A reticulocyte count is a blood test that measures how fast red blood cells called reticulocytes are made by the bone marrow and released into the blood. Reticulocytes are in the blood for about 2 days before developing into mature red blood cells. Normally, about 1% to 2% of the red blood cells in the blood are reticulocytes.
The reticulocyte count rises when there is a lot of blood loss or in certain diseases in which red blood cells are destroyed prematurely, such as hemolytic anemia. Also, being at high altitudes may cause reticulocyte counts to rise, to help you adjust to the lower oxygen levels at high altitudes. Click on the link above to read the rest of hte 4 page WebMD article