Towards a more accurate assessment of pleural fluid pH

Pleural effusion, defined as excess fluid in the pleural cavity, can complicate the course of a pathologically diverse range of common clinical conditions, including infectious and malignant disease of the lung, heart failure, cirrhosis, pancreatitis and rheumatoid arthritis.

Microbiological, cytological and chemical analysis of pleural fluid provides useful information for the clinician whose diagnostic challenge is the patient presenting with pleural effusion. Among the chemical tests commonly requested is pleural fluid pH measured using a blood gas analyzer.

According to the authors of a recent UK study there is no standardized protocol for collection of pleural fluid for pH, and the preanalytical variables that might affect the accuracy of measured pH have not been clearly defined. In a series of experiments, the effect of delay in analysis, presence of residual air, lidnocaine and heparin in the sample were all individually assessed for their effect on pleural fluid pH.

A change of 0.05 pH units was considered clinically significant. By this measure, results indicate a clinically significant increase in pleural fluid pH if measurement is delayed beyond 1 hour after sampling or there is any air in the sample. The presence of traces of lidnocaine in the sample causes a clinically significant decrease in pleural pH.

Although the presence of heparin in the sample was found to decrease pH, this change was not clinically significant. The authors conclude that for accurate measurement of pleural pH, air and lidnocaine must be excluded from the sample and pH must be measured within an hour of sampling, using a blood gas analyzer.