Influence of blood collection systems on the prothrombin time and international sensitivity index determined with human and rabbit thromboplastin reagents.

Three brands of blood collection tubes were studied for their influence on the prothrombin time (PT) and international sensitivity index (ISI) for 5 commercial thromboplastin reagents. With all reagents, PTs were shorter in Vacutainer (Becton Dickinson Vacutainer Systems, Plymouth, England) samples than in S-Monovette (Sarstedt, Nümbrecht, Germany) or Venosafe (Terumo Europe, Leuven, Belgium) samples. ISI values were higher with Vacutainer samples than with S-Monovette or Venosafe samples. The ISI differences between the tubes were small for Thromborel-S (2.1%; Dade Behring, Marburg, Germany) and Hepato Quick (1.1%; Diagnostica Stago, Asnières, France; Roche Diagnostics Nederland, Almere, the Netherlands) but greater for Neoplastin Plus (5.5%; Diagnostica Stago; Roche Diagnostics Nederland), Simplastin HTF (8.3%; bioMérieux, Durham, NC), and Innovin (8.8%; Dade Behring). The PT and ISI differences between the tubes could be explained mostly by the effect of magnesium ion contamination in the sodium citrate solutions. When PT ratios were transformed into international normalized ratios (INRs) using crossover ISI (ie, samples collected with one type of tube and ISI determined with another collection system for the PT reagent), the differences in mean INRs could be approximately 10%. For ISI calibration of reference thromboplastins, blood collection tubes should be used with minimal divalent metal ion contamination of the citrate solution.