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Idiopathic pulmonary fibrosis and nonspecific interstitial pneumonia should stay separate

Dept of Medicine, University of California, San Francisco, CA, USA.

To the Editors:

We read with interest the recent perspective by Maher et al. 1, challenging the current definition of idiopathic pulmonary fibrosis (IPF). Questioning dogma is critical to progress in science and the authors should be commended for doing so.

We agree that the relationship between disease entity (i.e. clinical diagnosis) and histopathological pattern is more complex than simply equating IPF with usual interstitial pneumonia (UIP), and that distinguishing idiopathic UIP from idiopathic nonspecific interstitial pneumonia (NSIP) can sometimes be difficult, even for expert clinicians, radiologists and pathologists 2. Indeed, the American Thoracic Society/European Respiratory Society Consensus Statement made these points quite deliberately 3.

However, we disagree with the authors' contention that "idiopathic UIP and idiopathic NSIP, sharing a common clinical phenotype, form a spectrum of disease with a common pathogenesis" 1. This blanket statement assumes a homogeneity to idiopathic NSIP and idiopathic UIP that is contrary to the evidence. The idea of a distinct clinical phenotype for idiopathic NSIP is supported by demographic, serological and survival differences seen when IPF and idiopathic NSIP patients are compared 4–6, and by the differences, not similarities, seen in gene expression profiles (six of 10 published NSIP profiles are distinct from UIP while four are similar) 7, 8.

It is our belief that idiopathic nonspecific interstitial pneumonia represents a collection of conditions including occult connective tissue disease, hypersensitivity pneumonia and, perhaps, truly idiopathic cases, and should not be lumped together clinically with idiopathic pulmonary fibrosis. We share the hope of Maher et al. 1 that new and emerging methods of categorising disease will allow us to better understand the relationship of histopathology to pathogenesis and to refine future definitions and diagnostic criteria.

Statement of interest

None declared.

REFERENCES

1. Maher TM, Wells AU, Laurent GJ. Idiopathic pulmonary fibrosis: multiple causes and multiple mechanisms?. Eur Respir J 2007;30:835–839.[Abstract/Free Full Text]
2. Flaherty KR, King TE Jr, Raghu G, et al. Idiopathic interstitial pneumonia: what is the effect of a multidisciplinary approach to diagnosis?. Am J Respir Crit Care Med 2004;170:904–910.[Abstract/Free Full Text]
3. American Thoracic Society. Idiopathic pulmonary fibrosis: diagnosis and treatment. International consensus statement. American Thoracic Society (ATS), and the European Respiratory Society (ERS). Am J Respir Crit Care Med 2000;161:646–664.[Free Full Text]
4. Kinder BW, Collard HR, Koth L, et al. Idiopathic nonspecific interstitial pneumonia: lung manifestation of undifferentiated connective tissue disease?. Am J Respir Crit Care Med 2007;176:691–697.[Abstract/Free Full Text]
5. Daniil ZD, Gilchrist FC, Nicholson AG, et al. A histologic pattern of nonspecific interstitial pneumonia is associated with a better prognosis than usual interstitial pneumonia in patients with cryptogenic fibrosing alveolitis. Am J Respir Crit Care Med 1999;160:899–905.[Abstract/Free Full Text]
6. Bjoraker JA, Ryu JH, Edwin MK, et al. Prognostic significance of histopathologic subsets in idiopathic pulmonary fibrosis. Am J Respir Crit Care Med 1998;157:199–203.[Web of Science][Medline] [Order article via Infotrieve]
7. Selman M, Pardo A, Barrera L, et al. Gene expression profiles distinguish idiopathic pulmonary fibrosis from hypersensitivity pneumonitis. Am J Respir Crit Care Med 2006;173:188–198.[Abstract/Free Full Text]
8. Yang IV, Burch LH, Steele MP, et al. Gene expression profiling of familial and sporadic interstitial pneumonia. Am J Respir Crit Care Med 2007;175:45–54.[Abstract/Free Full Text]

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