Peripheral blood morphologic findings in patients with COVID‐19

Abstract

Dear Editors, Our understanding of the novel coronavirus named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for the coronavirus disease 2019 (COVID-19), is rapidly expanding.1 Peripheral blood count abnormalities are diverse and can include neutrophilia and lymphopenia, or thrombocytopenia as well as thrombocytosis.2 Although there have been several case reports or small series documenting some of the morphological features seen in peripheral blood smears of patients with COVID-19,3-7 larger series are lacking and the range of features appears to be expanding. We studied peripheral blood smears in a cohort of patients with COVID-19, with the aim of expanding the current understanding of peripheral blood morphologic findings in these patients. This study was performed with local ethical board approval. We analyzed 78 peripheral blood smears from 27 distinct COVID+ patients at the Massachusetts General Hospital (Boston, MA) that were reviewed using CellaVision DM software (Sysmex America, Lincolnshire, IL) over a two-week period from April 6 to 17, 2020. For comparison, we analyzed smear findings in a control group of 14 confirmed COVID-19-negative patients randomly selected among those with blood smears available in CellaVision during the same time frame who had a total white blood cell count (WBC) of 10-20 x109/L. We gathered clinical information, including whether the patient required admission to the intensive care unit (ICU) or extracorporeal membrane oxygenation (ECMO) during their hospitalization and status (hospitalized, discharged, or dead) at the time of last follow-up. Various WBC parameters and peripheral blood morphologic findings were compared between the COVID-19 patients and the control group. Statistical analyses were conducted using Fisher’s exact test for categorical variables and Student’s t test for continuous variables. P-values <.05 were considered statistically significant. Our COVID-19 patient cohort had a mean age of 53 years (range 28-80) with more men than women (17:10). The majority of patients (22/27, 81.5%) were in the ICU, and 5/27 (18.5%) had been on ECMO. At the time of this study, only 4/27 (15%) had been discharged, and one patient had died. The main complete blood count (CBC) abnormalities in COVID+ patients included a mild leukocytosis (mean WBC 11.34 x109/L), anemia (mean Hgb 103 g/L), and absolute neutrophilia (mean 8.7 × 109/L) (Table 1). Despite having a higher proportion of patients in the ICU, COVID-19 patients showed significantly lower WBC and absolute neutrophil counts (ANCs) compared to control group, indicating a relatively mild leukocytosis. Absolute lymphocyte counts (ALCs) were also lower in COVID-19 patients compared to control group, but this difference did not reach statistical significance. Within the COVID-19 group, comparison of hospitalized versus discharged patients showed that the latter had an overall lower WBC count, lower ANC, and higher hemoglobin, although these differences did not reach statistical significance (data not shown). 103 (23.0) 101 (24.8) 31.7 (6.64) 31.1 (7.55) 88.1 (7.13) 89.8 (6.52) 286 (129) 289 (304) 8.7 (3.9) 12.5 (3.0) Absolute lymphocytes (109/L) 1.05 (0.54) 1.41 (1.22) Absolute monocytes (109/L) 0.88 (0.80) 1.14 (0.64) Absolute eosinophils (109/L) 0.23 (0.29) 0.28 (0.48) 0.03 (0.06) 0.05 (0.09) Common morphologic findings that emerged across multiple COVID+ patients included a myeloid left shift, atypical reactive lymphocytes with plasmacytic morphology and/or circulating plasma cells (Figure 1), large or giant platelets, and disintegrating neutrophils appearing as smudged cells (Figure 2). All these features were significantly more common in COVID-19 patients compared to control group (Table 1). The findings of left-shifted myeloid lineage cells and enlarged platelets both point to early release of immature forms from the bone marrow, reflecting a stress response that may be seen in several infectious and inflammatory conditions. The morphologic features that differed most significantly between the two groups of patients were circulating plasmacytic/plasma cells and smudged neutrophils. Although circulating plasma cells may be seen in the setting of various infectious and inflammatory states, this finding was most specific to the COVID-19 group in our study, present in 17/27 patients (63%), and not seen in any control group (P < .0001). This is an interesting finding given the ability of SARS-CoV-2 in inducing antibodies known to neutralize the virus in vitro, and the keen interest both in using convalescent plasma to treat COVID-19 and serologic testing for such antibodies as a potential marker for immunity.8 Whether the presence of circulating plasma cells in a patient with active infection is predictive for future efficacy of convalescent plasma therapy or immunity against reinfection remains to be determined and represents an area worthy of future study. Given the relatively simple and ubiquitous nature of peripheral blood smear preparation and review, its potential as a diagnostic and prognostic tool may have utility in a variety of healthcare settings, including those in low-/middle-income countries. We also found that smudged neutrophils, while nonspecific, appear to be a highly sensitive peripheral blood morphologic finding in COVID-19 patients, seen in 96% of patients versus 50% of controls (P = .0004). Although this finding could be related to the neutrophilia seen in infected patients,2 control group had an overall higher ANC yet had significantly fewer smudged neutrophils. Notably, the control group had smears prepared in the same manner and during the same time frame as COVID-19 patients in our study, arguing against a laboratory-derived artifact of smear preparation. Possible explanations for the apparent increase in neutrophil fragility in COVID-19 patients may be related to hypercoagulability and cytokine overactivation in the setting of disseminated intravascular coagulation (DIC), a common complication among hospitalized COVID-19 patients.9, 10 Additional factors, such as ECMO and secondary bacterial infection, could further contribute to neutrophil fragility. Given that many of these factors are more commonly seen in ICU patients, the finding of smudged neutrophils may represent a nonspecific finding in ICU patients regardless of the underlying diagnosis. Interestingly, however, some studies have pointed to a role for neutrophil extracellular trap (NET) dysregulation in the pathogenesis of COVID-19.11 These enzymatic webs are normally released by neutrophils in combating infections, but, when dysregulated, are implicated in increased inflammation and formation of microvascular thrombi. NET dysregulation could therefore provide a theoretical mechanistic etiology to our observation of higher numbers of smudged neutrophils in COVID-19 patients. Although increased smudged neutrophils have not been previously reported in COVID-19 patients, a prior study showed the presence of circulating apoptotic cells,3 which may reflect a different stage or mechanism of neutrophil degeneration. In summary, we identified several peripheral blood morphological findings in COVID-19 patients, many of which were attributable to bone marrow stress in the setting of infection. Among these features, the presence of circulating plasmacytic/plasma cells was found to be relatively specific for COVID-19 when compared to control group with mild-to-moderate leukocytosis, while fragmented or smudged neutrophils emerged as a highly sensitive morphologic marker for COVID-19. Both features likely represent by-products of the host antiviral immune response. Further study is needed to investigate their underlying relationship to COVID-19 pathogenesis and determine whether their presence in peripheral smears of infected patients correlates with outcome, including the ability to recover from infection and the development of neutralizing antibodies that may have a role in protective immunity. There was no external funding or grant for this study. The authors have no competing interests. Sam Sadigh devised the project, collected and analyzed data, and wrote the manuscript. Lucas R. Massoth collected and analyzed data. Bianca B. Christensen, Jonathan A. Stefely, and Joan Keefe collected data. Aliyah R. Sohani devised and supervised the project, analyzed data, and wrote the manuscript.

Topics & Concepts