Results
Among 3925 patients consecutively discharged between March 1st and December 31st, 2021, 12% (n=464) suffered hospital mortality. Survivor vs. non-survivor patient and clinical characteristics within initial 24h of hospitalization are summarized in
Table 1. The median age respectfully of non-survivors was 71 years compared to 67 for survivors. Younger patients were more likely to be in the survivor group. 67% of patients who died were male compared to 33% females hence males were more likely to suffer in patient mortality compared to females. Survivors exhibited a median of 3 compared to 5 comorbidities in non-survivors. The 9 most prevalent comorbidities were: hypertension, obesity, neurologic diseases, previous thromboembolic disease, diabetes mellitus type 2, renal failure (chronic kidney disease stage III or higher), chronic pulmonary disease (including chronic obstructive pulmonary disease, emphysema, and pulmonary fibrosis), heart failure and iron deficiency anemia. Patients with higher values of CRP, ferritin, LDH, D-dimer, PT and INR on admission were more likely to suffer in hospital mortality
BMI-Body mass index; WBC-White blood cells; RDW-Red cell distribution width; RDW-SD- Red cell distribution width-standard deviation; ANC-Absolute neutrophil count; ALC-Absolute lymphocyte count; APC-Absolute platelet count; CRP-C-reactive protein; LDH-Lactate dehydrogenase; INR-International normalized ratio.
CBC parameters obtained within 24 hours of admission were analyzed. Patients with a higher WBC count were more likely to be in the non-survivor group (median of 7.6 for non survivors compared to 6.9 for survivors). Patients with increased segmented neutrophils (81%) were more likely to be in the nonsurvivor group, compared to patients with decreased neutrophils (75%) who were more likely to survive. Consequently, patients with increased bands (4% as compared to 3%) as well as increased ANC (5.97 compared to 5.10) were more likely to be in the non-survivor group. Patients with increased RDW-CV (14.0 compared to 13.6) were also more likely to be in the non-survivor group. Patients with decreased platelets (190 compared to 205) were more likely to be in the non-survivor group. Similarly, patients with decreased lymphocytes (10 compared to 14) and decreased monocytes (4 compared to 8) were more likely to be in the non-survivor group.Computed ANC/ALC and APC/ALC ratios were higher in the non-survivor group. Other components of the CBC including hemoglobin concentration, eosinophils and RDW-SD were also analyzed. Patients with slightly higher hemoglobin were more likely to die, but these results were not statistically significant.
CBC-Complete blood count; R2- Coefficient of determination; LDH-Lactate dehydrogenase; CRP-C-reactive protein; ANC-Absolute neutrophil count; RDW-CV-Red cell distribution width-coefficient of variation
Table 2a demonstrates the results of bootstrap forest (BF) modeling for inflammatory markers alone. The R
2 was 53% which indicated that the model represented 53% of the variance in hospital mortality. Relative contribution of the model components included LDH (45%), CRP (30%) and ferritin (26%).
Table 2b demonstrates the results of bootstrap forest modeling for CBC parameters alone. The R
2 was 63%. Within this model, platelet counts, monocytes, RDW-CV, ANC, lymphocytes and segmented neutrophils contributed an average of 15% each to the overall explained variance. Bands contributed 10%
Table 2c demonstrates the results of bootstrap forest modeling for CBC parameters combined with inflammatory markers. The R
2 was 69%. Within this model, LDH contributed the most to the overall explained variance at 21%. Monocytes and RDW-CV each contributed 11%. Overall, the use of inflammatory markers alone demonstrated the lowest positive correlation with mortality with R
2 of 53%. The use of CBC parameters alone demonstrated positive correlation with mortality with R
2 of 63%. The highest R
2 was generated when CBC parameters were combined with inflammatory markers. This model demonstrated positive correlation to mortality with R
2 of 69%.
Longitudinal box plots illustrated in
Figure 1,
Figure 2 and
Figure 3 again show survivor vs non survivor characteristics. These 7 CBC parameters are illustrated because they were retained in the BF modeling in
Table 2. The box plots demonstrate that differential patterns observed at presentation are sustained with variations in magnitude, but statistically significant (p<.0125) across up to 5-days hospitalization. Namely, lymphopenia, thrombocytopenia and monocytopenia, all predict a poorer outcome as do elevated RDW-CV, neutrophils, ANC and bands.
RDW-Red cell distribution width-coefficient of variation
ANC-Absolute neutrophil count
Discussion
This study sought to identify baseline cell counts and proportions reported in or calculated from a complete blood count that may provide prognostic information in hospitalized patients with laboratory confirmed SARS-CoV-2 infection. Despite the widespread incidence of this disease, little is known about the underlying pathogenicity that predicts for which patients suffer mortality or develop critical disease versus mild disease. A possible first step in unraveling this mystery would be to characterize risk factors at presentation. Thus, enabling early identification of patients exhibiting excess risk for progression to death.
Inflammatory markers including serum levels of ferritin, LDH and CRP have been proposed to carry prognostic significance [
4]. Coagulopathy and overt disseminated intravascular coagulation appear to be associated with high mortality rates [
4]. Our findings suggest use of putative COVID-19 inflammatory markers alone to predict mortality demonstrated an explanatory variance (R
2) in mortality of 53%. Whereas a CBC parameter ensemble (signature) provided 10% greater explained variance. Integration of CBC ensemble and inflammatory marker signatures increased explanatory power to 69%.
