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Pleural Fluid PH

Blood Sciences Test


Specimens need to be taken anaerobically into a heparinised blood gas syringe and sent to the laboratory immediately. The laboratory must be verbally informed prior to sending the sample, and out of hours the night duty BMS should be notified by bleep.

Special Instructions

NB Only specimens received within 1 hour of sample collection should be analysed.

Test Usage

A double membrane known as the pleura encloses each lung; in between this double layer i.e. inner and outer pleura is the pleural cavity. The pleural cavity is filled with a watery liquid – the pleural fluid. This fluid aids in the movement of the lungs during breathing and in healthy individuals rarely exceeds 10ml per lung.

Pleural fluid pH is normally about 7.6 because of bicarbonate accumulation in the pleural cavity (compared to blood which has a pH of ~7.4).

Various conditions affect pleural fluid dynamics resulting in an accumulation of excess fluid in the cavity. This is known as a pleural effusion.

Pleural fluid pH is performed on all non-purulent effusions to differentiate between empyemas and parapneumonic effusions.

  • pH <7.20 – indicative of an empyema requiring drainage
  • pH 7.21-7.29 – indicative of a complicated parapneumonic effusion which may require drainage
  • pH >7.29 – indicative of a simple parapneumonic effusion which usually responds to antibiotics

Turnaround time

1 day


Local test

Cannot be added on to an existing request

Specimen Labelling Procedure
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