Lung nontumor
Other nonneoplastic disease
Acute fibrinous and organizing pneumonia

Topic Completed: 1 January 2018

Minor changes: 13 December 2019

Revised: 30 January 2019, last major update January 2018

Copyright: (c) 2003-2018,, Inc.

PubMed search: Acute fibrinous and organizing pneumonia

Related topics: Acute respiratory distress syndrome (ARDS) / diffuse alveolar damage (DAD)

Akira Yoshikawa, M.D.
Andrey Bychkov, M.D., Ph.D.
Page views in 2019: 3,629
Page views in 2020 to date: 2,395
Cite this page: Yoshikawa, A. and Fukuoka, J. Acute fibrinous and organizing pneumonia. website. Accessed August 3rd, 2020.
Definition / general
Essential features
  • It is a newly proposed subacute interstitial pneumonia similar to organizing pneumonia or organizing diffuse alveolar damage
  • Histologically, remarkable fibrin deposition (or "fibrin balls") along with plugs of organizing pneumonia in air spaces are characteristic
  • Also called acute fibrinous organizing pneumonia
  • Usually bilateral or sometimes unilateral lobes of the lung
  • In the initial study, acute fibrinous and organizing pneumonia (AFOP) was described as a possible variant of diffuse alveolar damage because of its similar aggressive behavior and mortality rate (Arch Pathol Lab Med 2002;126:1064)
  • However, recent studies and case reports have found that the clinical course and prognosis of AFOP is better and closer to that of organizing pneumonia (J Clin Pathol 2015;68:441, Chin Med J (Engl) 2015;128:2701)
  • Nowadays, AFOP is considered a histological variant of organizing pneumonia or a different type of lung disease similar to organizing pneumonia, which sometimes follows an aggressive course
  • Some idiopathic AFOP may be due to infection of undiagnosed bacteria
Clinical features
  • Most patients present with mild to moderate subacute respiratory failure (J Clin Pathol 2015;68:441, Chin Med J (Engl) 2015;128:2701)
    • Fever, fatigue and malaise
    • Cough
    • Dyspnea
    • Sputum or sometimes hemoptysis
    • Duration of symptoms before diagnosis is 1 - 4 weeks
  • Some patients may follow fulminant course, need mechanical ventilation and die of the disease, similar to diffuse alveolar damage (Arch Pathol Lab Med 2002;126:1064, Medicine (Baltimore) 2016;95:e4073)
  • Abnormal chest auscultation
    • End inspiratory fine crackles in affected lobes
  • Mild to moderate restrictive or obstructive pattern in pulmonary function tests (Chin Med J (Engl) 2015;128:2701)
    • Decreased total lung capacity (TLC)
    • Decreased forced vital capacity (FVC)
    • Decreased diffusing capacity of the lung for carbon monoxide (DLCO)
  • Based on clinical features, radiology and histology
    • No unique clinical or radiological findings have been identified to date
    • Open chest lung biopsy is recommended
      • Transbronchial lung biopsy or computed tomography guided needle lung biopsy may be diagnostic if clinical and radiological features are suggestive enough
    • Acute fibrinous and organizing pneumonia can be a background pattern with other disease present
      • If the specimen is too small and the main lesion is not included, acute fibrinous and organizing pneumonia can be underdiagnosed (Int J Clin Exp Pathol 2014;7:4493)
  • Increased C reactive protein
  • Increased serum surfactant proteins A and D
  • Increased serum ferritin may predict prognosis
  • Occasional positive sputum bacterial culture
  • Negative serum antibodies of connective tissue diseases and hypersensitivity pneumonitis
Radiology description
  • Simple chest radiography
    • Bilateral or unilateral ground glass opacity and consolidation
  • High resolution computed tomography (Radiographics 2013;33:1951):
    • Variable images, similar to organizing pneumonia
      • Typically, patchy mixture of ground glass opacity and consolidation
      • Size varies from a few centimeters to a whole lobe
    • Rapidly progressive variant may show bilateral diffuse opacity, similar to diffuse alveolar damage
Radiology images

Images hosted on other servers:

Chest radiograph

Acute fibrinous organizing pneumonia in left upper lobe

AFOP in right lower lobe

Multifocal opacities

Before and after corticosteroid therapy

Before disease onset and before / after corticoid therapy

Prognostic factors
Case reports
  • In general, corticosteroid pulse therapy with / without cyclophosphamide improves the symptoms and prognosis (Arch Pathol Lab Med 2002;126:1064, J Clin Pathol 2015;68:441, Chin Med J (Engl) 2015;128:2701)
  • Treatment for underlying cause is also important for secondary acute fibrinous and organizing pneumonia
    • Antibiotics are effective for acute fibrinous and organizing pneumonia induced by bacterial infection
    • It is questionable if antibiotics can be a general therapeutic choice or not
  • Mechanical ventilation may be necessary for aggressive type
Gross description
  • Multiple patchy consolidated lesions
  • Ill defined, soft to firm gray areas
  • Mild increase in weight
  • Alveoli are filled with reddish fibrinous exudates
Microscopic (histologic) description
  • Major findings
    • Dominant findings of intra-alveolar fibrin, so called "fibrin ball"
      • Involves more than 20% of the alveolar spaces in the lesion
      • Neutrophils are usually scanty or absent
    • Organizing pneumonia: fibroblastic plugs in alveolar sacs and ducts with loose collagen matrix
    • Diffuse and patchy distribution
  • Minor findings
    • Mild to moderate interstitial changes
      • Lymphoplasmacytic infiltrate
      • Alveolar septal expansion with myxoid connective tissue
      • Limited within areas of fibrinous lesion
    • Type 2 pneumocyte hyperplasia
  • Pertinent negative findings; need to rule out secondary causes and other lung disease if present
    • Hyaline membranes
    • Eosinophilic inflammation
    • Extensive bronchopneumonia or abscess
    • Granulomatous inflammation
    • Vasculitis including capillaritis
    • Areas of necrosis
    • Marked dense fibrosis or honeycombing
  • See J Clin Pathol 2015;68:441
Microscopic (histologic) images

Images hosted on PathOut server:

Images contributed by Akira Yoshikawa, Nagasaki University Hospital:

Case 1: acute fibrinous and organizing pneumonia
Suspected due to drug induced lung injury; 56 year old man

Low power

Medium power

High power

Case 2: idiopathic acute fibrinous and organizing pneumonia
87 year old man (nonsmoker) presented 2 months ago with fever,
dyspnea on exertion and bilateral opacities on chest radiograph
Piperacillin / tazobactam was not effective; transbronchial lung biopsy
was performed and 2 samples were taken from the right upper lobe

Low power

Medium power

Elastica van Gieson staining

Fibrin balls

Images hosted on other servers:

Images contributed by Dr. Yale Rosen:

Acute fibrinous and organizing pneumonia

Other images:

Fibrin balls in air spaces

Fibrin balls in air spaces

Organizing pneumonia of AFOP


Prominent intra-alveolar fibrin

Positive stains
Negative stains
Differential diagnosis
Board review style question #1
Which two of the following findings are against the histological diagnosis of acute fibrinous and organizing pneumonia?

  1. Eosinophilic infiltration
  2. Hyaline membranes
  3. Lymphocytic infiltration
  4. Organizing pneumonia
  5. Type 2 pneumocyte hyperplasia
Board review answer #1
A and B. Eosinophilic infiltration suggests eosinophilic pneumonia. Hyaline membranes suggest diffuse alveolar damage.
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