Abstract
Dear Editor:
Literature indicates that glucose measurements in centrifuged serum tubes may be clinically viable up to 96 h as well as use of centrifuged lithium–heparin tubes should be unadvisable. 1 In this respect, current guidelines recommend tubes with separator; this recommendation is not, however, followed by all laboratories, due to problems encountered with gel on their devices. 2 Indeed, the gel separator can generate several issues such as adsorption of certain analytes by the gel, and also fouling of analyzers by clogging the aspiration needles. 3,4 These issues may justify the avoidance of a gel separator, and in this vein, this study addresses glucose stabilities in centrifuged serum and plasma tubes that do not contain a gel separator.
The glucose stability on transition from 0 to 12 h preservation at room temperature was evaluated on 104 unsealed paired samples of plasma from heparin tube without gel (4 mL HT without gel; Becton Dickinson, Franklin Lakes, New Jersey) and serum from clot activator tube without gel (4 mL CAT; Becton Dickinson). Two tubes (HT and CAT) were collected from each patient. All tubes were sent to the laboratory by pneumatic tube system (PTS) and immediately centrifuged at 2500 g for 15 min in our laboratory. Glucose was measured immediately and 12 h after centrifugation, using Roche COBAS 8000 device (Roche Diagnostics, Rotkreuz, Switzerland). Then, glucose levels at 0 (t0) and 12 h (t12) were compared by relative bias calculation. At t0, glucose levels were 5.85 ± 1.56 mmol/L for plasma and 6.092 ± 2.42 mmol/L for serum (mean ± standard deviation [SD]). After 12 h of storage, glucose levels were 3.98 ± 1.75 mmol/L for plasma and 5.29 ± 2.57 mmol/L for serum (mean ± SD). Initial glucose levels at t0 between plasma and serum were not significantly different (P-value = 0.20 by paired t-test), glucose levels at t12 between plasma and serum were significantly different (P-value <0.0001 by paired t-test). Decrease mean biases of 34% and 14% were observed after 12 h for plasma and serum, respectively.
In practice, the limitation of our study, that tubes are unsealed and stored at room temperature, is met by many laboratories managing daily a high number of tubes for which reseal and storage at 4°C is hampered and even omitted due to the lack of available time and resources. For this reason, the present results may be useful. Interestingly, our data are consistent with those of Balboni et al., 1 who also showed a stability of glucose in serum better than in plasma. For other limitations, we did not perform multiple time points between immediate measurements and 12 h storage. However, as shown by Lippi et al., 5 glucose decrease is also related to the number of leukocytes. In the latter study, glucose stability was checked for 2 h and showed that serum use better preserves stability of glucose. Our results are in agreement with previous study and complete it with a larger number of patients and a longer storage time for samples. 5 We hypothesize that the difference observed between serum and plasma is linked to reduction in glycolysis by leukocytes since residual leukocyte count in serum should be lower than that in plasma (as shown by Balboni et al. 1 ). These results suggest that clotting could lead to a pseudo barrier by mechanical separation between cells and medium. Furthermore, the use of serum has other advantages, which also could be explained by this hypothetic mechanical separation by the clot as, for instance, correction of pseudohyperkalemia in case of hyperleukocytosis after transportation by PTS. 6,7
In conclusion, our results suggest that glucose stability is better in centrifuged serum tubes than in lithium–heparin tubes without gel unseal, which could be useful for glucose monitoring but probably should not be used for diagnosis of diabetes regarding our limits. The American Diabetes Association recommends use of plasma rather than serum for glucose measurement, and the number of laboratories that follow this recommendation is growing up. Use of serum has some limitations in the laboratory as increase of turnaround time related to clotting could also lead to preanalytical problems through fibrin that could block instrument needles and lead to incorrect results. However, despite these limitations, serum should not be precluded and could be useful, notably in some doubtful results (i.e., related to leukocytosis or difficulty to send the tube quickly). Indeed, along with previous studies, this study highlights that serum could increase separation with cells and could have benefit to avoid some interference related to hyperleukocytosis.