Regular Article A comparative study of conventional versus new, magnesium-poor Vacutainer® Sodium Citrate blood collection tubes for determination of prothrombin time and INR Anton M.H.P. van den Besselaar a, ⁎, Iris J.H. van Vlodrop b , Paul B. Berendes b , Christa M. Cobbaert c a Department of Thrombosis and Hemostasis, Leiden University Medical Center, Leiden, The Netherlands b Department of Clinical Chemistry, Albert Schweitzer Hospital, Dordrecht, The Netherlands c Department of Clinical Chemistry, Leiden University Medical Center, Leiden, The Netherlands. abstract article info Article history: Received 12 February 2014 Received in revised form 28 March 2014 Accepted 18 April 2014 Available online 26 April 2014 Keywords: Blood specimen collection Prothrombin Time Anticoagulant drugs Therapeutic drug monitoring Magnesium Introduction: Conventional Vacutainer® Sodium Citrate blood collection tubes contain a relatively high concen- tration of contaminating magnesium ions, which may result in shortening of the prothrombin time (PT) and the International Normalized Ratio (INR). Recently the manufacturer of Vacutainer® Sodium Citrate tubes intro- duced new tubes with a magnesium-poor stopper. The magnesium concentration in the new low-Mg tubes is significantly lower than that in the conventional plastic tubes. The purpose of the present study was to compare PT and INR determined in specimens drawn with the new tubes to those drawn with the conventional tubes. Materials and Methods: Venous blood specimens were collected from 22 healthy persons and 65 patients treated with vitamin K-antagonists using conventional Vacutainer® Sodium Citrate tubes and new, low-Mg Vacutainer® tubes. PT and INR were determined with four thromboplastin reagents, i.e., three brands of recombinant human tissue factor and one brand of combined rabbit brain reagent. Magnesium concentrations were determined in the citrate plasmas with a colorimetric method. Results: The differences in PT, INR and International Sensitivity Index (ISI) between the two tubes were significant when using three recombinant human thromboplastin reagents, but were not significant when using the rabbit thromboplastin. The PT and INR differences between the tubes correlated with the magnesium concentration dif- ferences (P b 0.001). The INR bias between the four reagents was greater for specimens drawn with conventional tubes than the INR bias for specimens drawn with the new tubes. Conclusion: Agreement of INR between reagents is improved by using magnesium-poor tubes. © 2014 Elsevier Ltd. All rights reserved. Introduction The prothrombin time (PT) and International Normalized Ratio (INR) are determined by many clinical laboratories for monitoring of treatment with vitamin K-antagonists. The result of the PT test depends on the preanalytical conditions of the specimen. An important preanalytical factor is the system used for drawing the blood specimen. Many commercial blood collection systems are composed of an evacu- ated tube containing a solution of sodium citrate, and a rubber stopper. Previous studies have shown that the International Sensitivity Index (ISI) of a thromboplastin reagent, which is a measure of its responsive- ness to the defect induced by vitamin K-antagonists, may depend on the blood collection system used [1,2]. Furthermore it has been shown that some vacuum tubes are contaminated with magnesium resulting in a shortening of the PT [3–7]. It has been proposed that the magnesium contamination in sodium citrate solutions for blood collection should be less than 1.0 mmol/L [7]. Manufacturers became aware of the undesirable effects of magnesium contamination and started to develop new, magnesium-poor tubes. Recently, the manufacturer of plastic Vacutainer® Sodium Citrate tubes has replaced the conventional rubber stopper by a low-magnesium stopper. A prototype of a magnesium-poor Vacutainer® Sodium Citrate tube has been evaluated in 2011 [8]. By the end of 2012, a new low- magnesium Vacutainer® Sodium Citrate tube was introduced on the market. At that time, the conventional plastic tube was still being used by clinical laboratories. The primary purpose of the present study was to evaluate the new plastic tube by comparison to the conventional tube, focusing on the PT and INR of healthy persons and patients treated with vitamin K-antagonists. In this study we used three PT reagents made of recombinant human tissue factor and one reagent made of Thrombosis Research 134 (2014) 187–191 ⁎ Corresponding author at: Department of Thrombosis and Hemostasis, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands. Tel.: +31 71 5261894; fax: +31 71 5266868. E-mail address: a.m.h.p.van_den_besselaar@lumc.nl (A.M.H.P. van den Besselaar). http://dx.doi.org/10.1016/j.thromres.2014.04.016 0049-3848/© 2014 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Thrombosis Research journal homepage: www.elsevier.com/locate/thromres