What is the next step for a 42-year-old patient with a family history of diabetes mellitus, symptoms of fatigue and polydipsia, and a random blood sugar of 7.2 mmol/L?

Next Step: Confirm Diabetes Diagnosis with Fasting Plasma Glucose

The first next step is to obtain a fasting plasma glucose (FPG) test to confirm the diagnosis of diabetes mellitus. 1, 2

Rationale for Fasting Plasma Glucose

• The patient's random blood sugar (RBS) of 7.2 mmol/L (approximately 130 mg/dL) is below the diagnostic threshold for diabetes, which requires a random plasma glucose ≥11.1 mmol/L (200 mg/dL) when classic symptoms are present 3

• Although the patient has classic symptoms of diabetes (fatigue and polydipsia) plus a family history, the RBS value alone is insufficient to diagnose diabetes 3, 1

• The American Diabetes Association recommends that in the absence of unequivocal hyperglycemia (random glucose ≥200 mg/dL with symptoms), the diagnosis must be confirmed with a fasting plasma glucose ≥7.0 mmol/L (126 mg/dL) on a separate occasion 3, 1

Diagnostic Algorithm

Step 1: Order Fasting Plasma Glucose

• The patient should fast for at least 8 hours before blood draw 3
• If FPG ≥7.0 mmol/L (126 mg/dL), diabetes is confirmed 1, 2
• If FPG is 5.6-6.9 mmol/L (100-125 mg/dL), this indicates impaired fasting glucose (prediabetes) 3

Step 2: Consider Additional Testing

• An HbA1c ≥6.5% can also confirm diabetes and provides baseline glycemic control information 3, 1
• If FPG and HbA1c are discordant (one above and one below diagnostic threshold), repeat the abnormal test for confirmation 3, 2

Why Not Other Tests First?

• A 2-hour oral glucose tolerance test (OGTT) is more cumbersome and less practical than FPG for initial diagnosis 3
• The American Diabetes Association emphasizes FPG as the preferred diagnostic test due to convenience and reproducibility 4, 5
• Random glucose testing alone (which was already done) is only diagnostic if ≥11.1 mmol/L with symptoms 3, 6

Important Clinical Considerations

• Do not delay testing: The patient has classic symptoms and a family history, making diabetes highly likely despite the borderline RBS 1, 6
• Avoid premature treatment: While the clinical picture is suggestive, formal diagnosis requires meeting established glucose criteria before initiating diabetes medications 3, 1
• Single exception: If the patient had a random glucose ≥11.1 mmol/L (200 mg/dL) with classic symptoms, diabetes could be diagnosed immediately without confirmatory testing 3

After Diagnosis Confirmation

Once diabetes is confirmed with FPG ≥7.0 mmol/L:

• Check for ketones to rule out diabetic ketoacidosis, especially if glucose is markedly elevated 1
• If glucose is ≥13.9 mmol/L (250 mg/dL) with symptoms of polyuria, polydipsia, and weight loss, initiate insulin therapy immediately while also starting metformin 1
• If metabolically stable with glucose <13.9 mmol/L and asymptomatic, metformin is the initial pharmacologic treatment if renal function is normal 1