[Performance limiting causes in single use unit systems for the measurement of PO2 and PCO2 in blood]

Having had the opportunity of testing two of the three commercially available single use unit systems (i-STAT and OPTI-1), we have listed a series of performance limiting factors which they share, even though, their detectors being based on different methodologies, they have specific causes of dysfunction. PO2 detectors face the worse problems for different reasons. Temperature control is critical: the cassette or at last the detectors, the internal solution and the specimen have to be heated to 37 C, thermostabilisation to one tenth of degree centigrade is a must. Gas equilibration between liquid phases, the outside ambiance and the plastic material can raise problems: a decrease of the external barometric pressure alters the result, an absence of oxygen buffer capacity can induce modifications of the PO2 in the solutions. The transition of the detectors from a dry to a wet stage can provoke changes of the final result in case of accidental pre-humidication of the cassette. The presence of a single internal solution generates problems linked to a single point calibrage procedure which is a source of variability. The general quality of the cassette can only be evaluated by numerous internal electronic checks: increasing the severity of the controls improves quality but increases the number of rejected cassettes. External quality control through classical procedures are not applicable and finally the quality of the cassettes depends primarily on rigorous manufacturing conditions.