Pleural fluid cytology revealed no malignant or mesothelial cells. A second chest X-ray, taken after the intercostal drain insertion, additionally showed a right pneumothorax, revealed after drainage of several litres of purulent fluid.

His blood tests on admission were as follows:

Question 1: What is the differential diagnosis of a chronic exudative pleural effusion, considering the negative Gram stain and routine bacterial culture?

Answer to Q1

A chronic, exudative pleural effusion with a negative Gram stain and routine bacterial culture, should prompt the following differential diagnosis, amongst other things:

Note: some of the above may be cultured on routine bacterial culture (e.g. Nocardia) although the sensitivity for their detection by this method is low, and therefore they should remain considerations at this stage.

A CT scan of the chest revealed a significant residual right pneumothorax (despite the intercostal drain), and collapse/consolidation of all 3 lobes of the right lung, with mediastinal shift to the left:

An alert clinician noted that the fluid, while grossly purulent, had an unusual brown-orange colour (see picture below) and was odourless.

Question 2: An ‘empyema’ that looks like this is virtually pathognomonic for which diagnosis?

Answer to Q2

The pleural fluid shown has the appearance of “anchovy paste”, and this is virtually pathognomonic for amoebiasis.

The “anchovy paste” appearance is the result of a combination of necrotic and apoptotic material, and haemolysis of red cells that have bled into the cavity.

Question 3: Which is the most appropriate diagnostic test that can be performed to confirm this diagnosis?

Answer to Q3

The most appropriate diagnostic test at this stage would be serology for amoebiasis.

More than 95% of both amoebic liver abscesses and thoracic amoebiasis develop detectable antibodies. Such testing may be performed by indirect haemagglutination assays (IHA) or ELISA assays .

Microscopy may demonstrate trophozoites from aspirated material, but sensitivity is poor.

Antigen testing and molecular diagnostics (PCR) on aspirated material are both highly sensitive and, especially in the case of PCR, highly specific. However, they are expensive and generally limited to reference laboratories in Southern Africa. In the context of this case, they would be unlikely to add anything to serology, which is much cheaper and more readily available.

Question 4: Primary amoebic empyemas are very rare. Where else should one look in the body to see if there is another (primary) focus of infection?

The abdomen should be scanned to look for a ruptured amoebic liver abscess. Almost all amoebic empyemas are the result of such an event¹.

Although both an abdominal ultrasound and a contrasted abdominal CT scan were initially reported as normal, alert infectious diseases doctors insisted on a review of the scans.