Blood count · 10^9/L

Leukocytes (WBC)

Leukocytes (white blood cells) are cells of the immune system — the body's main defense against infections (bacteria, viruses, fungi) and the removal of damaged cells. They are produced in the bone marrow, circulate in the blood, and patrol the tissues. The total count is the sum of five subtypes, each with its own role.

See my Leukocytes (WBC) explained →

Ranges we useNORIP (Nordic Reference Interval Project)

Rating	Range	What it means
Talk to a doctor	below 3.5 10^9/L
Good	3.5 – 8.8 10^9/L	Within the reference range.
Worth improving	8.8 10^9/L and above

Source: NORIP (Nordic Reference Interval Project) · A multicentre study of reference intervals for haemoglobin, basic blood cell counts and erythrocyte indices in the adult population of the Nordic countries (Nordin G et al.) · 2004 · open

Why it matters

Elevated with infection/inflammation; low in some conditions.

If it’s low

Low leukocytes (leukopenia) mean fewer immune cells and greater susceptibility to infection.

If it’s high

Elevated leukocytes (leukocytosis) mean more white blood cells than usual — the body is often bolstering its defenses. A mild transient rise is often benign; more important are the magnitude and persistence.

What to do

A mild transient rise during infection is common — repeat after ~4 weeks, when you are well, to see the true baseline. A marked or persistent abnormality belongs to a doctor (differential blood count).