year-old patient presents with:
- FBS of 7.2 mmol/L (approximately 130 mg/dL) - which exceeds the diagnostic threshold of 126 mg/dL (7.0 mmol/L) for diabetes 2, 1
- Classic symptoms of occasional fatigue and thirst, which are consistent with hyperglycemia 1, 3
- Strong family history with maternal diabetes, placing him at higher risk 2, 1
Why Confirmation Testing is Required
The American Diabetes Association mandates that in the absence of unequivocal hyperglycemia with acute metabolic decompensation, diagnostic criteria must be confirmed by repeat testing on a different day. 1, 2 This is critical because:
- A single abnormal test result is insufficient for diagnosis unless the patient presents with hyperglycemic crisis 1
- Laboratory error must be ruled out through confirmation 2
- The same test (FBS in this case) should preferably be repeated for better concordance 2
Diagnostic Thresholds to Confirm
Upon repeat testing, the results will determine the diagnosis:
- Diabetes mellitus: FBS ≥126 mg/dL (7.0 mmol/L) on two separate occasions 2, 1, 3
- Prediabetes (Impaired Fasting Glucose): FBS 100-125 mg/dL (5.6-6.9 mmol/L) 1, 4
- Normal: FBS <100 mg/dL (5.6 mmol/L) 1
Why Other Options Are Incorrect
Option B (tolerate symptoms and return if worse) is dangerous and inappropriate because:
- The patient already has an elevated FBS above the diabetes threshold 1
- Classic symptoms of hyperglycemia are present 1, 3
- Delaying diagnosis risks progression to complications and acute hyperglycemic crisis 5, 6
Option C (start metformin immediately) is premature because:
- Diagnosis has not been confirmed with repeat testing as required by guidelines 1, 2
- Treatment should only begin after diagnostic confirmation 1
- The patient may have prediabetes rather than diabetes, which would alter the management approach 4
Clinical Pearls and Pitfalls
Common pitfall: Starting treatment based on a single abnormal glucose value without confirmation can lead to overdiagnosis and unnecessary medication exposure. 1
Important consideration: If this patient had presented with random plasma glucose ≥200 mg/dL (11.1 mmol/L) WITH classic symptoms of hyperglycemia, diagnosis could be made immediately without repeat testing. 1, 3 However, his FBS of 130 mg/dL, while elevated, requires the standard confirmation protocol.
Timing of repeat test: The repeat FBS should be performed on a different day, with the patient fasting for at least 8 hours before the test. 1, 3