Coronavirus disease 2019 patients are at high risk of thrombosis related to endothelium injury, low blood flow and marked hypercoagulability [1] . Recently, a high prevalence of lupus anticoagulant (LA) was reported in the COVID-19 patients [2] , immediately questioned by the possibility of false positive testing given the marked elevation in C-reactive protein (CRP) levels attributed to the major pulmonary or systemic inflammation in these patients [3] . Interestingly, a strong association between thrombosis and the presence of LA in critically ill COVID-19 was suggested but not demonstrated [4] . Moreover, the contribution of anticardiolipin and anti-2glycoprotein-I antibodies to COVID-19-associated thrombosis was suggested in three patients with multiple cerebral infarctions, although no information on their detection and IgA/IgG titers was given [5] . Altogether, only few data were provided regarding anticardiolipin and anti-2-glycoprotein-I antibodies [2, 4] . Therefore, we aimed to investigate the prevalence of elevated antiphospholipid antibodies, namely LA, anticardiolipin IgG/IgM and anti-2-glycoprotein-I IgG, and their possible association with thrombotic complications in COVID-19 patients.
We conducted a prospective single-center observational study including all consecutive critically ill COVID-19 adults admitted from March 23 to April 15, 2020. SARS-CoV-2 infection was diagnosed using standard RT-PCR technique (Cobas-SARS-CoV-2 kits ® , Roche, France). This study was part of the ICU-COVID cohort registry approved by our institutional ethics committee. Duplex ultrasound was systematically performed once weekly to diagnose proximal and distal lower extremity deep vein thrombosis. If suspected, pulmonary embolism was confirmed using computed-J o u r n a l P r e -p r o o f and anti- 2 -glycoprotein-I IgG were quantified using chemiluminescence assays (Acustar ® , Werfen). Differences with p < .05 were considered significant.
Seventy-four consecutive mechanically ventilated patients were included. On admission, they received prophylactic (73%) or therapeutic (27%) enoxaparin or UFH. None had received any other anticoagulant drug before ICU admission. Thrombotic events were reported in 28 patients (38%) and included 26 deep vein thrombosis, 4 pulmonary embolisms, 1 stroke and 1 extensive venous catheter thrombosis. Patients with thrombosis exhibited significantly higher plasma D-dimer (p = .0003), serum creatinine (p = .02) and serum lactate dehydrogenase (p = .03), as well as a trend to more marked hypoxemia (p = .08; Table 1 ).
Overall, antiphospholipid antibodies, namely LA and/or elevated anticardiolipin IgG/IgM and/or elevated anti- 2 -glycoprotein-I IgG, were present in 88% of the patients. LA, based on dRVVT system, was positive in 63 patients (85%) but not associated with thrombotic complications (p = .7; Fig.1 ). Noteworthy, our dRVVT results could be interpreted since UFH or enoxaparin anti-Xa activity was systematically measured and found < 0.9 IU/mL in all samples, thus excluding heparin interference with dRVVT results, due to the presence in the reagent of an heparin quenching agent effective until 1.0 IU/mL as specified by the manufacturer and checked locally for accreditation [6, 7] . 
Compared to other viral and bacterial infections known to trigger transient antiphospholipid antibodies [9] , LA prevalence was extremely high (85%) in critically ill COVID-19 patients, similar to Helms' study (87.7%) also conducted in ICU patients [4] . This elevated prevalence could be attributed to the cytokine storm-related inflammation and dysimmunity. By contrast to Helms et al. We found a moderately elevated prevalence of anticardiolipin/anti- 2 -glycoprotein-I antibodies (12%) in our COVID-19 patients, in the same order of magnitude as Harzallah'study prevalence (10%) [2] . One suggested mechanism is cross-reacting antibodies involving antigenic similarities between viral antigens and the host  2 -glycoprotein-I, used as molecular mimicry [9] . In critically ill COVID-19 patients, cross-reactivity may be facilitated by  2 -glycoprotein-I exposure subsequent to major endothelial injury. However, whether these antibodies could be responsible for increased thrombotic risk remains uncertain, depending on their titer, isotype and persistence. This association is clearly questionable in the three patients described by Zhang et al. [5] . In COVID-19 patients with thrombosis, persistence period of antiphospholipid antibodies over a 3-month period is required before considering the diagnosis of antiphospholipid syndrome.
Limitations of the current study include the relatively small number of patients, the single-center setting, and the short study period. However, to the best of our knowledge, this is the first study evaluating the prevalence of both LA and anticardiolipin/anti-2-glycoprotein-I antibodies in COVID-19 patients. Because we focused on critically ill COVID-19 patients, prevalence of antiphospholipid antibodies might be different in less severe patients.
In conclusion, we confirm that LA are highly prevalent but conclude that despite its high prevalence, LA are not associated with thrombosis occurrence reported in the COVID-19 patients. 
None J o u r n a l P r e -p r o o f confirm (Conf, high phospholipid concentration) results were normalized, i.e. expressed as ratios against reference plasma results. Final results were expressed as screen ratio/confirm ratio. Cut-off value was 1.20 for both screen ratio and screen ratio/confirm ratio, demonstrating the phospholipodependence. Figure 1 