Hospitalized patients with positive antiphospholipid antibodies who have low complement levels are at increased risk for death—a retrospective cohort study

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To investigate whether low complement levels can predict worse outcomes in patients hospitalized with positive anti-phospholipid antibodies. This was a retrospective cohort study. We obtained demographics, laboratory, and prognostic data of all consecutive patients hospitalized between 2007 and 2021, for whatever reason, with at least one positively abnormal anti-phospholipid antibody, who were also tested for complement levels (C3 or C4). We then compared the rates of long-term mortality, 1-year mortality, deep vein thrombosis, and pulmonary emboli between groups of low complement and normal complement levels. Multivariate analysis was used to control for levels of clinical and laboratory confounders. We identified 32,286 patients tested for anti-phospholipid antibodies. Of those patients, 6800 tested positive for at least one anti-phospholipid antibody and had a documented complement level. Significant higher mortality rates were found in the low complement group, with an odds ratio for mortality (OR 1.93 CI 1.63-2.27 p < .001). Deep vein thrombosis and pulmonary emboli rates were similar. Multivariate analysis confirmed that low complement was an independent predictor for mortality after controlling for age, sex, dyslipidemia, chronic heart failure (CHF), chronic kidney disease (CKD), and anemia. Our study results indicate that low complement is associated with significantly higher mortality rates in admitted patients with elevated levels of anti-phospholipid antibodies. This finding correlates with recent literature suggesting a vital role for complement activation in anti-phospholipid syndrome.