What is being tested?This test measures the number and percentage of reticulocytes in the blood and serves as an indicator of the adequacy of bone marrow red blood cell (RBC) production. Reticulocytes are immature red blood cells. They are produced in the bone marrow when
stem cells differentiate to the next stage of RBC development, eventually forming reticulocytes and finally mature RBCs. Most RBCs are fully mature before they are released from the bone marrow into the blood, but about 0.5 – 2% of the RBCs in circulation will be reticulocytes.
The body attempts to maintain a stable number of RBCs in circulation by continually removing old RBCs (approximately 120 days old) and producing new ones in the bone marrow. If this steady state is disrupted by an increased loss of RBCs or by decreased production, then the affected patient will develop anemia. Increased loss of red blood cells may be due to acute or chronic bleeding (hemorrhage) or hemolysis. The body compensates for this loss by increasing the rate of RBC production. When this happens, the number and percentage of reticulocytes in the blood increases until the balance is restored or until the production capacity of the marrow is reached.
Decreased RBC production may occur when the bone marrow is not functioning normally (due to a bone marrow disorder such as aplastic anemia or due to marrow suppression from a variety of causes including radiation and chemotherapy treatments for cancer), because of insufficient erythropoietin (a hormone produced by the kidneys to stimulate RBC production), or because of deficiencies in certain nutrients such as iron, vitamin B12, or folate. This decreased production leads to decreased numbers of RBCs in circulation, decreased amounts of hemoglobin (an oxygen-carrying protein inside the RBC), a decreased hematocrit (the amount of cells versus plasma in the blood), and a decreasing number of reticulocytes as old RBCs are removed from the bloodstream but not fully replaced.
Occasionally, both the reticulocyte count and the RBC count will be increased because of excess RBC production. This may be due to a variety of causes including inappropriately increased production of erythropoietin, disorders that chronically produce increased numbers of RBCs (polycythemia vera), and even smoking.