Similar studies have suggested that certain CBC parameters, including lymphocyte, platelet, neutrophil, and monocyte counts appear to correlate with the severity and mortality of COVID-19 cases. Kilercik and coinvestigators [
5] concluded that increased neutrophil-lymphocyte ratio (NLR) or ANC/ALC is a strong predictor for poor prognosis in COVID-19 patients. Other indices associated with severe illness in their study were decreased platelet counts and increased RDW as well as monocytopenia and lymphopenia. Tan and coinvestigators [
6] concluded that lymphopenia is an effective and reliable indicator of the severity and hospitalization in COVID-19 patients. Linssen and coinvestigators [
7] developed a parameter prognostic score to predict, during the first three days after presentation, which patients would recover without ventilation or deteriorate within a two-week timeframe. They found that lymphopenia is present for 7 days in non-critical illness group compared to 10 days in the critical illness group. The NLR is also increased in the critical illness group compared to the non-critical group. In addition, the plateletto- lymphocyte ratio (PLR) was abnormally elevated for both groups throughout, with values slightly higher in the critical illness group, but only until day 5, after which the non-critical and critical illness groups overlapped. Wang’s team [
8] concluded that WBC, neutrophil count, NLR, PLR, RDW-CV and RDW-SD parameters in patients in the severe illness group were significantly higher than those in the moderate illness group. Bellan and coinvestigators [
9] built a final multivariate model which confirmed that median age of 67 or greater as well as male gender, thrombocytopenia <166,000, NLR>4.68 and RDW >13.7 were independent predictors of in-hospital mortality. Hemoglobin levels were not associated with in-hospital mortality. Zheng and coinvestigators [
10] showed that the neutrophil count in patients with severe disease was higher than that in those with non-severe disease, while the platelet count in patients with the severe type was lower than that in those with the non-severe type. The authors also noted lymphopenia which is consistent with most studies discussed above. They were able to create a NLP (neutrophil-lymphocyte-platelet score) by assigning points to certain cutoffs for neutrophil counts, lymphocyte counts and platelet counts. In their study, a higher NLP was associated with a greater risk of COVID progression.
Taken together, our findings are consistent with others described above. Lymphopenia, thrombocytopenia, and monocytopenia are generally observed in non-survivors or patients who progressed to critical illness. In addition, elevated ANC/ALC (or NLR) and APC/ALC (or PLR) also manifest in patients who progress to critical illness, but these ratios are only sustained for the initial 120hrs after hospitalization.
The clinical manifestations of COVID-19 range from asymptomatic, to mild-moderate clusters of flu like symptoms such as fever, fatigue, myalgia, dry cough, dyspnea, and anorexia. Patients may manifest life threatening disease including ARDS requiring mechanical ventilation, sepsis, coagulopathy and multi system organ failure [
11]. The severity of COVID-19 has been attributed to a dysregulated innate immune response prompting a proinflammatory cytokine storm [
12]. No single definition of cytokine storm or the cytokine release syndrome is widely accepted however, a cytokine storm is a potentially fatal immune mediated condition characterized by high-level activation of immune cells and excessive production of massive inflammatory cytokines and chemical mediators [
13]. It is the main cause of disease severity and death in patients with COVID-19, and is related to high levels of circulating cytokines, severe lymphopenia, thrombosis, and massive mononuclear cell infiltration in multiple organs [
13]. Based on recent studies and currently available literature, there are several different hypotheses for the observed leukocytosis with lymphopenia that is observed in severe COVID-19 patients. During the inflammatory cytokine storm, elevated levels of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6), have been closely correlated with lymphopenia. Treatment with tocilizumab, an IL-6 receptor antagonist, increased the number of circulatory lymphocytes, further suggesting IL-6 increase is a key player in the lymphopenia development [
13]. Other studies have noted that COVID-19 infection can result in exhaustion of T cells. In one study, it was found that T cell counts are reduced significantly in COVID-19 patients, and the surviving T cells appeared functionally exhausted [
14]. Overall, lymphopenia and increased levels of certain cytokines, such as IL-6, have been closely associated with the disease severity and a remarkable decrease in T cell counts is almost always observed in severe cases [
13]. Additionally, another study demonstrated that the SARS-CoV-2 virus can infect macrophages and dendritic cells in human lymph nodes and spleens, which leads to tissue damage and lymphocyte reduction through the promotion of IL-6 secretion [
15].
Lymphopenia can occur during sepsis, viral infections, or infections due to other pathogens and therefore is not pathognomic to COVID-19 infection alone [
16]. However, a study from 2014 demonstrated that persistent lymphopenia was associated with increased mortality in sepsis [
17]. This is of particular importance in COVID-19 infections where the lymphopenia is not just transient but is often persistent and in our study is maintained for up to 120hours after hospitalization. Importantly, eliminating lymphopenia would also help to decrease the NLR and the PLR which may improve the likelihood of survival in patients.
A randomized clinical trial conducted showed that recombinant human granulocyte colony-stimulating factor (rhG-CSF) treatment for patients with COVID-19 with lymphopenia but no comorbidities did not accelerate clinical improvement, but the number of patients developing critical illness or dying may have been reduced [
18]. Further studies will need to be done to determine if repletion of lymphocytes and platelets can become an important element for consideration in decreasing the amount of moderately ill patients with covid who progress to critical illness or death.
Our study had several limitations that we attempted to minimize with statistical balancing of confounders:It was a single center study and most of the study population was not racially diverse or representative ofnational statistics. There were also some differences between the amount of male and female patients who were included although it can be argued that this is expected since multiple studies have shown that males are more likely to suffer critical illness requiring hospitalization from COVID-19 infection [
19]. In addition, patient enrolment and data collection occurred over several COVID-19 surges with presumed multiple variants undergoing temporally emerging treatment recommendations could have impacted survival. Data from out-patient settings including milder cases not requiring hospitalization were not compared.