Clinical Significance of CBC and WBC Morphology in the Diagnosis and Clinical Course of COVID-19 Infection

Olga Pozdnyakova, MD, PhD; Nathan T. Connell, MD, MPH; Elisabeth M. Battinelli, MD, PhD; Jean M. Connors, MD; Geoffrey Fell, MS; Annette S. Kim, MD, PhD


Am J Clin Pathol. 2021;155(3):364-375. 

In This Article


Our study demonstrates significant numerical and atypical WBC morphologic changes associated with SARS-CoV-2 infection and shows that the severity of changes is distinct not only between mild and severe disease but also between critically ill patients with and without COVID-19 infection. Our data comparing patients with COVID-19 requiring ICU stays support other published studies that found that COVID-19 clinical course is associated with markedly different peripheral blood WBC findings in response to viral infection, with the more severe disease presenting with more pronounced leukocytosis, neutrophilia, and lymphopenia.[11–14] However, to date, only a handful of publications with small cohort sizes have addressed the dynamic CBC changes during the course of COVID-19 in recovered and deceased patients.[15,16] Such studies are of crucial importance as monitoring CBC parameters changes could be a fast and simple method to predict disease outcome in hospitalized patients, ensuring earlier therapeutic interventions at the first signs of decline. Similar to the study by Pan et al,[15] we show that patients who ultimately died of disease continued to develop neutrophilia with percent changes from the baseline exceeding 30% in most cases. Granulocytes are stimulated by granulocyte colony-stimulating factor, which has been shown to be increased in critically ill COVID-19–positive patients. Most patients with fatal disease also demonstrate progressive lymphopenia, with only four (22%) patients demonstrating ALC normalization at demise. It has been suggested that viral-induced apoptosis may explain the lymphopenia.[17] Our preliminary data of bone marrow assessment from deceased COVID-19–positive patients show hypercellularity with marked myeloid hyperplasia, which could be a result of a hyperinflammatory state due to increased cytokines. In addition, the bone marrows of patients with COVID-19 display prominent hemophagocytosis (unpublished data), and it is interesting to contrast the inflammatory milieu seen in COVID-19 to that in hemophagocytic lymphohistiocytosis (HLH).[18,19] Marked elevation in cytokines in COVID-19 is associated with increased inflammatory markers, especially ferritin, a diagnostic criterion for HLH; however, the lack of severe cytopenias in most cases of COVID-19 despite other markers of macrophage activation suggests different underlying pathophysiologies at play.

WBC morphologic changes are also different between disease stages in COVID-19–positive patients. The initial stages and/or more mild disease are associated with exuberant coalescent monocyte vacuolization and expansion of atypical lymphocytes (Image 1), while disease progression and/or more severe disease are associated with loss of these changes in the setting of increasing neutrophilia and left-shifted myeloid maturation. Interestingly, these striking changes in monocyte morphology with extensive unusual cytoplasmic vacuolization that have also been described in a few prior studies and case reports were not contingent upon AMoC, which remained low to low/normal although it fluctuated.[2,5,20] Monocytopenia has been described in COVID-19–positive patients, with some studies showing even lower monocytes counts in the ICU patients, which we did not observe.[10,11] It has been suggested that peripheral blood monocytes may be reduced due to recruitment to sites of inflammation (ie, lungs) by CXCL10 and CCL2, which are significantly elevated in critically ill COVID-19–positive patients.[21] Regardless, the findings of different monocyte morphology may signify the presence of different monocyte subsets with different functional characteristics in mild and severe disease states. This notion is supported by the recent findings of the presence of a significantly higher percentage of CD14+ CD16+ inflammatory monocytes in peripheral blood of COVID-19–positive patients compared with normal controls; these nonclassical monocytes were characterized by higher forward scatter on flow cytometry and expression of CD68, CD80, CD163, and CD206, and they were enriched for in a more severe disease.[2] These inflammatory monocytes/macrophages secrete granulocyte macrophage colony-stimulating factor, inducing granulocytes and macrophage production, as well as interleukin 6, which is significantly elevated in the ICU patients and has been hypothesized to be a driving factor in monocyte differentiation to macrophages as opposed to dendritic cells inducing subsequent lung damage along with sustaining a hyperinflammatory state.[22,23] In addition, some studies have shown that monocytes develop alkaline vacuoles and that the pH of vacuoles may differ between monocyte subsets, explaining different monocyte morphology in different disease states.[24]

Interestingly, despite a clearly significant role that monocytes/macrophages play in sustaining a hyperinflammatory response in SARS-CoV-2 infection, it appears that not all patients with COVID-19 display monocyte morphologic changes in peripheral blood that we describe in our study. Nazarullah et al[6] have reported quantitative and qualitative peripheral blood changes in 12 COVID-19–positive patients and found that in their cohort, the most significant morphologic changes were related to nuclear morphology in neutrophils showing acquired Pelger-Huët anomaly and monolobate neutrophils. While we did find similar nuclear changes, they were present in a small number of neutrophils (predominantly <5% of neutrophils) and were not specific to COVID-19–positive patients; similar findings were present in COVID-19–negative ICU patients. However, we noticed another nuclear change in neutrophils in the form of Howell-Jolly body-like inclusions that was seen in 10% of patients with COVID-19. Similar to our findings, Nazarullah et al[6] have reported the presence of neutrophil toxic granulation and myeloid left shift, as well as the increased frequency of plasmacytoid lymphocytes in COVID-19–positive patients.

We recognize that analysis of morphologic changes in WBC is operator dependent and as a result could be subjective. In an attempt to standardize morphologic review of peripheral blood smears in patients infected with COVID-19, we have performed an exploratory analysis of morphology-associated research parameters, which are measured but not reported with every CBC with automated differential at our institution. Our analysis revealed significant differences in the research parameters related to morphology of neutrophils, monocytes, and lymphocytes between COVID-19–positive and COVID-19–negative ICU patients. Our experience with these parameters is scant, limiting the interpretation of these findings. However, the presence of different parameter values between COVID-19–positive and COVID-19–negative patients and the absence of such difference between COVID-19–positive ICU and non-ICU patients suggests that some of these changes could be attributed to the SARS-CoV-2 virus. For example, one could wonder if the differences in monocyte RNA content, as suggested by the significantly higher MO-Y value in COVID-19–positive ICU patients compared with COVID-19–negative ICU patients (124.09 vs 115.15, P = .0227), could be due to possible direct infection of monocytes by COVID-19, similar to the SARS-CoV virus.[25] It would be of great interest to investigate the research morphology-associated parameters in other viral or parasitic infections known to induce peripheral blood morphologic changes. There have been a few studies exploring research hematology analyzer parameters predominantly in the setting of infection. The study by Henriot et al[26] showed that the lymphocyte research parameters AS-Lymph % and Re-Lymph %, measured on the Sysmex XN-10, may be useful in distinguishing between viral and bacterial infections in febrile children. Some research hematology parameters have been investigated as possible predictors of sepsis. For example, the analysis of neutrophil and monocyte volume parameters measured on the UniCel DxH 800 analyzer concluded that the inclusion of monocyte distribution width along with WBC count improves detection of sepsis at the time of the ICU admission.[27,28]

In conclusion, the newly diagnosed hospitalized patients with COVID-19 should undergo a comprehensive manual WBC differential analysis to look for morphologic predictors of poor outcome, as well as daily CBC with manual WBC differential to monitor for numerical and morphologic alterations suggesting potential clinical deterioration due to disease progression.