What is the most likely diagnosis for a 10-year-old patient with polyuria (excessive urine production), polydipsia (excessive thirst), an elevated Hemoglobin A1c (HbA1c) level, and a low C-peptide level?

Type 1 Diabetes Mellitus

The most likely diagnosis is Type 1 diabetes mellitus (Answer A), based on the combination of classic hyperglycemic symptoms (polyuria/polydipsia), elevated HbA1c, and critically low C-peptide indicating severe insulin deficiency.

Diagnostic Reasoning

C-peptide as the Key Discriminator

• The C-peptide level of 0.09 is profoundly low, indicating near-complete loss of endogenous insulin production, which is the hallmark of Type 1 diabetes 1
• C-peptide is secreted in equimolar amounts with insulin and reflects pancreatic beta-cell function; levels this low essentially exclude Type 2 diabetes and MODY, both of which retain significant insulin production capacity 1

HbA1c Interpretation in Pediatric Diabetes

• The HbA1c of 6.8% confirms chronic hyperglycemia, and research demonstrates that HbA1c >6.35% in children with polyuria/polydipsia has 100% sensitivity and specificity for Type 1 diabetes when combined with typical symptoms 2
• This HbA1c level, while not dramatically elevated, is diagnostic when interpreted alongside the clinical presentation and C-peptide 2

Clinical Presentation Analysis

• Polyuria and polydipsia are classic presenting symptoms of Type 1 diabetes in children, reflecting osmotic diuresis from hyperglycemia 1, 2
• The age of 10 years falls within the peak incidence range for Type 1 diabetes onset in childhood 1

Why Other Diagnoses Are Excluded

Type 2 Diabetes (Answer B)

• Type 2 diabetes would show preserved or even elevated C-peptide levels due to insulin resistance with maintained beta-cell function 1
• While obesity has made Type 2 diabetes more common in children, the profoundly low C-peptide definitively excludes this diagnosis 1

MODY (Answer C)

• MODY typically presents with mild hyperglycemia and preserved C-peptide levels, as these are genetic defects in beta-cell function rather than autoimmune destruction 1
• The severely depleted C-peptide is inconsistent with MODY pathophysiology 1

Stress-Induced Hyperglycemia (Answer D)

• Stress hyperglycemia is transient and would not produce an elevated HbA1c (which reflects 2-3 months of glycemic control) 2
• C-peptide would be normal or elevated in stress states, not profoundly suppressed 1

Critical Diagnostic Pitfall

• The relatively modest HbA1c of 6.8% should not mislead clinicians into dismissing diabetes—the combination with low C-peptide and classic symptoms is diagnostic 2
• Some children present early in their disease course before HbA1c becomes markedly elevated, but the insulin deficiency (low C-peptide) is already established 1, 2