Are changes in blood-ethanol concentration during storage analytically significant? Importance of method imprecision

Knowledge about the stability of drugs and metabolites in biological fluids is important information when the analytical results are evaluated and interpreted. This study examines changes in blood-ethanol concentration (BEC) during the storage of specimens for up to 12 months at 4 degrees C.

Venous blood samples were taken from drunk drivers in evacuated glass tubes containing sodium fluoride and potassium oxalate as chemical preservatives. The concentrations of ethanol in blood were determined in duplicate by headspace gas chromatography on arrival at the laboratory and again after storage in a refrigerator at 4 degrees C for up to 12 months.

The relationship between the standard deviation (SD) of analysis of ethanol at concentration intervals of 0.2 mg/g (BEC) was defined by the linear regression equation SD=0.00243+0.0104 BEC (r=0.99). At a mean BEC of 1.64 mg/g, the SD was 0.019 mg/g which corresponds to a coefficient of variation of 1.1%. The mean decrease in BEC (+/-SD) between first and second analysis was 0.105+/- 0.0686 mg/g (t=19.3, d.f.=158, p<0.001) and the loss of alcohol was positively correlated with the duration (days) of storage (r=0.44, p<0.001), although with large inter-tube variations. A correlation also existed (r=0.23, p<0.01) between the loss of ethanol and the starting BEC. When blood samples (n=49) were opened 17 times to remove aliquots for analysis over 6.5 months, the BEC decreased by 0.217+/-0.054 mg/g compared to a fall of 0.101+/-0.076 mg/g in tubes kept unopened. None of the blood samples showed a significant increase in BEC after storage.

To be considered analytically significant, the BEC had to decrease by 0.013 (2.6%), 0.028 (1.9%) and 0.045 mg/g (1.8%), respectively at starting concentrations of 0.5, 1.5 and 2.5 mg/g.