If an automated count is not available, a reticulocyte count can be done manually following special staining of the blood film. In manual examination, activity of the bone marrow can also be gauged qualitatively by subtle changes in the numbers and the morphology of young RBCs by examination under a microscope. Newly formed RBCs are usually slightly larger than older RBCs and show polychromasia. Even where the source of blood loss is obvious, evaluation of erythropoiesis can help assess whether the bone marrow will be able to compensate for the loss and at what rate. When the cause is not obvious, clinicians use other tests, such as: ESR, serum iron, transferrin, RBC folate level, hemoglobin electrophoresis, renal function tests (e.g. serum creatinine) although the tests will depend on the clinical hypothesis that is being investigated. When the diagnosis remains difficult, a bone marrow examination allows direct examination of the precursors to red cells, although is rarely used as is painful, invasive and is hence reserved for cases where severe pathology needs to be determined or excluded. Red blood cell size
In the morphological approach, anemia is classified by the size of red blood cells; this is either done automatically or on microscopic examination of a peripheral blood smear. The size is reflected in the mean corpuscular volume (MCV).
Rh disease, one of the causes of hemolytic disease of the newborn
Transfusion reaction to blood transfusions
Mechanical trauma to red blood cells
Microangiopathic hemolytic anemias, including thrombotic thrombocytopenic purpura and disseminated intravascular coagulation
Infections, including malaria
Heart surgery
Haemodialysis
Parasitic
Trypanosoma congolense alters the surfaces of RBCs of its host and this may explain T. c. induced anemia

Blood loss
Anemia of prematurity, from frequent blood sampling for laboratory testing, combined with insufficient RBC production
Trauma or surgery, causing acute blood loss
Gastrointestinal tract lesions, causing either acute bleeds (e.g. variceal lesions, peptic ulcers) or chronic blood loss (e.g. angiodysplasia)
Gynecologic disturbances, also generally causing chronic blood loss
From menstruation, mostly among young women or older women who have fibroids
Many type of cancers, including colorectal cancer and cancer of the urinary bladder, may cause acute or chronic blood loss, especially at advanced stages
Infection by intestinal nematodes feeding on blood, such as hookworms and the whipworm Trichuris trichiura 
Iatrogenic anemia, blood loss from repeated blood draws and medical procedures.The roots of the words anemia and ischemia both refer to the basic idea of "lack of blood", but anemia and ischemia are not the same thing in modern medical terminology. The word anemia used alone implies widespread effects from blood that either is too scarce (e.g., blood loss) or is dysfunctional in its oxygen-supplying ability (due to whatever type of hemoglobin or erythrocyte problem). In contrast, the word ischemia refers solely to the lack of blood (poor perfusion). Thus ischemia in a body part can cause localized anemic effects within those tissues.