The prevalence of anemia in patients admitted to the intensive care unit (ICU) reaches 66%. Moreover, numerous patients develop anemia during ICU hospitalization. In fact, anemia is the most common hematologic disease in the ICU.

The majority of patients hospitalized in the ICU present with acute systemic inflammation, so called systemic inflammatory response syndrome (SIRS). These patients may develop anemia of inflammation (AI). In crtitically ill patients AI may present acutely (acute systemic inflammation) or chronically (comorbidities associated with prolonged systemic inflammation), here we describe both presentations of AI as ‘anemia of critical illness’ (ACI). The second most frequent type of anemia in critically ill patients is iron-deficiency anemia (IDA). A mixed type of anemia (ACI + IDA) may also be present in these patients.

The three major pathophysiological mechanisms leading to ACI are: iron restriction, decreased erythropoiesis, and
decreased erythrocyte lifespan. Cytokines synthesized during SIRS induce the production of hepcidin that inhibits the
only transmembrane iron exporter (ferroportin) present in the duodenum and macrophages.

Etiological classification of anemia in critically ill patients poses a significant challenge to clinicians, as there is a multitudeof tests available, and there are various reference ranges for these tests reported in the literature in the patient population in question. Pure ACI or mixed ACI + IDA can be diagnosed using a single laboratory test — complete blood count with analysis of reticulocytes — which provides Hb concentration in erythrocyte and reticulocyte.

The management of ACI incorporates discontinuation with erythropoiesis-stimulating agent causing anemia, reduction
of iatrogenic blood loss, parenteral iron, and combined therapy of parenteral iron with erythropoiesis-stimulating
agents in approved indications.