Comparison of glucose determinations on blood samples collected in three types of tubes

Because of the metabolism of serum glucose in collection tubes containing blood samples, serum glucose levels may be found to decrease over time. Several types of collection tubes have been designed to, at least partially, block glucose metabolism by red blood cells in blood collection tubes that may not be analyzed immediately after blood collection. These include red-top collection tubes with serum separator, grey-top tubes with a fluoride glycolysis inhibitor, and heparin-containing green-top tubes which prevent clot formation. As part of a quality assurance project, we investigated whether glucose levels differed in the three tube types from each of 18 volunteers on a prolonged standing of 4 hours. We then determined the glucose concentrations of all three tubes from each of the 18 volunteers. We used refrigerated samples over a five-day period to determine if the initial values were reproducible. Surprisingly, after standing for four hours at room temperature, we found that the glucose levels in the three tubes from each volunteer were statistically indistinguishable from one another using the two-tailed paired t-test. Also, a linear regression analysis showed that the values of glucose for the three pairs of two tube types were closely correlated with one another, with correlation coefficients of >0.97, slopes close to 1, and Y-intercepts close to 0. These results suggest that blood collection in any of these tubes will render similar values for serum glucose even after standing for four hours. The tubes were then refrigerated at 4°C and re-analyzed after another six hours and then once per day for the next four days. Beginning at the first day at the six-hour determination, the glucose levels in the red- and grey-top tubes were statistically indistinguishable from one another but not in the red- and green-top tubes and in the grey- and green-top tubes. This was due to a steady decrease in the glucose levels in the green-top tubes. The glucose levels in the red- and grey-top tubes from each volunteer remained constant over the five-day period so that the coefficients of variation (CV) were low. In contrast, due to the decrease of glucose levels in the green-top tubes, the CVs for repeated glucose determinations in these tubes were high. Interestingly, a regression analysis of the glucose values for all three sets of paired tubes showed high (> 0.97) correlation coefficients and slopes close to 1. However, a regression analysis of the glucose values in the red- and green-top and grey- and green-top tubes at day five showed Y-intercepts of about -32 suggesting that there is a constant decrease of glucose in the green-top tubes that amounts to approximately 6 mg/dL per day over five days. These results suggest that red-top tubes with serum separator or grey-top tubes with a fluoride glycolysis inhibitor may be used for reproducible glucose determinations.