Koudriavtseva and colleagues (1) measured levels of different biomarkers in healthy controls, patients with remitting multiple sclerosis (MS), and patients with relapsing MS. The Quantikine enzyme-linked immunosorbent assay (ELISA) (R&D Systems, Minneapolis, MN, Cat. No. DCF300) was used to measure plasma tissue factor (TF) (S. Lorenzano, personal communication). The authors found that patients with relapsing MS had lower levels of plasma TF compared to either patients with remitting MS or the control group. It was concluded that plasma TF is a promising biomarker and possible therapeutic target in relapsing-remitting MS.A previous clinical study called Comparison of Acute Treatments in Cancer Hemostasis (CATCH) used the same Quantikine ELISA to measure plasma TF levels in 805 patients with cancer and acute symptomatic venous thromboembolism treated with anticoagulants (2). The mean and median values for TF were 72.5 pg/mL and 50.3 pg/mL, respectively. Plasma TF was identified as a biomarker of recurrent venous thromboembolism in this group of patients. We were invited to write a commentary on the paper (understanding the pathway), in which we raised concerns about the ability of the Quantikine ELISA to accurately measure levels of TF in human plasma (3).Recently, we assessed the ability of four commercial human TF ELISAs, including the Quantikine ELISA, to detect TF in various biological samples, including human plasma (4).