Reticulocytes
What it measures, and what changes the number.
Reticulocytes are young red blood cells recently released from the bone marrow. They still contain traces of genetic material, which lets automated analyzers or a stained blood smear count them. The result reflects how actively the marrow is producing new red cells.
The test is usually read with the red blood cell count, hemoglobin and hematocrit, and often with iron studies, vitamin B12 and folate, because together they help show whether red cell production is keeping pace with loss or breakdown.
It can be reported as a percentage of red cells, as an absolute count, or both, so the unit on the report matters. Counts from automated analyzers and manual smears can differ. Samples are drawn into an EDTA tube and are best analysed promptly. Spanish-language reports may print it as Reticulocitos.
Often read alongside: Red blood cells, Hemoglobin, Hematocrit, Ferritin, Vitamin B12