For the first time, the influence of COVID-19 on blood microrheology was studied. For this, the method of filtering erythrocytes through filters with pores of 3.5 μm was used. Filterability was shown to significantly decrease with the increasing severity of the patient’s condition, as well as with a decrease in the ratio of hemoglobin oxygen saturation to the oxygen fraction in the inhaled air (SpO₂/FiO₂). The filterability of ≤ 0.65, or its fast decrease during treatment, were indicators of a poor prognosis. Filterability increased significantly with an increase in erythrocyte count, hematocrit and blood concentrations of hemoglobin, albumin, and total protein. The effect of these parameters on the erythrocyte filterability is directly opposite to their effect on blood macrorheology, where they all increase blood viscosity, worsening the erythrocyte deformability. The erythrocyte filterability decreased with increasing oxygen supply rate, especially in patients on mechanical ventilation, apparently not due to the oxygen supplied, but to the deterioration of the patients’ condition. Filterability significantly correlates with the C-reactive protein, which indicates that inflammation affects the blood microrheology in the capillaries. Thus, the filterability of erythrocytes is a good tool for studying the severity of the patient’s condition and his prognosis in COVID-19.

The hematological effects of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are important in COVID-19 pathophysiology. However, the interactions of SARS-CoV-2 with platelets and red blood cells are still poorly understood. There are conflicting data regarding the mechanisms and significance of these interactions. The aim of this review is to put together available data and discuss hypotheses, the known and suspected effects of the virus on these blood cells, their pathophysiological and diagnostic significance, and the potential role of platelets and red blood cells in the virus’s transport, propagation, and clearance by the immune system. We pay particular attention to the mutual activation of platelets, the immune system, the endothelium, and blood coagulation and how this changes with the evolution of SARS-CoV-2. There is now convincing evidence that platelets, along with platelet and erythroid precursors (but not mature erythrocytes), are frequently infected by SARS-CoV-2 and functionally changed. The mechanisms of infection of these cells and their role are not yet entirely clear. Still, the changes in platelets and red blood cells in COVID-19 are significantly associated with disease severity and are likely to have prognostic and pathophysiological significance in the development of thrombotic and pulmonary complications.

Complement system is a part of the immune system that provides organism cells with protection against foreign pathogens. Various kinds of defects in the complement system can induce development of serious diseases. This review summarizes different malfunctions of the complement system and also shows how these defects can result in development of severe diseases.

Hereditary hemolytic anemias (HHAs) have quite similar clinical manifestations, complicating differential diagnosis and determining treatment, such as splenectomy. For the differential diagnosis of hereditary spherocytosis (HS), pyruvate kinase deficiency (PKD), and hereditary stomatocytosis (HSt), we proposed to measure erythrocyte filterability through membrane filters with a pore diameter of 3 or 3.5 μm. Our modified method provides improved reproducibility reducing the variation in results caused by differences in the filters used. The filterability of erythrocytes significantly differs in patients with HS, HSt and PKD (medians and 1.5 IQR were: 0.045 [0; 0.51] (n=88); 0.685 [0.46; 0.79] (n=12) and 0.75 [0.55; 0.86] (n=28) rel. un., respectively). The specificity of the filterability test for HS diagnosis was 100%, comparable to the best available tests, while its sensitivity (85.2%) was somewhat lower than that of some alternative methods. Only the filterability test could identify a second subgroup in HS patients (14.8%) with higher filterability (close to normal), but their HS diagnosis was confirmed by the decreased filterability by 3-µm pore filter. This subgroup requires further detailed study. The proposed filterability measurement method is promising for diagnosing HS. It has high sensitivity and specificity and can be performed quickly without expensive equipment.

Hereditary hemolytic anemias (HHAs) have very similar clinical manifestations, complicating differential diagnosis and determining treatment strategies, such as splenectomy. To differentiate between hereditary spherocytosis (HS), pyruvate kinase deficiency (PKD), and hereditary stomatocytosis (HSt), we proposed measuring erythrocyte filterability using membrane filters with a pore diameter of 3 or 3.5 μm. Our modified method provides improved reproducibility, reducing the scattering in results caused by differences in the filters used. Erythrocytes’ filterability differs significantly between patients with HS, HSt, and PKD (medians and 95% confidence intervals were 0.025 [0.004; 0.100] (n = 86); 0.645 [0.520; 0.730] (n = 10), and 0.760 [0.700; 0.782] (n = 26), respectively). The specificity of the filterability test for the HS diagnosis was 99.1%, comparable with the best available tests, while its sensitivity (85.4%) was slightly lower than that of some alternative methods. Only the filterability test was able to identify a second subgroup of HS patients (14.0%) with higher filterability (close to normal), but the diagnosis of HS in these patients could be confirmed by reduced filterability on a 3-μm filter. This subgroup requires further detailed study. The proposed method for measuring filterability is promising for diagnosing HS. It has high sensitivity and specificity and can be performed quickly without expensive equipment.