Red cell count

Many laboratories print separate ranges for women (around 3.8 to 4.8) and men (around 4.5 to 5.5); use the range your lab printed.

This is the number of red blood cells in a given volume of blood. The analyser counts the cells directly, and then uses this count together with the haemoglobin and haematocrit to work out the average size and haemoglobin content of each cell.

It is part of the full blood count. On its own it is less informative than haemoglobin, but doctors look at it when the red cells are unusually small or large, because the count and the haemoglobin can then move in different directions.

• Dehydration on the day of the test
• Smoking
• Living at altitude
• Thalassaemia trait, where the cells are small but numerous
• Conditions where the bone marrow makes too many red cells

• Blood loss
• Low iron, vitamin B12 or folate
• Pregnancy
• Long-term kidney disease
• Bone marrow conditions and some medicines

• Hydration on the day changes it slightly
• Small differences between analysers are common
• It is a little lower in the general population in older age

No fasting needed. Interpreted together with haemoglobin and the red cell indices rather than alone.

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