Anaemia is a condition in which the blood has an insufficient concentration of haemoglobin or an insufficient red-cell mass for the physiological requirements of the individual, with the precise diagnostic threshold depending on factors such as age, sex, pregnancy and laboratory reference standards. Haemoglobin is the iron-containing protein in red blood cells responsible for transporting oxygen from the lungs to tissues. Anaemia can therefore reduce oxygen-carrying capacity, although the physiological response depends on how rapidly the condition develops and whether other compensatory mechanisms are available. Anaemia is a finding or clinical condition with many causes rather than a single disease.
Anaemia can be classified by mechanism and by red-cell characteristics. Major mechanisms include inadequate red-cell production, increased red-cell destruction and blood loss. Reduced production can result from iron deficiency, deficiencies affecting cell formation, chronic disease, kidney dysfunction or bone-marrow disorders. Increased destruction is termed haemolysis and can arise from inherited abnormalities, immune processes, infections or other causes. Blood loss may be obvious, such as major trauma or heavy menstrual bleeding, or occult, such as chronic gastrointestinal bleeding. Identifying the mechanism is more informative than treating the haemoglobin value in isolation.
Iron deficiency is one of the most common causes of Anaemia worldwide. Iron is required for haemoglobin synthesis, so inadequate dietary intake, impaired absorption or increased physiological requirements can produce iron depletion. Chronic blood loss is an important cause in adults, and the appropriate investigation depends on age, sex, symptoms and clinical context. Iron deficiency can exist before haemoglobin falls sufficiently to meet conventional definitions of Anaemia. Laboratory evaluation commonly considers haemoglobin, mean corpuscular volume, ferritin and other iron-related measurements, although ferritin can rise during inflammation and must therefore be interpreted in context.
Anaemia can also arise from deficiencies of vitamin B12 or folate, which impair normal DNA synthesis during red-cell production. These deficiencies can produce larger-than-normal red cells and may be accompanied by abnormalities in other blood-cell lines. Vitamin B12 deficiency is particularly important because neurological complications can occur and may become persistent if treatment is delayed. Causes include inadequate intake, malabsorption, autoimmune processes and certain medications or gastrointestinal conditions. Folate deficiency has different causes and management considerations. Correct diagnosis matters because simply giving iron will not correct Anaemia caused by these mechanisms.
Chronic inflammatory conditions can produce Anaemia through altered iron handling, reduced erythropoietin responsiveness and impaired red-cell production. Kidney disease can contribute because the kidneys normally produce erythropoietin, a hormone stimulating red-cell production in bone marrow. Bone-marrow disorders can reduce production of red cells and other blood-cell types. Haemolytic Anaemia involves shortened red-cell survival, with the body attempting to compensate by increasing production. The resulting laboratory pattern can include changes in reticulocyte count, bilirubin, lactate dehydrogenase and other markers, depending on the cause.
Symptoms of Anaemia depend on severity, speed of development, underlying cause and individual physiological reserve. Common manifestations can include fatigue, weakness, reduced exercise tolerance, breathlessness and pallor. More severe or rapidly developing Anaemia can cause dizziness, rapid heartbeat, chest discomfort or worsening of pre-existing cardiovascular disease. Some people with gradually developing Anaemia have relatively few symptoms because physiological compensation develops over time. Symptoms alone cannot establish the cause, and similar complaints can arise from many other conditions. Blood testing is therefore generally necessary when Anaemia is suspected.
Investigation of Anaemia begins with a blood count and then uses the pattern of results to guide further testing. Mean corpuscular volume can classify red cells as microcytic, normocytic or macrocytic, although these categories overlap and should not be treated as diagnoses. Reticulocyte measurements help indicate whether bone marrow is responding appropriately. Additional tests may assess iron status, vitamin levels, kidney function, inflammation, haemolysis or blood loss. Clinical history is equally important because diet, menstrual history, gastrointestinal symptoms, medications, pregnancy, family history and chronic illness can point towards different mechanisms.
Treatment of Anaemia is directed at its underlying cause. Iron replacement can correct confirmed iron deficiency, while vitamin replacement addresses appropriate nutritional deficiencies. Treating bleeding, managing chronic disease, correcting kidney-related mechanisms or addressing haemolysis may be necessary in other cases. Blood transfusion can rapidly increase oxygen-carrying capacity in selected severe or symptomatic situations, but it does not necessarily correct the underlying cause and carries its own risks. A common misconception is that all Anaemia means iron deficiency; assuming this can delay diagnosis of gastrointestinal bleeding, nutritional deficiency, kidney disease, inherited conditions or other significant disorders.
