**Question:** Criteria for diagnosis of diabetes mellitus is

A. Fasting plasma glucose ≥7.0 mmol/L
B. Random plasma glucose ≥11.1 mmol/L in an individual with classic symptoms of diabetes
C. Plasma glucose ≥11.1 mmol/L 2 hours after a 75g oral glucose tolerance test (OGTT)
D. HbA1c ≥6.5% (48 mmol/mol)

**Correct Answer:** D. HbA1c ≥6.5% (48 mmol/mol)

**Core Concept:**
Diabetes mellitus is a group of chronic conditions characterized by high blood glucose levels. The term "diabetes mellitus" comes from the Latin words for "sweet urine" due to the characteristic smell of glucose in the urine of affected individuals. There are three main types of diabetes:

1. Type 1 Diabetes: Autoimmune disease where the pancreas cannot produce insulin.
2. Type 2 Diabetes: Insulin resistance and/or inadequate insulin secretion, leading to high blood glucose levels.
3. Gestational Diabetes: High blood glucose levels during pregnancy due to hormonal changes.

**Why the Correct Answer is Right:**
HbA1c, also known as glycated hemoglobin, is a marker of long-term glucose control in the blood. It represents the average blood glucose levels over the past 2-3 months. An HbA1c level of ≥6.5% (48 mmol/mol) is considered diagnostic for diabetes mellitus. This is because a high HbA1c indicates chronic hyperglycemia and uncontrolled blood glucose levels.

**Why Each Wrong Option is Incorrect:**

A. Fasting plasma glucose ≥7.0 mmol/L (126 mg/dL) is a single measurement taken after an overnight fast. While it can indicate diabetes, it does not provide a comprehensive picture of blood glucose control over time, which is essential for diagnosis.

B. Random plasma glucose ≥11.1 mmol/L (200 mg/dL) in an individual with classic symptoms of diabetes is a single measurement without any context about the patient's symptoms or fasting and postprandial glucose levels. The presence of symptoms alone is not sufficient for diagnosis of diabetes.

C. Plasma glucose ≥11.1 mmol/L (200 mg/dL) 2 hours after a 75g oral glucose tolerance test (OGTT) is a single measurement taken after a glucose load. It provides information about the blood glucose response to a glucose load but lacks information about the patient's fasting and postprandial glucose levels, which are crucial for diagnosis.

**Clinical Pearls:**

Glycated hemoglobin (HbA1c) testing is a more reliable diagnostic tool for diabetes compared to fasting plasma glucose or the oral glucose tolerance test (OGTT) because it provides a comprehensive picture of blood glucose control over time. The World Health Organization (WHO) recommends HbA1c ≥6.5% (48 mmol/mol) for diabetes diagnosis in adults, considering various factors like lifestyle, medications, and other co-