The development of anemia due to bleeding follows a pattern determined by the origin and severity (acute or chronic) of hemorrhage. In the acute phase, with loss of blood volume, there may be no significant changes in hemoglobin level or hematocrit. Only after hemodilution occurs as a compensatory mechanism to restore blood volume will a reduced hemoglobin concentration and hematocrit be detected. For this reason, a single hemoglobin determination is not sufficient to determine or exclude blood loss in the acute situation and therefore should be regularly repeated.
Anemia resulting from an episode of acute bleeding may be normochromic and normocytic until the reticulocyte response occurs. This response, which normally occurs after 24 to 48 h, may be delayed if some underlying bone marrow disease is present. After the increase in reticulocytes, which may be accompanied by a mild leukocytosis and/or thrombocytosis, the anemia will have a macrocytic aspect because of the predominance of younger red cells which have a larger volume than old erythrocytes.
Chronic blood loss due to hemorrhage from a discrete lesion may remain clinically undetected and eventually result in a microcytic anemia due to depletion of the body's iron reserves. This phase will be accompanied by a normal or low reticulocyte count.
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