Autoimmune hemolytic anemia, COVID-19, and the importance of a complete immunohematologic evaluation

Abstract

Kimura and colleagues recently published an interesting report describing the occurrence of autoimmune hemolytic anemia (AIHA) and pure red cell aplasia (PRCA) in the setting of coronavirus disease 2019 (COVID-19) [1]. This combination of immune phenomena has rarely been described; however, the co-occurrence of multiple autoimmune disorders in the setting of COVID-19 is not unexpected given the immune dysregulation observed in a substantial percentage of patients with this infection. Nevertheless, the diagnosis of AIHA secondary to COVID-19, while not novel [2], is still rare, and sufficient laboratory data must be presented to definitively conclude this unusual event. The authors reportedly diagnosed the patient in question with warm AIHA on the basis of abnormal hemolytic biomarkers and a direct antiglobulin test (DAT) positive for IgG. Unfortunately, aside from the reported “positive DAT (IgG-mediated)”, the authors provide no definitive evidence to conclude this was an immune-mediated hemolytic process. Importantly, the authors fail to describe the testing methodology employed for the DAT, which is crucial in assessing potential immune-mediated processes, as differing methodologies (e.g., tube, gel, solid phase, flow cytometry) vary significantly in both their sensitivity and specificity [3]. This variability can often lead to false positive or false negative results. Additionally, the authors failed to report any additional immunohematologic tests or results, including plasma or elution studies. This is important, as a clinically significant warm autoantibody would typically show pan-reactivity in a patient's plasma when tested against donor reagent red blood cells (RBCs), and would be able to be eluted from the patient's RBCs and similarly react with all reagent RBCs. The absence of reactivity in plasma, and particularly in the eluate, would suggest an alternative cause of immune-mediated hemolysis such as a drug-induced process. Moreover, the authors failed to comment on recent transfusion history or medication use, both of which could also result in a positive DAT, and similarly could potentially mediate hemolysis via a drug-induced or delayed alloantibody-mediated hemolytic process if the patient had undergone recent transfusion. Another point to consider is that a significant proportion of individuals may display a positive DAT at baseline without evidence of hemolysis, including a small fraction of healthy blood donors (0.1%) and a larger percentage of hospitalized patients (up to 15%) [2]. Most notably, up to 50% of patients with COVID-19 may have a positive DAT without an underlying hemolytic process [4]. Causes of false-positive DATs, or positive DATs without hemolysis include various conditions that predispose patients to increased binding of immunoglobulins to RBCs, including hematologic disorders such as sickle cell disease [5]. Further, conditions with elevated serum immunoglobulin levels are associated with an increased incidence of positive DATs [5]. Other causes include high serum protein, antiphospholipid antibodies, reticulocytosis, medications including intravenous immune globulin, and certain infectious agents [5]. In this context, it is important to report the strength of the DAT reaction, which is typically graded on a 1+ to 4+ scale, while microscopic (i.e., m) or weakly-positive (i.e., w+) DATs are usually considered negative. Although variability among both technologists and laboratories likely exists in interpreting DAT strength, a positive DAT secondary to non-specific immunoglobulin or protein binding generally displays weaker agglutination. Conversely, the opposite is not necessarily true (i.e., a ‘true’ DAT is not always 4+); however, some authors have shown that DAT strength may indeed correlate with the presence of in vivo hemolysis [6]. Thus, the DAT reactivity strength is a helpful component in assessing its significance when evaluating patients for potential autoimmune hemolysis. All of these considerations must be taken into account when assessing a patient for a potential immune-mediated hemolytic condition, as the DAT is simply designed to determine if immunoglobulin G (IgG) and/or complement (C3) is bound to an individual's RBCs. The DAT itself does not automatically implicate an autoimmune hemolytic process, and care should be taken to provide additional evaluation and contextual information in these cases. Nevertheless, I do not necessarily disagree that the authors' case represents AIHA in the setting of mild COVID-19 without significant inflammation; however, this is not novel, as we have previously described patients with significant immunosuppression and an absence of underlying inflammation having the ability to develop severe AIHA [7]. Thus, I concur with the authors that additional biologic mechanisms of autoimmunity and pathogen-mediated cross-reactivity are likely contributing factors, and further investigation into these mechanisms will undoubtedly elucidate novel etiologies of autoimmune phenomena. In summary, this case contributes to the literature regarding COVID-19-associated autoimmune processes, and highlights the importance of further investigation into immune-mediated cytopenias secondary to infectious agents. However, readers and researchers are cautioned, as they should ensure sufficient laboratory testing is performed and reported to establish a definitive diagnosis of an autoimmune process, particularly when an unusual condition is in question. Funding No funding was received. Declaration of competing interest The author has no conflicts of interest.