In chronic bronchitis, the pathological hallmark is an increase in the Reid index. What does the Reid index measure?

Explanation

## Reid Index in Chronic Bronchitis ### Definition and Measurement **Key Point:** The **Reid index** is defined as the **ratio of the thickness of the mucous gland layer to the total thickness of the bronchial wall**. $$\text{Reid Index} = \frac{\text{Mucous gland layer thickness}}{\text{Total bronchial wall thickness}}$$ ### Normal vs. Abnormal Values - **Normal Reid index:** < 0.4 (or approximately 40%) - **Chronic bronchitis:** Reid index > 0.5 (or approximately 50%) - This indicates **mucous gland hypertrophy and hyperplasia** ### Pathological Basis **High-Yield:** Chronic irritation from cigarette smoke and air pollutants leads to: 1. Hypertrophy of mucous glands 2. Hyperplasia of mucous glands 3. Increased mucus production 4. Disproportionate increase in gland layer relative to total wall thickness ### Clinical Significance **Mnemonic:** **REID** = **R**atio **E**valuates **I**nflammatory **D**amage **Clinical Pearl:** The Reid index is a **histological diagnostic criterion** for chronic bronchitis and helps differentiate it from emphysema, which shows parenchymal destruction without glandular changes. ### Diagnostic Criteria for Chronic Bronchitis - Clinical: Productive cough for ≥3 months/year for ≥2 consecutive years - Pathological: Reid index > 0.5 with mucous gland hyperplasia - Absence of other explanations for chronic cough [cite:Robbins 10e Ch 15]