Reticulocytes

Some laboratories use 25 to 125 or 50 to 100; use the range your lab printed.

Reticulocytes are red blood cells that have left the bone marrow within the last day or two. They still carry traces of the machinery used to make haemoglobin, which is what the analyser detects. Counting them shows how fast the marrow is making new red cells. This is the absolute number in a given volume of blood.

Doctors request it when the haemoglobin is low, to see whether the bone marrow is responding by making more red cells (as after bleeding or when red cells are being broken down) or is not responding (as with low iron, B12 or folate, or a marrow problem). It is also used to check that iron or vitamin replacement is working.

• Recent blood loss
• Conditions where red cells are broken down early (haemolysis)
• The first weeks after starting iron, vitamin B12 or folate replacement
• Recovery of the bone marrow after chemotherapy
• Living at altitude
• Erythropoietin injections

• Low iron, vitamin B12 or folate
• Long-term kidney disease
• Bone marrow conditions
• Chemotherapy and radiotherapy
• Long-term inflammatory illness
• Some infections that suppress the marrow, such as parvovirus

• A rise shortly after blood donation or a bleed is the expected response
• A rise in the first fortnight of iron or vitamin replacement is expected
• Analysers measure it with limited precision, so day-to-day differences are common

No fasting needed. Interpreted alongside the haemoglobin; a count that is normal when the haemoglobin is low is itself informative.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.