Measurement of oxygen and carbon dioxide partial pressures in synovial fluid after tonometry

A commercially available gas analyser was used to measure tension of oxygen (PO2) and carbon dioxide (PCO2) in synovial fluid samples after tonometry. Measured values of PCO2 were close to the expected (median difference 0.2 kPa, range -0.4 to 0.4) within the analysed concentration range of 4-10 kPa. No consistent difference between measured and expected values of PO2 were found for oxygen in the range 3-11 kPa (median difference 0.1 kPa, range -0.3 to 1.2). For oxygen tensions below 3 kPa, however, the measured values invariably overestimated the actual PO2, the errors ranging from 0.3 to 1.9 kPa, median 1.1. The importance of proper handling of samples was investigated and storage for 1 h at 0 degrees C in plastic syringes resulted in elevation of the PO2 levels measured (range of elevation 0.2 to 3.6 kPa, median 1.15), whilst no significant differences were found when stored in glass syringes. Within the limits stated, commercially available gas analysers may thus be used to investigate these parameters related to local tissue metabolism in effusive joint conditions.