Question 13.3 from the first paper of 2008 presented the candidates with a characteristic film, and asked them to "list 4 clinical signs typically found on chest examination". Up until 2020, this had been the only engagement with pleural effusions the college has had, which is surprising given how much one can ask about. Is it transudative or exudative? What caused it? What are the radiological features? What tests would you order? And so forth. Acknowledging this, they ultimately asked a detailed question about pleural fluid analysis in Question 16 from the first paper of 2020.
The best resources for this:
- The atelectasis page from LITFL
- Light's own 1972 paper about his exudate vs transudate criteria (1972)
- Light's "Pleural effusion" article for NEJM (2002)
Of the formal published literature, nothing beats this BTS guidelines statement from Thorax (2003)
That Xray from the SAQ again:
Causes of pleural effusion:
Clinical features of pleural effusion
- Unilaterally decreased chest expansion
- Stiny dull percussion note
- Absent breath sounds
- Whispering pectoriloquy above level of effusion
- Bronchial breath sounds above level of effusion
- Apical impulse shift to left
Radiological features of a pleural effusion
What's that opacity on the Xray? It is denser than air, that's all you can say. The photons didn't penetrate it very well. Could be anything dense: could be tumour, could be pus, could be blood, could be semen. Anything.
The features listed in Radiopedia are:
- blunting of the costophrenic angle
- blunting of the cardiophrenic angle
- fluid within the horizontal or oblique fissures
- 200ml of fluid is required to make a normal erect PA film look abnormal
- About 50ml is enough to blunt the costophrenic angles
Indications for thoracocentesis
- Do I even need to tap it, you might ask. Can't I just dry them out? You sure can. Another R.W. Light paper can be quoted: Approximately 75 percent of effusions due to congestive heart failure resolve within 48 hours with aggressive diuresis.
- Unilateral effusion
- Bilateral effusion which fails to respond to medical therapy
- Suspicion of empyema (or the expectation that one will form)
- Difficulty weaning ventilation (increased effort of breathing)
- Diagnosis (i.e. is it malignant?)
The appearance of the fluid gives clues, according to another paper by Light.
- Bloody fluid suggests malignancy (or haemothorax, duh)
- Straw-coloured fluid suggests transudate
- Milky-coloured fluids suggests chylothorax.
Biochemical features of pleural effusion
This is really the list of things you might want to order:
- Pleural fluid protein and LDH; Serum protein and LDH. Light's own 1972 paper about his criteria is available online. Of course at that stage he never actually called them "Light's criteria". However, 30 years later in his article on pleural effusions for NEJM Richard W Light does refer to his own criteria as "Light's criteria".
- In short, your effusion is exudative if:
- The fluid to serum protein ratio is greater than 0.5
- The fluid LDH is over 200 IU/L
- the fluid LDH to serum LDH ratio os greater than 0.6
- In short, your effusion is exudative if:
- Glucose: an extremely low pleural fluid glucose suggests that something is consuming it. Low pleural fluid glucose suggests TB, pneumonia or malignancy.
- pH: this is a weird one. Everybody orders pleural fluid pH, and few understand what significance it has. According to the 2000 guidelines from CHEST, pH can determine the need for therapeuric drainage. Anormal pleural pH is about 7.60; a pH of <7.20 is equivalent to a positive gram stain in terms of identifying an effusion which requires drainage. Oesophageal rupture can also cause a low pleural pH.
- Amylase: this is elevated in pancreatitis-related effusion and in oesophageal rupture
- Cholesterol: this reveals the effuion as a chylothorax
- Cell count
Cellular features of a pleural effusion
- Lymphocytosis = malignancy or tuberculosis. This cell count clue comes from another paper by Light. In his case series, of 31 exudative effusions with a lymphocytic predominance, 30 were due either to tuberculosis or neoplasm.
- Neutrophilia = parapneumonic effusion or PE.
- Eosinophilia (more than 10% eosinophils) usually means there has recently been blood or air in the pleural space; however weird causes include drugs and environmental toxins (dantrolene, bromocriptine, nitrofurantoin, exposure to asbestos) or autoimmune causes eg. Churg–Strauss syndrome.
Appearance of pleural fluid
In much the same way as the analysis of ascitic fluid can include a careful examination of its appearance, looking at your pleral fluid sample using your normal human eyes is possible. The question is, does it achieve anything? The answer may not surprise you. CICM examiners certainly think it does, as they included an appearance seciton in their answer to Question 16 from the first paper of 2020. Villena et al (2004) asked this question in a titanic 7-year-long prospective study from Madrid, during which 715 10-ml samples (that's 7 litres of pleural fluid) were scrutinised by numerous clinicians. The samples were described as "watery", "serous", "blood tinged", "bloody", "purulent", "milky" "turbid" and "brown". Their appearance, it turned out, had virtually no relationship with their aetiology. "The appearance of the fluid should not be overemphasized as a diagnostic test", the investigators grated sourly. However, in order to please the cour of examiners, the following list (from Question 16) should be regurgitated into future exam answers:
- Clear, straw-coloured – more likely transudate (although still may be exudate)
- Blood-stained – malignancy, pulmonary infarction
- Yellow/green – rheumatoid Pus – empyema
- Turbid – inflammatory exudate