A transudate has a protein content of less than 25g/L and an exudate 35g/L (Oxford Handbook of Clinical Medicine1).

Light's Criteria for Pleural Effusions:2

The effusion is a exudate if any one (or more) of the following is present

1.   Fluid protein : Serum total protein >0.5

2.   Fluid LDH : Serum LDH >0.6

3.   Fluid LDH : Upper limit of normal serum LDH >2/3


The effusion may well be an empyema if the pH < 7.2


Three causes of a transudate are:

1. Cardiac failure 2. Cirrhosis 3. Nephrotic syndrome


Three causes of an exudate are:

1. Cancer 2. Empyema 3. Rheumatoid arthritis


Investigations should be aim to find a cause of the effusion. A diagnostic aspirate can be sent for pH, LDH, glucose, protein, MC&S, amylase and cytology.

Just because an effusion is present, it doesn’t mean it needs to be drained. History, examination and diagnostic aspiration are important first steps. If the patient is compromised by the effusion or there is a suspicion of empyema, then it needs draining. If there is gross ascites secondary to liver disease, this should be drained first and the pleural effusion may well resolve without the need for a second drain with its inherent risks. If the effusion is secondary to heart failure, then this should be treated appropriately before any drainage is considered, which should be a last resort or for a compromised patient.

On a PA radiograph an effusion is seen as a uniform radiodense opacity obliterating the costophrenic angle and hemidiaphragm and extending up with an apparent meniscus on its lateral margin. If the whole lung is opacified, there may be mediastinal shift to the opposite side. The meniscus is not due to fluid tracking up around the lung – imagine a washing tub filled with water and a slightly smaller, unfilled washing tub pushed in. If a horizontal radiograph is taken the densest parts would be the bottom (where the beam shoots through all the water at the base) and the sides – these are the parts of the effusion you see on the PA radiograph. There is usually a clear horizontal fluid level on the PA radiograph.

On the supine radiograph all the fluid is evenly spread across the lung, so you see no fluid level. Also, the lung is not entirely white at as there is aerated lung above, there is an even increase in opacification across the whole lung.

An AP radiograph shows findings between a PA and a supine radiograph. The AP is usually semi-erect and the fluid is greatest at the base, but extends upwards so that the fluid level is at an angle to the x-ray beam causing a gradual decrease in opacification from base to apex.


1. Longmore M, Wilkinson I, Davidson E, Foulkes A, Mafi A. Oxford Handbook of Clinical Medicine. 8th ed. OUP Oxford; 2010.


2. Light RW. Clinical practice. Pleural effusion. N. Engl. J. Med. 2002;346(25):1971-1977.