An empyema can resemble a pleural effusion and can mimic a peripheral pulmonary abscess, although a number of features usually enable distinction between the two (see empyema vs. lung abscess). Features that help distinguish a pleural effusion from an empyema include:
Shape and location
- form an obtuse angle with the chest wall
- unilateral or markedly asymmetric whereas pleural effusions are (if of any significant size) usually bilateral and similar in size 4.
- lenticular in shape (bi-convex), whereas pleural effusions are crescentic in shape (i.e., concave towards the lung)
Findings on CT
Features suggestive of an empyema include:
- enhancing thickened pleura (see split pleura sign) whereas pleural effusion has thin imperceptible pleural surfaces
- locules of gas absent unless recent thoracocentesis
- obvious septations
- associated consolidation
- associated adjacent infection (e.g. subdiaphragmatic abscess)
Biochemistry / Microbiology
- pH is < 7.0
- glucose level < 40mg/dL 4
- white cells or organisms present
- 1 Landay MJ, Conrad MR. Lung abscess mimicking empyema on ultrasonography. AJR Am J Roentgenol. 1979; 133(4): 731-4. AJR Am J Roentgenol [pubmed citation]
- 2. Stark DD, Federle MP, Goodman PC, Podrasky AE, Webb WR. Differentiating lung abscess and empyema: radiography and computed tomography. AJR Am J Roentgenol. 1983; 141(1): 163-7. AJR Am J Roentgenol [pubmed citation]
- 3. Kazerooni Ella A and Barry H Gross. The Core Curriculum: Cardiopulmonary Imaging. 1st ed. Lippincott Williams & Wilkins, 2003. ISBN: 0781736552.
- 4. Collins Jannette and Eric J. Stern. Chest radiology. Lippincott Williams & Wilkins, 2007. ISBN: 0781763142, 9780781763141.