In this nested case-control study, we evaluated haematological and morphological parameters of hospitalised patients with real-time polymerase chain reaction verified COVID-19 infection compared to patients with similar symptomatology but without COVID-19 infection. Seventy-four COVID-19 positive and 228 COVID-19 negative patients were evaluated with routine haematological parameters. Severe disease was defined as death and/or need of intensive care treatment. Twenty-seven COVID-19 positive and 18 COVID-19 negative patients were furthermore included for morphological evaluation using smear examination. Significant differences were found for platelet indices and white blood cell parameters. Thus, platelet count and plateletcrit was lower in COVID-19 patients, whilst mean platelet volume, platelet distribution width, and platelet large cell ratio was significantly higher than in non-COVID-19 patients. Leukocyte, neutrophil, immature granulocyte, lymphocyte, monocyte, eosinophil, and basophil count was lower in COVID-19 patients. No significant differences were found for red blood cell count, haemoglobin, haematocrit or mean corpuscular haemoglobin for COVID-19 versus non-COVID-19 patients. COVID-19 patients with a severe disease course had higher levels of immature granulocytes, but lower lymphocyte and platelet counts compared to patients with non-severe COVID-19. In terms of morphology, 14.8% of COVID-19 patients had a normal smear examination, compared to 83.3% of non-COVID-19 patients. Hypogranulated neutrophils were more frequent in COVID-19 patients (p < .001), but non-COVID-19 patients had higher levels of reactive lymphocytes, compared to COVID-19 patients. In conclusion, several haematological morphological abnormalities are more frequent in patients with COVID-19 disease, and several findings indicate that platelets play a fundamental role in the pathophysiology of the disease.
Keywords: Haematology; automated blood counts; infectious diseases; internal medicine; medical biochemistry.