La anemia se define como una concentración baja de hemoglobina en la sangre. Se detecta mediante un análisis de laboratorio en el que se descubre un nivel de hemoglobina en la sangre menor de lo normal. Puede acompañarse de otros parámetros alterados, como disminución del número de glóbulos rojos, o disminución del hematocrito, pero no es correcto definirla como disminución de la cantidad de glóbulos rojos, pues estas células sanguíneas pueden variar considerablemente en tamaño, en ocasiones el número de glóbulos rojos es normal y sin embargo existe anemia. La anemia no es una enfermedad, sino un signo que puede estar originado por múltiples causas. La deficiencia de hierro precede a la aparición de la anemia ferropénica, la cual es la causa principal de todas las formas posibles de anemia. Se caracteriza por un descenso de las cifras de hemoglobina, hematíes pequeños con poca cantidad de hemoglobina en su interior y cifras bajas de hierro en los depósitos (descenso de la ferritina). Se produce principalmente por patologías gastrointestinales o, en el caso de las mujeres jóvenes, por menstruaciones habitualmente muy abundantes; no obstante, se comete con frecuencia el error de atribuir la presencia de anemia ferropénica a los sangrados menstruales, sin hacer otras evaluaciones. Por lo general, la anemia se maneja inadecuadamente en la mayoría de los pacientes, puesto que no se suele investigar su origen o la evaluación es incompleta, limitándose el médico a administrar tratamientos con hierro oral. La hemoglobina es una molécula que se encuentra en el interior de los glóbulos rojos de la sangre y sirve para transportar el oxígeno hasta los tejidos. Por ello cuando existe anemia severa, los tejidos y órganos del organismo no reciben suficiente oxígeno, la persona se siente cansada, su pulso está acelerado, tolera mal el esfuerzo y tiene sensación de falta de aire.

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Anemia, also spelled anaemia, is usually defined as a decrease in the amount of red blood cells (RBCs) or hemoglobin in the blood. It can also be defined as a lowered ability of the blood to carry oxygen. When anemia comes on slowly, the symptoms are often vague and may include feeling tired, weakness, shortness of breath or a poor ability to exercise. Anemia that comes on quickly often has greater symptoms, which may include confusion, feeling like one is going to pass out, loss of consciousness, or increased thirst. Anemia must be significant before a person becomes noticeably pale. Additional symptoms may occur depending on the underlying cause. There are three main types of anemia: that due to blood loss, that due to decreased red blood cell production and that due to increased red blood cell breakdown. Causes of blood loss include trauma and gastrointestinal bleeding, among others. Causes of decreased production include iron deficiency, a lack of vitamin B12, thalassemia, and a number of neoplasms of the bone marrow. Causes of increased breakdown include a number of genetic conditions such as sickle cell anemia, infections like malaria, and certain autoimmune diseases. It can also be classified based on the size of red blood cells and amount of hemoglobin in each cell. If the cells are small, it is microcytic anemia. If they are large, it is macrocytic anemia while if they are normal sized, it is normocytic anemia. Diagnosis in men is based on a hemoglobin of less than 130 to 140 g/L (13 to 14 g/dL), while in women, it must be less than 120 to 130 g/L (12 to 13 g/dL). Further testing is then required to determine the cause. Certain groups of individuals, such as pregnant women, benefit from the use of iron pills for prevention. Dietary supplementation, without determining the specific cause, is not recommended. The use of blood transfusions is typically based on a person's signs and symptoms. In those without symptoms, they are not recommended unless hemoglobin levels are less than 60 to 80 g/L (6 to 8 g/dL). These recommendations may also apply to some people with acute bleeding. Erythropoiesis-stimulating medications are only recommended in those with severe anemia. Anemia is the most common disorder of the blood, affecting about a quarter of the people globally. Iron-deficiency anemia affects nearly 1 billion. In 2013, anemia due to iron deficiency resulted in about 183,000 deaths – down from 213,000 deaths in 1990. It is more common in females than males, among children, during pregnancy, and in the elderly. Anemia increases costs of medical care and lowers a person's productivity through a decreased ability to work. The name is derived from Ancient Greek: ἀναιμία anaimia, meaning "lack of blood", from ἀν- an-, "not" + αἷμα haima, "blood".

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