Reticulocyte Count

The reticulocyte count measures newly released, RNA-containing red cells in peripheral blood. It reflects marrow erythroid output and decides whether an anaemia is hypoproliferative or the result of loss or destruction with an appropriate marrow response.

Automated counting uses fluorescent nucleic acid dyes and flow cytometry. Manual counting uses supravital staining with new methylene blue.

Indications. Every unexplained anaemia, suspected haemolysis, response to iron, vitamin B12 or folate treatment, recovery after chemotherapy or transplant, monitoring of erythropoietin therapy.

Specimen. EDTA blood, tested within 24 hours. Delay lowers the percentage as RNA degrades.

Reference values. Adults: 0.5 to 2.5% of red cells, with an absolute count of 25 to 100 x10⁹/L. Newborns: 2 to 6% at birth, falling towards adult values over the first 2 weeks.

Corrected reticulocyte count. Reticulocyte percentage multiplied by the patient haematocrit and divided by 0.45. It adjusts for a low red cell mass.

Reticulocyte production index. Corrected percentage divided by the maturation time in days (1.0 at haematocrit 0.45, 1.5 at 0.35, 2.0 at 0.25, 2.5 at 0.15). An index over 2 indicates an appropriate marrow response, and under 2 an inadequate one.

Immature reticulocyte fraction. Early marker of marrow recovery and engraftment. The reference range is analyser specific.

Reticulocyte haemoglobin content. Haemoglobin content of reticulocytes. Values under about 28 pg suggest iron restricted erythropoiesis and are useful when ferritin is unreliable, for example in inflammation or kidney disease. Cut offs are analyser specific.

Raised count with anaemia (appropriate response). Haemolysis (autoimmune, hereditary, microangiopathic, mechanical), acute blood loss, response to iron, vitamin B12 or folate replacement, marrow recovery after chemotherapy or transplant, high altitude, erythropoietin therapy.

Low or inappropriately normal count with anaemia (hypoproliferative). Untreated iron, vitamin B12 or folate deficiency, anaemia of chronic disease, chronic kidney disease, aplastic anaemia, pure red cell aplasia, marrow infiltration, myelodysplasia, endocrine disease, ineffective erythropoiesis (megaloblastic anaemia, thalassaemia), chemotherapy.

Timing after treatment. Reticulocytes rise 3 to 4 days after starting iron, B12 or folate and peak at day 5 to 10. After haemorrhage the count rises over 3 days and peaks at 7 to 10 days.

Critical findings needing immediate action. Severe anaemia with reticulocytopenia in a patient with chronic haemolysis (sickle cell disease, hereditary spherocytosis) indicates aplastic crisis, usually from parvovirus B19.

Pitfalls.

• The percentage is falsely high when the red cell count is low, so use the absolute count or the corrected index.
• Howell-Jolly bodies, Heinz bodies, Pappenheimer bodies, malaria parasites, platelet clumps and nucleated red cells can be miscounted.
• Manual counts are imprecise. At least 1000 cells should be counted, ideally by two observers.
• Recent transfusion dilutes the count.

Special groups. Neonatal values are age specific, and preterm infants have higher counts. Mild reticulocytosis is normal in pregnancy.

Follow up tests. Blood film, direct antiglobulin test, haptoglobin, lactate dehydrogenase, bilirubin, ferritin, vitamin B12, folate, parvovirus B19 PCR, bone marrow examination.