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OBJECTIVES: D-dimer test has extensive usage as a component of diagnostic algorithms in exclusion of deep vein thrombosis and pulmonary embolism. Today, it has been reported to have also prognostic value for SARS-CoV-2/ COVID-19. Due to differences among methods, specificity of the antibodies used in reagents and reporting units there may be discordant results among laboratories. In this study investigation of the conformity of d-dimer results on various analytical systems using external quality control data is aimed. MATERIALS and METHODS: Peer group means of four different batches (3 samples for each) analyzed on ACL Top(IL HemosIL D-Dimer-HS), ACL 8000(IL HemosIL D-Dimer), Sysmex CS(Siemens Innovance), ACL Top(IL HemosIL D-Dimer-HS 500), Stago(STA-Liatest D-DI Plus) and BioMerieux VIDAS (dedicated reagent) in a cycle of Bio-Rad External Quality Assessment Program(US) were evaluated with grouping according to D-dimer unit (DDU) and fibrin equivalent unit (FEU). For systems in DDU group [ACL Top (IL HemosIL D-Dimer-HS), ACL 8000 (IL HemosIL D-Dimer)] difference between the systems was assessed using Mann-Whitney U test and inFEU group with Kruskal-Wallis test for each of the batches. A statistical significance level of p <0.05 was established. RESULTS: Peer group mean values of DDU reporting systems were ranged between 0.119-0.189mg/L for level 1, 0.259-0.318mg/L for level 2, 0.695- 0.762mg/L for level 3 and 0.871-1.09mg/L for level 4.The difference between the systems was insignificant (p>0.05). Peer group mean intervals in FEU group were observed as; 0.229-0.324mg/L for level 1, 0.577-0.920mg/L for level 2, 1.310-2.540mg/L for level 3, and 2.160-3.820mg/L for level 4. Significant difference was observed for four levels in FEU group (p<0.05). CONCLUSIONS: It was thought that conformity in DDU group may be due to more traceable results obtained with different reagents of the same manufacturer. Since harmonization could not be achieved in D-dimer measurement, it is important to inform clinicians that different results can be obtained depending on method. [ABSTRACT FROM AUTHOR]