It is the test to determine how fast is reticulocyte, i.e., red blood cells are being made by the bone marrow and is released in the blood. Before getting converted into the mature red blood cells, reticulocyte remains in the blood for a time period of two days. The count of the reticulocytes increases in blood during certain disease which causes the death of the red blood cells at premature stage, as in haemolytic anaemia. When we move towards higher altitude, reticulocyte count increases in order to facilitate more oxygen supply to the tissues to compensate the low pO2. The normal range of the reticulocyte in the blood is 0.5-1.55 in normal human adults. The reticulocyte count is used to estimate the normal degree of the effective erythropoiesis, production of red blood cells, which can be reported as the reticulocyte percentage or absolute reticulocyte count. The rate of the reticulocyte percentage is high in case of anaemia and the true bone marrow response to the anaemia might be hidden.