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Analytical comparison of AxSYM, HemosIL DD HS and Innovance D‐dimer immunoassays with the Vidas D‐dimer

Sir, d-dimer is a family of fibrin fragments which form and circulate in the blood immediately after a thrombotic event (Stein et al., 2004). Although the optimal strategy for management of patients with suspected venous thromboembolism (VTE) is dependent on local expertise and cost, diagnostic algorithms including clinical assessment and d-dimer testing have been validated in several trials. Although none of the d-dimer assays had positive likelihood values that greatly increased the certainty of diagnosis, it was concluded that negative results are strong evidence against deep vein thrombosis (DVT) or pulmonary embolism (PE) (Stein et al., 2004; Qaseem et al., 2007; Wells, 2007; Stein et al., 2007; Toh & Hoots, 2007; Institute for Clinical Systems Improvement, 2006; Le Gal et al., 2006). This hypothesis is supported by a recent meta-analysis, which concluded that the enzyme-linked immunofluorescence assay (ELFA) has the highest overall sensitivity for both DVT and PE (>96%), thus resulting in a more confident exclusion of the disease and reducing the need for additional expensive and invasive imaging analysis (Di Nisio et al., 2007). Moreover, in the 2001, the Scientific and Standardization Committee of the International Society on Thrombosis and Hemostasis proposed new diagnostic criteria for diagnosing overt disseminate intravascular coagulation (DIC), including also a fibrin biomarker, later identified in d-dimer (Toh & Hoots, 2007). Although several quantitative and rapid tests were made available to clinical laboratories over the past decades, considerable variability has been reported between results obtained using different methods. Therefore, the aim of this study was to compare the analytical performances of three novel quantitative d-dimer immunoassays with the reference VIDAS ELFA. The study design included 200 consecutive patient samples, which have been referred to our laboratory from the emergency department with a clinical suspicion of VTE. Blood samples were drawn in 4.5-ml siliconized vacuum tubes containing 0.109 mol/l buffered trisodium citrate (Terumo Europe N.V., Leuven, Belgium). Immediately after collection, samples were gently mixed by inverting the tubes 4–6 times, transported to the laboratory within 30 min from venipuncture, centrifuged at 1500 g for 20 min at room temperature and divided in four identical aliquots. The aliquots were further assayed simultaneously with the current reference assay by VIDAS ELFA (Task Force Report 2002, van der Graaf et al., 2000, Perrier et al., 1999), and three novel commercially available immunoassays. VIDAS d-dimer (bioMerieux, Marcy l’Etoile, France) is based on a sandwich-type enzyme-linked immunofluorescent assay automated on the VIDAS immunoanalyzer (Pittet et al., 1997). The HemosIL D-Dimer HS are latexenhanced turbidimetric immunoassays developed from Instrumentation Laboratory (IL, Milano, Italy) (Moerloose et al., 2005). The AxSYM d-dimer assay (Axis-Shield Diagnostics, Dundee, UK) is based on Microparticle Enzyme Immunoassay (MEIA) technology (Ghanima & Sandset, 2007). The Innovance d-Dimer (Dade-Behring, Marburg, Germany) is a particle-enhanced, immunoturbidometric assay for the quantitative determination of d-dimer on the BCS analyzer (Dade-Behring). All d-dimer assays were calibrated employing proprietary calibrators and an identical lot of reagent was used throughout the study period for each assay. Measurements were performed in duplicate and results averaged. The recommended cut-offs for the different immunoassays are VIDAS <500 ng/ml, AxSYM d-dimer <500 ng/ml, IL D-Dimer HS <243 ng/ml and Innovance d-dimer <500 ng/ml, respectively. The lower and upper detection limits of the assays are comprised between 45 and 10 000 ng/ml for VIDAS, 50 and 450 000 ng/ml for AxSYM, 21 and 197 000 ng/ml for HemosIL HS, 170 and 25 000 ng/ml for Innovance, respectively. Results were compared by Passing & Bablok regression analysis and Spearman rank correlation. The bias of results against the reference VIDAS ELFA was evaluated by Bland & Altman plots and results were expressed as percentage mean bias and 95% confidence interval (CI) limits of agreement. Receiver operator characteristics (ROC) curve analysis was used to assess the analytical efficiency of each technique against the reference VIDAS ELFA. Results of this evaluation are shown in Table 1. The results of 29 out of the 200 samples, which were laying LETTER TO THE EDITOR INTERNATIONAL JOURNAL OF LABORATORY HEMATOLOGY