Diagnostic Approach to Anaemia in Critical Care

In the history, note pre-existing co-morbidities including renal and cardiac impairment. Also note medications, diet and symptoms suggestive of blood loss. Examine for jaundice, lymphadenopathy or organomegaly.

If there is a sudden unexpected drop in Hb, this may be a sampling error; consider repeating the full blood count (FBC).
Is the anaemia acute or chronic? This may inform the diagnosis, likely tolerance of anaemia and treatment strategy. Only 10–15% of ICU patients have chronic anaemia prior to admission [4].
Is the anaemia isolated or are there other cytopenias? Thrombocytopenia (Chapter 3) is also a common finding (~40% of ICU patients) and will therefore often coexist with anaemia in critically ill patients. Common causes of both include sepsis, organ failure and acute blood loss, but important differentials include disseminated intravascular coagulation (DIC) (Chapter 9) and thrombotic microangiopathies (TMAs) (Chapter 11). Review the clotting and blood film. Haemophagocytic lymphohistiocytosis (HLH) is an important cause of fever and cytopenias that can present with single or multiple organ failure (see succeeding text under extrinsic causes of extravascular haemolysis).
Pancytopenia may be an artefact (i.e. dilution during sampling) or result from BM failure or infiltration, infection, HLH, hypersplenism, drugs, autoimmune disease or megaloblastic anaemia. If pancytopenia is present, examine for splenomegaly and request a blood film and haematinics. If the cause remains unclear, the advice of a haematologist should be sought as a BM examination may be required.

If the cause of an isolated anaemia remains unclear, it is worth classifying on the basis of the mean cell volume (MCV) as this helps to direct further investigation (Figure 1.1).

c1-fig-0001 — **Figure 1.1** Investigation of anaemia in critical care. MCV, mean cell volume; DIC, disseminated intravascular coagulation; TMAs, thrombotic microangiopathies; HLH, haemophagocytic lymphohistiocytosis; BM, bone marrow; AoCD, anaemia of chronic disease; PNH, paroxysmal nocturnal haemoglobinuria.

Normocytic anaemia (MCV within the normal range)

Anaemia is most commonly normocytic in critically ill patients and usually multifactorial [4]. Iatrogenic reasons include frequent blood sampling and haemodilution (intravenous fluid administration). Occult blood loss can occur as a result of gastrointestinal mucosal inflammation and contributed to by coagulation defects, thrombocytopenia and uraemia. Another important reason is secondary anaemia, also termed anaemia of chronic disease (AoCD) or anaemia of inflammation. This is the most common cause of anaemia in hospitalized patients and can be caused by infection, cancer, autoimmune disease or chronic kidney disease [5]. This results in a functional iron deficiency, whereby adequate iron stores are present but the availability of iron for erythropoiesis is reduced. Hepcidin, a peptide produced by the liver in response to inflammatory cytokines, may be at least in part responsible for this phenomenon. Typically, ferritin is elevated and serum iron and transferrin saturation are low. Iron supplementation usually produces no improvement (in contrast to true iron deficiency). In addition to functional iron deficiency, inflammation results in reduced EPO production, reduced marrow sensitivity to EPO and shortened red cell survival.

Check haematinics as combined deficiencies can result in a normal MCV. Additionally, due to the acute phase response, a normal ferritin does not exclude iron deficiency. Consideration should be given to the possibility of occult or recent bleeding. The anaemia is likely to resolve with improvement of the underlying conditions. Meanwhile, supportive management (including transfusion when required; see Chapter 17) is the mainstay of therapy, and blood sampling should be kept to a minimum.

Microcytic anaemia (MCV below the normal range)

This is often a result of iron deficiency (blood loss, malabsorption or dietary deficiency). It can also occur during prolonged and severe illness as an extreme form of AoCD. Less commonly, microcytosis can be due to a haemoglobinopathy (e.g. thalassaemia trait), acaeruloplasminaemia, myelodysplasia, hyperthyroidism, lead poisoning and some rare congenital conditions (e.g. congenital sideroblastic anaemia).

Only gold members can continue reading. Log In or Register to continue