title: A comparative study of conventional versus new, magnesium-poor Vacutainer® Sodium Citrate blood collection tubes for determination of prothrombin time and INR.
abstract: BACKGROUND
  Conventional Vacutainer® Sodium Citrate blood collection tubes contain a relatively high concentration 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 introduced 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.
  
  
  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 differences (P<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.
  
  
  CONCLUSIONS
  Agreement of INR between reagents is improved by using magnesium-poor tubes.
authors.affiliation: Department of Thrombosis and Hemostasis, Leiden University Medical Center, Leiden, The Netherlands. Electronic address: a.m.h.p.van_den_besselaar@lumc.nl.
authors.forename: Anton M H P
authors.initials: AM
authors.lastname: van den Besselaar
authors.affiliation: Department of Clinical Chemistry, Albert Schweitzer Hospital, Dordrecht, The Netherlands.
authors.forename: Iris J H
authors.initials: IJ
authors.lastname: van Vlodrop
authors.affiliation: Department of Clinical Chemistry, Albert Schweitzer Hospital, Dordrecht, The Netherlands.
authors.forename: Paul B
authors.initials: PB
authors.lastname: Berendes
authors.affiliation: Department of Clinical Chemistry, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Christa M
authors.initials: CM
authors.lastname: Cobbaert
mesh_terms: D000818:Animals; D001800:Blood Specimen Collection / Q000295:instrumentation* / Q000379:methods; D016903:Drug Monitoring; D005260:Female; D006801:Humans; D019934:International Normalized Ratio / Q000295:instrumentation*; D008297:Male; D011517:Prothrombin Time / Q000295:instrumentation*; D011817:Rabbits
keywords: Anticoagulant drugs; Blood specimen collection; Magnesium; Prothrombin Time; Therapeutic drug monitoring