Reticulocytes (retics) are immature red blood cells. While not uncommon to be circulating in the blood stream in small quantities, an elevated number of retics can indicate a number of disease states and patient conditions. Physicians typically order retic counts when a patient has a low hemoglobin and hematocrit, bone marrow production is being assessed, anemia is suspected, or the patient is receiving chemotherapy.

Before red blood cells are released into the blood stream by the bone marrow, their nucleus is removed on their way out, making room for hemoglobin to fill the vacated space. Since hemoglobin has a high affinity for oxygen, the more hemoglobin in a red blood cell, the more oxygen it can deliver to all body systems. 

Retics contain fragments of the nucleus and typically constitute less than one percent of a patient's circulating red blood cells. However, when the body senses a sudden urgency for more red blood cells---for example, if the patient has lost a significant amount of blood or is becoming anemic for other reasons---it tends to release red blood cells into the circulatory system before they're fully mature. When this occurs, fragments of the nucleus remain in the cell as it enters the circulatory system. When a patient's blood has an elevated number of retics, it tells physicians their patient's body senses a blood loss and an urgent need to deliver more cells to transport oxygen, even if those cells are immature. 

 Anemic patients are expected to have elevated reticulocytes in their blood, signifying the body's natural mechanism to overcome anemia is working, and the bone marrow is functionally replenishing the body's red blood cells. Anemia without elevated reticulocytes could indicate aplastic anemia or other bone marrow abnormalities.

 Retics are drawn in EDTA tubes and are stable at room temperature for 24 hours. Stability at refrigerated temperatures is up to 72 hours. The immature cells can be detected and counted by observing a stained blood smear, but are most often performed by the same hematology instrument that conducts a CBC on the same tube, or by automated flow cytometry.

References

1. LabTestsOnline. American Association for Clinical Chemistry. AACC. Accessed 4/3/2020.
2. Wu A. Tietz Clinical Guide to Laboratory Tests---Fourth Edition. Elsevier. St. Louis, Missouri. 2006.