Reticulocyte Count

Reticulocytes are red blood cells just one or two days out of the bone marrow — still carrying traces of RNA from their maturation. A normal person has 0.5–2.5% reticulocytes in circulation. The clinically critical use is in evaluating any anaemia: a HIGH reticulocyte count means the marrow is responding correctly to blood loss or haemolysis (blood is being destroyed or lost, and the marrow is making more); a LOW reticulocyte count in an anaemic patient means the marrow is failing or starved of inputs (iron, B12, folate deficiency, marrow infiltration, drug suppression). Standardised using the 'reticulocyte production index' (RPI) which corrects for the haematocrit and the early-release of immature cells.

• Acute or chronic blood loss — marrow ramps up production; this is the normal response.
• Haemolytic anaemia — peripheral destruction; reticulocytes can exceed 10%.
• Response to treatment of deficiency anaemia — after starting iron / B12 / folate, reticulocyte count rises within days as marrow recovers.
• High-altitude acclimatisation — physiological response to lower oxygen.
• Sickle cell disease, hereditary spherocytosis — chronic compensated haemolysis.

• Iron, B12 or folate deficiency — marrow has run out of substrate; reticulocytes are inappropriately low for the degree of anaemia.
• Aplastic anaemia — marrow failure; very low or zero reticulocytes.
• Marrow infiltration — leukaemia, lymphoma, metastatic cancer.
• Chemotherapy, radiation — direct marrow suppression.
• Chronic kidney disease — low erythropoietin → low marrow stimulation.
• Severe infection, chronic inflammation — cytokine-driven suppression.