To the Editor: Patients with coronavirus disease 2019 have a profound hypercoagulable state, and complicating venous thrombotic events are common. [1][2][3] Abnormalities in coagulation screening measures, including a prolonged activated partial-thromboplastin time (aPTT), have been reported in patients with This finding could be seen as a reason to avoid the use of anticoagulation at both therapeutic and prophylactic doses.A prolonged aPTT may indicate a clottingfactor deficiency or the presence of an inhibitor of coagulation that is either specific (e.g., antibody to factor VIII) or nonspecific (e.g., lupus anticoagulant). Lupus anticoagulant can affect in vitro tests of blood coagulation but typically is not associated with bleeding. As part of the antiphospholipid syndrome, lupus anticoagulant is associated with a thrombotic risk. We investigated the cause of prolonged aPTT in patients with Covid-19.Blood specimens obtained from 216 patients who were positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) were received for coagulation screening, and 44 (20%) were found to have a prolonged aPTT. The specimens from 9 patients were excluded, and those from 35 patients were investigated further. (Details of the methods are provided in the Supplementary Appendix, available with the full text of this letter at NEJM.org.)A summary of the results is provided in Table 1. The median age was 57 years, and 24 patients were male. Pulmonary embolism was confirmed in 1 patient, and clinically suspected thrombosis was present in 1 patient. No clinically significant bleeding or arterial thromboses were reported.No patients were found to have deficiencies in factor VIII or factor IX. In 5 patients, marginal reductions in factor XI were found that were un-

ISRCTN55509212.

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