A Review of Morphologic Findings in Peripheral Blood Smears of COVID-19 Patients

Peripheral smear examination is a simple and cost-effective test that is routinely performed while monitoring patients diagnosed with COVID-19. We sought to summarize the peripheral blood morphologic findings in patients with COVID-19 infection.

A systematic review was conducted using a standardized keyword search on Medline database (PubMed), med RXIV, Google Scholar, EMBASE, and SCOPUS for studies discussing peripheral blood smear or morphologic blood findings in patients diagnosed with COVID-19.

A total of 28 studies were included in the review. Normocytic normochromic anemia was the most frequently encountered red blood cell finding. Neutrophilia was seen in most of the studies. A variety of morphological changes were observed in neutrophils, including pyknotic nuclei, variable shapes, toxic granules, and cytoplasmic vacuolization. Hyposegmented neutrophils, pseudo-Pegler Huet forms, and hypogranular forms were common findings reported by many studies. Lymphopenia was reported by most studies. Lymphocytes showed numerous morphological changes, including reactive forms, Downey forms, increased large granular lymphocytes, and plasmacytoid cells. The presence of giant platelets was seen frequently.

The peripheral blood in COVID-19 shows a spectrum of findings, mostly reactive changes in neutrophils, monocytes, lymphocytes, and platelets. Increased neutrophil/lymphocyte ratio and higher neutrophil counts have been associated with poor prognosis, which potentially could help triage patients, but this needs to be confirmed in larger studies.