**Abstract**

SARS-CoV2 infection has devastating consequences on healthcare systems and has caused 3 million deaths by April 2021. Identifying patients at risk of death is a priority. Moderate–severe COVID-19 cases seem to associate a cytokine release that follows endothelial injury, triggering a hyperinflammatory and procoagulant state in which leukocytes and platelets are protagonists. Our group has published some reports about the usefulness of the hemogram in COVID-19. Hemogramderived ratios, mainly the neutrophil-to-lymphocyte ratio (NLR) and the novelty neutrophil-to-platelet ratio (NPR), obtained on admission and their rate of change during hospitalization, can easily detect patients with high risk of mortality. Hemogram is a tool available to all hospitals and analyzing the hemogram-derived ratios would provide much more information than could be extracted by evaluating the counts in isolation. We now know that in COVID-19 it is essential to start early anti-inflammatory treatment when patient deteriorates and the hemogram could be a good indicator of this situation. More comprehensive studies are needed to determine how useful these hemogram-derived ratios and prognostic scores are. In the next chapter we will present information related to this aspect as well as our group's research on the usefulness of the hemogram in COVID-19.

**Keywords:** COVID-19, neutrophil-to-platelet ratio, NPR, neutrophil-to-lymphocyte ratio, NLR, hemogram-derived ratios

#### **1. Introduction**

COVID-19 is a systemic disease, in which all organs can be affected. Several studies have emphasized an anomalous immune response as the starting point for hypercoagulability phenomena, endothelial damage and macro- and microthrombosis, which would trigger life-threatening consequences in patients. White blood cell populations (monocytes, lymphocytes and neutrophils) play a crucial role in the systemic inflammatory response and platelets have a direct function in the thrombotic response.

Differential blood cell counts can be measured simply, are cost-effective and reliable, and therefore can be used as markers of severity of the immune and inflammatory, and even the procoagulant response. In this context, white blood cells and platelets as circulating biomarkers involved in inflammatory and thrombotic responses could potentially predict clinical outcomes of patients with COVID-19.

Various hemogram parameters, including hemogram-derived ratios, have been used to try to identify patients with worse prognosis for COVID-19. In the following chapter the reader will find information related to this aspect as well as our group's research on the usefulness of the hemogram in COVID-19.
