**Question:** Diabetes mellitus is best diagnosed by

A. Fasting plasma glucose (FPG)
B. Oral glucose tolerance test (OGTT)
C. Random plasma glucose (RPG)
D. Hemoglobin A1C (HbA1C)

**Core Concept:** Diabetes mellitus is a group of chronic, systemic disorders characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both. These conditions are associated with long-term damage, dysfunction, and failure of various organs and systems. The diagnostic criteria for diabetes mellitus include impaired fasting glucose (IFG), impaired glucose tolerance (IGT), and diabetes mellitus.

**Why the Correct Answer is Right:** Fasting plasma glucose (FPG) is the most appropriate option for diagnosing diabetes mellitus due to its simplicity, ease of access, and cost-effectiveness. FPG is a single blood test taken after an 8-hour fasting period, measuring glucose levels postprandial. A fasting plasma glucose level of ≥7.0 mmol/L (126 mg/dL) is diagnostic for diabetes mellitus.

**Why Each Wrong Option is Incorrect:**

B. Oral glucose tolerance test (OGTT) is a more comprehensive test involving blood glucose measurements at several time points after an oral glucose load. While it is valuable in confirming gestational diabetes and diagnosing diabetes in pregnant women, it is not the first-line diagnostic tool due to its complexity, time-consuming nature, and increased risk of hypoglycemia.

C. Random plasma glucose (RPG) is a single blood glucose measurement taken at any time and is not considered reliable for diagnosing diabetes mellitus. A random plasma glucose level of ≥11.1 mmol/L (200 mg/dL) is indicative of diabetes, but it is not as accurate as FPG or OGTT.

D. Hemoglobin A1C (HbA1C) is a valuable tool for monitoring glycemic control over the past 2-3 months but is not as sensitive for diagnosing diabetes mellitus as FPG or OGTT. An HbA1C level of ≥6.5% (48 mmol/mol) is indicative of diabetes, but it is not the first-line test for diagnosis.

**Clinical Pearls:**

- Diabetes mellitus is a group of diseases characterized by chronic hyperglycemia due to defects in insulin secretion, insulin action, or both.
- A fasting plasma glucose level of ≥7.0 mmol/L (126 mg/dL) is diagnostic for diabetes mellitus according to World Health Organization (WHO) criteria.
- The American Diabetes Association (ADA) recommends a fasting plasma glucose level of ≥7.0 mmol/L (126 mg/dL) or a 2-hour plasma glucose level of ≥11.1 mmol/L (200 mg/dL) during an oral glucose tolerance test (OGTT).
- A random plasma glucose level of ≥11.1 mmol/L (200 mg/dL) is indicative of diabetes mellitus, but it is not the first-line diagnostic test.
- HbA1C level