**Question:** Most reliable test for screening of diabetes mellitus -

A. Fasting plasma glucose test
B. Postprandial plasma glucose test
C. Oral glucose tolerance test (OGTT)
D. Hemoglobin A1c (HbA1c) test

**Correct Answer:** D. Hemoglobin A1c (HbA1c) test

**Core Concept:**
Diabetes mellitus is a chronic disorder characterized by hyperglycemia due to defects in insulin secretion, insulin action, or both. The screening of diabetes mellitus aims to detect individuals with impaired glucose tolerance or impaired fasting glucose, which may progress to diabetes if left untreated.

**Why the Correct Answer is Right:**
Hemoglobin A1c (HbA1c) test is considered the most reliable test for screening diabetes mellitus because it provides an average blood glucose level over the past 2-3 months. This test is less affected by day-to-day fluctuations in blood glucose levels, making it a more stable indicator of long-term glycemic control.

**Why Each Wrong Option is Incorrect:**

A. Fasting plasma glucose test (FPG) is a useful test for screening diabetes, but it only measures glucose levels after an overnight fast. This may not accurately represent an individual's overall glucose control over the past months.

B. Postprandial plasma glucose test (PPG) measures glucose levels after a meal and is also less affected by day-to-day fluctuations, but it does not represent long-term glycemic control like HbA1c test.

C. Oral glucose tolerance test (OGTT) measures glucose levels at multiple timepoints after taking a specific oral glucose load. While it provides valuable information, it is more complex, time-consuming, and invasive compared to HbA1c test, making it less suitable for large-scale screening in clinical practice.

**Clinical Pearl:**

Hemoglobin A1c (HbA1c) test is widely used in clinical practice for diabetes screening, diagnosis, and monitoring treatment efficacy due to its advantages:

1. Represents average blood glucose levels over a longer period (2-3 months)
2. Less affected by day-to-day glucose fluctuations
3. Provides a single value for diagnosis and monitoring
4. Available in most clinical laboratories
5. Easy to interpret in terms of WHO diagnostic criteria and American Diabetes Association guidelines

**Explanation of the Core Concept:**

Diabetes mellitus is a group of diseases characterized by chronic hyperglycemia due to defects in insulin secretion, insulin action, or both. The HbA1c test is a valuable tool for diagnosing, monitoring, and managing diabetes due to its advantages over other tests.

**Why Core Concept is Important:**

Adequate screening and diagnosis of diabetes are crucial for early intervention, preventing complications, and improving patients' quality of life. The HbA1c test provides a single value that can diagnose diabetes and monitor its treatment efficacy, making it essential in clinical practice.

**Clinical Pearls:**

1. HbA1c test is recommended for diabetes