SARS-CoV-2 can infect multiple organs and result in MODS, and this virus causes COVID-19 [11, 12]. Although most patients had no syndromes or mild syndromes, fatal MODS can develop rapidly within a few days in severe cases [13]. Therefore, COVID-19 treatments essentially require practical evaluation of the disease condition and expected judgment of disease progression, and both the evaluation and judgment urgently require laboratory evidence for clinical guidance [14].
According to our paired detection results of sputum and throat swab samples of the same patients assessed 68 times, the accuracy of sputum detection is significantly higher than that of throat swabs. Upper respiratory tract samples are now widely used to detect viral RNA for the diagnosis of COVID-19, we must also remind that throat swabs are more suitable for broad-spectrum screening. For suspected cases, sputum and throat swab samples must be combined to improve the detection rate.
COVID-19 disease progression (incubation, syndromic, and recovery periods) involves virus-host interactions through which the host immune system recognizes and presents the virus-specific antigen to effective T and B cells and thereby clears the virus. In this process, pyroptosis of infected epithelial cells can release many DAMPs and PAMPs thereby attracting lymphocyte infiltration [6]. Furthermore, extensive and severe infection sites could rapidly attract excessive lymphocyte infiltration within a short time, thereby reducing the quantity of blood lymphocytes [15]. Specifically, this reduction was mainly attributed to the lymphocyte decrease and NLR increase in blood cell counts, and recovered patients usually had lymphocyte restoration. Moreover, the cell count of neutrophils increased 5–9 days after viral infection and then gradually decreased. This rise and fall of neutrophil counts may be associated with bacterial infection that stimulates the bone marrow to produce neutrophils instantly, and bacterial infections usually occur one week after the onset of viral infections. Therefore, a continuous reduction in lymphocyte counts and increased NLR indicate a worsening trend of disease progression, and an increased number of neutrophils suggests potential bacterial infection in COVID-19.
In this study, we also found that inflammatory cytokines and serum markers were correlated with COVID-19 tissue damage and lymphocyte counts. LDH is a cytoplasmic glycolytic enzyme expressed in almost every tissue and could represent the extent of tissue damage in COVID-19 [16, 17], in which severe pneumonia has a high level of LDH [18] and associated DAMPs and PAMPs. Alveolar macrophages can recognize the DAMPs and PAMPs released by the pyroptosis of endothelial cells, thereby initiating cytokine secretion (e.g., IL-1β, IL-18, and TNF-α). The IL-1β, TNF-α, and Toll-like receptor signaling pathways can activate innate immune cells and effective T cells to produce IL-6 [19], which circulates to the liver and induces an extensive range of acute-phase proteins, such as CRP, serum amyloid A (SAA), haptoglobin, fibrinogen, and α1-antichymotrypsin [20]. In addition, IL-6 can promote the final maturation of B cells into antigen-specific antibody-producing plasma cells [21]. Therefore, excessive inflammation [22], which occurs as a high level of LDH, may result in macrophage pyroptosis and lymphocyte exhaustion [23], and a large amount of IL-6 is produced in this process. Increased IL-6 expression [24] was correlated with high levels of CRP, SAA, and D-D (fibrinogen degradation product) and lymphocyte reduction (decreased lymphocyte count and increased NLR). It is difficult to define which one is the earliest and decisive factor, but our data demonstrate that CRP, LDH, and D-dimer levels are significantly higher in severe patients than in mild patients in the first two weeks. These indicators may lead to the formation of the cytokine storm in severe patients and COVID-19 exacerbation.
The inflammatory cytokines and serum markers analyzed in this study had individual specificity and may be used to evaluate specific disease progression time points, which may help better understand disease progression [25]. However, this study contained only a limited sample size, which is a limitation. Therefore, studies with more samples are warranted for further validation.