# Idiopathic Interstitial Pneumonias Classification

> Idiopathic interstitial pneumonias for NEET-PG Medicine compares UIP, NSIP, DIP, RB-ILD, COP and AIP by HRCT pattern, histology, steroid response and prognosis.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/idiopathic-interstitial-pneumonias-classification
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: Harrison's Principles of Internal Medicine; Davidson's Principles and Practice of Medicine
- How to cite: "Idiopathic Interstitial Pneumonias Classification", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/idiopathic-interstitial-pneumonias-classification

## Direct answer

Idiopathic interstitial pneumonias are six major diffuse lung diseases of unknown cause, classified by HRCT pattern, histology and clinical course as UIP, NSIP, DIP, RB-ILD, COP and AIP. Idiopathic pulmonary fibrosis is the UIP pattern and has the worst prognosis, whereas several of the others respond to corticosteroids. Diagnosis requires excluding known causes such as drugs, occupational exposure and connective tissue disease first.

## What you must remember

- **UIP:** The histological and HRCT pattern of idiopathic pulmonary fibrosis, with basal subpleural reticulation, honeycombing, temporal heterogeneity and fibroblastic foci; the worst prognosis.
- **NSIP:** The most common pattern in connective tissue disease, with bilateral basal ground glass and reticulation; cellular forms respond to steroids, while fibrotic forms do worse.
- **COP:** Patchy, often migratory consolidation, with the reverse halo sign on HRCT and Masson bodies on histology; steroid-responsive with a good prognosis.
- **DIP:** Seen in smokers, with ground glass and lower-zone predominance, and pigmented macrophages filling alveolar spaces uniformly on histology.
- **RB-ILD:** Also a smoking-related pattern, with centrilobular nodules and upper-zone ground glass, and pigmented macrophages in respiratory bronchioles; improves on stopping smoking.
- **AIP:** An acute, ARDS-like illness with bilateral ground glass and consolidation, diffuse alveolar damage on histology and very high mortality.
- **Diagnosis:** Exclude known causes first, then use HRCT, since a typical UIP pattern in the right clinical setting may not need biopsy, which is reserved for indeterminate patterns.

## Matching the HRCT pattern to the histology

Each idiopathic interstitial pneumonia is recognised by pairing an HRCT pattern with a histological pattern, because the pair predicts both the response to treatment and the prognosis. The table summarises the features most often tested in the exam.

| Pattern | HRCT key feature | Histology key feature | Prognosis and treatment |
|---|---|---|---|
| UIP (IPF) | Basal subpleural reticulation with honeycombing | Temporal heterogeneity with fibroblastic foci | Poor; antifibrotics, not steroids |
| NSIP | Bilateral basal ground glass with reticulation | Uniform temporal pattern; cellular or fibrotic | Variable; cellular forms respond to steroids |
| COP | Patchy consolidation, often migratory; reverse halo | Intraluminal plugs with Masson bodies | Good; steroid-responsive |
| DIP | Ground glass with lower-zone predominance | Pigmented macrophages filling alveolar spaces | Good with smoking cessation and steroids |
| RB-ILD | Centrilobular nodules, upper-zone ground glass | Pigmented macrophages in respiratory bronchioles | Good with smoking cessation |
| AIP | Bilateral ground glass and consolidation, ARDS-like | Diffuse alveolar damage | Very poor; high mortality |

Prognosis tracks the balance between fibrosis and cellular inflammation, so UIP sits at one end and the smoking-related and organising forms at the other. In practice, a typical UIP pattern in the right clinical setting can be diagnosed without biopsy, while an indeterminate HRCT points towards histology.

## High-yield viva angles

Most errors come from labelling every fibrosing lung disease as IPF, or from treating IPF with the steroid regimen that works for other patterns.

- **Trap:** Labelling every fibrosing interstitial disease as IPF. **Correct:** IPF is the UIP pattern only, and only after known causes such as drugs, occupational exposure and connective tissue disease are excluded.
- **Trap:** Treating IPF with corticosteroids. **Correct:** Steroids are not beneficial in IPF and are not recommended, while antifibrotic drugs are the standard therapy.
- **Trap:** Overlooking smoking cessation in DIP and RB-ILD. **Correct:** Stopping smoking is the cornerstone of management for these patterns, and many patients improve after it.
- **Trap:** Calling a patchy, migratory consolidation recurrent pneumonia and missing COP. **Correct:** COP presents this way, and the reverse halo sign on HRCT should prompt the diagnosis.

## Frequently asked questions

### Which idiopathic interstitial pneumonia is idiopathic pulmonary fibrosis?

Idiopathic pulmonary fibrosis is the idiopathic interstitial pneumonia with the UIP pattern on HRCT and histology. It has the worst prognosis of the group and is treated with antifibrotic drugs rather than corticosteroids.

### What is the reverse halo sign in cryptogenic organising pneumonia?

The reverse halo sign is a central ground-glass opacity surrounded by a ring of consolidation on HRCT. It is classically associated with cryptogenic organising pneumonia, though it can also be seen in other conditions.

### Why does cellular NSIP have a better prognosis than idiopathic pulmonary fibrosis?

Cellular NSIP has a better prognosis than idiopathic pulmonary fibrosis because its histology shows a uniform temporal pattern and it responds to corticosteroids. Fibrotic NSIP can be harder to treat, so the histological subtype matters.

### Which idiopathic interstitial pneumonia shows Masson bodies on histology?

Masson bodies, plugs of granulation tissue within bronchioles and alveolar ducts, are a classic histological feature of cryptogenic organising pneumonia. Their presence separates it from the other idiopathic interstitial pneumonias.