DOI 10.1515/cclm-2013-1006      Clin Chem Lab Med 2014; 52(5): e87–e89

Letter to the Editor Dominika Szoke*, Cristina Valente and Mauro Panteghini

Better blood collection tubes for plasma glucose: ready for prime time? Keywords: citrate acidification; glycolysis inhibition; sodium fluoride. *Corresponding author: Dominika Szoke, Laboratorio Analisi Chimico-Cliniche, Ospedale Luigi Sacco, Via GB Grassi 74, 20157 Milano, Italy, Phone: +39 02 3904 2290; Fax: +39 02 5031 9835, E-mail: [email protected] Dominika Szoke, Cristina Valente and Mauro Panteghini: Clinical Biochemistry Laboratory, ‘Luigi Sacco’ University Hospital and Department of Biomedical and Clinical Sciences, University of Milan Medical School, Milan, Italy

To the Editor,

The 2011 guidelines of the National Academy of Clinical Biochemistry for laboratory analysis in diabetes mellitus (DM) recommend minimizing influence of glycolysis on plasma glucose determination by immediately placing the sample tube in ice-water slurry and separating plasma from cells within 30 min from drawing [1]. In recognizing that this approach is often impractical in daily practice, the use of a blood collection tube containing a rapidly effective glycolysis inhibitor, such as citrate buffer, is considered an acceptable alternative for preventing misclassification of patients [1]. In this context, we read with interest Gambino’s view confirming sodium fluoride failure to control glycolysis and recommending the introduction in every laboratory of citrate-acidized fluoride tubes for obtaining more reliable plasma glucose results [2, 3]. In raising the importance of preventing glucose loss from blood samples prior to analysis, Bruns has, however, highlighted the lack of definitive knowledge on the extent of influence that true stabilization of glucose by using “citrate” tubes has in the definition of diagnostic cutpoints for DM and subsequent classification of patients [4, 5]. Even if blood collection tubes containing citrate buffer are commercially available for several years [Venosafe tubes with fluoride-citrate mixture (VFC), Terumo Europe,

code no. VF-053SFC], we were actually able to identify only one study evaluating the impact of the introduction of these tubes on diagnostic test results when compared with more traditionally used approaches, such as use of fluoride additive [6]. Furthermore, although there is no doubt that fluoride tubes, currently used in many laboratories in our country and worldwide, do not fully prevent the glycolysis effect, we are unaware of the amount of experimental bias between glucose results obtained by using fluoride tubes and VFC in a common outpatients’ setting and of the impact of the tube replacement in this population. Therefore, in planning to introduce VFC in our daily practice, we performed a comparison between in-use tubes for glucose testing, containing sodium fluoride/oxalate (BD Vacutainer, Becton Dickinson, code no. 368920) (BDV), and VFC, and aimed to establish the potential impact of the tube replacement on our population setting. Paired blood samples from 35 DM outpatients and 15 apparently healthy volunteers were collected into both BDV and VFC tubes. To simulate common practice, in which samples are sent in batches to our laboratory from remote collection sites that do not have centrifugation facilities, blood samples were stored at room temperature (22 ± 2 °C) for 165  min on average ( ± 20 min) before centrifugation. Glucose concentrations were measured in triplicate using GLUC3 enzymatic hexokinase method on Integra 800 platform (Roche Diagnostics). The study was done in compliance with the Helsinki Declaration of 1975, as revised in 2008, and informed consent of participants was obtained. Glucose concentrations ranged from 4.1 to 10.7 mmol/L in BDV and from 4.5 to 11.1 mmol/L in VFC, respectively. In statistical evaluations, VFC results were considered as reference. Passing-Bablok regression analysis yielded a correlation coefficient of 0.9979 [95% confidence interval (CI) 0.9964–0.9988], with a slope of 1.00 (95% CI 0.98–1.02) and an intercept of −0.41 mmol/L (95% CI −0.51 to −0.26). The presence of a constant bias independent of glucose concentration was confirmed by plotting sample differences (Figure 1).

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e88      Szoke et al.: Better blood collection tubes for plasma glucose

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Figure 1 Difference plots showing bias between plasma glucose concentrations as measured in BD Vacutainer fluoride/oxalate and Terumo Venosafe with fluoride-citrate (FC) mixture tubes. An average negative bias of −0.41 mmol/L [95% confidence interval (CI) −0.37 to −0.44) and −6.68% (95% CI −6.06 to −7.31) for fluoride/ oxalate tubes was detected. Solid lines represent mean bias values and dashed lines represent 95% CI.

To estimate the impact of introducing VFC tubes on our population setting, we retrieved all fasting plasma glucose results obtained in 2012 for outpatients by using BDV tubes. A total of 14,238 results were obtained and then recalculated by applying the BDV = 1.00 VFC – 0.41 equation. This resulted in a different clinical classification for 3269 subjects (23%); particularly, for 2574 measurements (18%) desirable glucose concentrations (