Of time and temperature, plastic and glass: specimen handling in the blood-gas laboratory.

Character actor Walter Burke spoke his famous line about “plastics” just about the time that syringes made of plastic were being widely introduced to the medical marketplace. Plastics have taken hold in just about every aspect of modern life, including health care. Plastics equate to disposability, a characteristic that makes them valuable for eliminating cross-contamination in almost every area of the hospital and clinic. Intravenous bottles have become intravenous bags, and glass syringes have become just syringes, almost always plastic. However, there is at least one medical use in which plastic does not have an advantage over glass, and that is for storage of whole blood to be used for blood-gas analysis. Almost as soon as plastic syringes appeared in hospitals, differences in blood-gas tensions of O2 and CO2 were observed with specimens collected in plastic versus glass syringes.1 For over 30 years, investigators have described the effects of gas diffusion through the walls of plastic syringes.2–5 The PO2 of blood stored in a glass syringe decreases slightly over time, primarily as a result of continuing cellular metabolism. Room-air blood-gas specimens drawn and stored in plastic syringes show the opposite pattern: the PO2 increases, despite ongoing metabolism in erythrocytes and their kin.6 The degree of difference is largely a result of the differences in gas partial pressures between the sample and the environment. Samples with PO2 values in the normal range tend to show PO2 increases, whereas samples with high PO2 values (eg, in shunt studies) decrease toward the ambient partial pressure of O2. Notably, these differences can be clinically important if the blood-gas analysis is delayed. The PCO2 displays similar but smaller differences between plastic and glass syringes, and in the opposite direction. The traditional technique for reducing the effects of cell metabolism in blood-gas samples has been to place the specimen in an ice-water bath. This works remarkably well for specimens in glass syringes; the changes in bloodgas tension in samples kept on ice for an hour or longer are clinically insignificant. The deleterious changes in gas tension observed in plastic syringes seem to be exaggerated when those same syringes are placed in a cooling bath.7 There are several explanations for this phenomenon. The solubility of oxygen almost doubles when the specimen is cooled from 37°C to 4°C. Hemoglobin’s affinity for O2 increases with cooling even more dramatically, with a marked leftward shift of the dissociation curve. The combined effects of increased solubility and increased hemoglobin affinity enhance the influx of O2 through the walls of the plastic syringe. When the specimen is warmed back to 37°C in the blood-gas analyzer, these effects reverse, releasing O2 into solution and causing a falsely increased PO2 measurement.