The repeatability of quantitative D-Dimer assays in clinical laboratories.
Nowadays, assessment of D-Dimer is widely used for the exclusion of venous thrombosis. For the proper use of cut-off levels information about the analytical performance of D-Dimer test should be known. Recently the within- and between-laboratory variation of quantitative D-Dimer assays was investigated (1). Here we studied the repeatability of quantitative D-Dimer assay using data of the external quality assessment programme of the ECAT Foundation.In 2005, three different plasma samples with a D-Dimer level close to the cut-off level were distributed twice to the participants in different surveys. The repeatability was defined as a coefficient of variation (the difference between the measurements divided by the mean value of both measurements). There is an average repeatability of 8.4% (range: 7.3 - 10.5%, n=156-178). The average repeatability for different methods is comparable, notably for the Vidas (bioMerieux) method 7.2% (range: 6.3 - 8.3%, n= 47-62), for the AutoDimer (Trinity) method 8.0% (range: 6.5 - 9.2%, n=13-15), for the Tinaquant (Roche Diagnostics) method 7.6% (range: 6.2 - 9.9%, n=37-41) and for the STA Liatest (Diagnostica Stago) 9.4% (range: 6.5 - 13.4%, n=40-43). Based on the observed repeatability the 95%-range of uncertainty around the cut-off level was calculated. This is for the Vidas method 428 - 572 ng/mL, for the AutoDimer method 151 - 209 ng/mL, for the Tinaquant method 424 - 576 ng/mL and for the Liatest 406 - 594 ng/mL.Samples with a test result above the upper limit of the uncertainty range can be defined as positive, while test results below the lower limit of the uncertainty range can be defined as negative. Samples with a test result within the uncertainty range should be repeated before a proper clinical decision can be made. When the average of both measurements is below or above the cut-off level the sample can de defined as negative or positive, respectively. (1) M. Spannagl, F. Haverkate, H. Reinauer, PO. Meijer. Blood Coag and Fibrinol 2005; 16: 439 - 443.