The Reid Index is a mathematical relationship that exists in a human bronchus section observed under the microscope. It is defined as ratio between the thickness of the submucosal mucus secreting glands and the thickness between the epithelium and cartilage that covers the bronchi. The Reid index is not of diagnostic use in vivo since it requires a dissection of the airway tube, but it has value in post mortem evaluations and for research.
- RI is the Reid Index
- wall is the thickness of the airway wall between the epithelium and the cartilage's perichondrium
- gland is the thickness of the mucous producing gland at the location of inspection.
A normal Reid Index should be smaller than 0.4, the thickness of the wall always more than double the thickness of the glands it contains. A greater value is the most pathognomonic indicator of chronic bronchitis.
Hyperplasia and hypertrophy of mucous glands as in chronic bronchitis causes them to be present at deeper levels in the bronchial wall and thicker in size, thus increasing the Reid Index beyond the normal value.