WBC blood test: what a high or low white blood cell count means

What the WBC count measures

White blood cells (leukocytes) are made in the bone marrow and help your body fight infection and disease. The WBC count is the total number in a sample of blood, reported in thousands per microliter.

There are five types: neutrophils, lymphocytes, monocytes, eosinophils and basophils. A CBC with differential counts each one; see the CBC guide and the WBC differential lab guide.

What a high WBC can mean

Clinicians usually call a count above about 11,000 per microliter in a non-pregnant adult leukocytosis. Common causes:

• Infection, bacterial or viral.
• Physical or emotional stress: surgery, hard exercise, injury, burns. Stress hormones release white cells stored along vessel walls.
• Smoking and obesity.
• Inflammatory and autoimmune conditions, such as rheumatoid arthritis or inflammatory bowel disease.
• Medicines, including corticosteroids, lithium and some asthma inhalers (beta agonists).
• Allergic reactions and pregnancy. The upper limit rises to about 13,200 in the third trimester.
• Less common: leukemia and other blood cancers. A count above 100,000 is almost always one of these.

Timing helps. A rise over hours to days points to infection or stress, while one lasting weeks to months points to chronic inflammation or, less often, a blood disorder.

What a low WBC can mean

A low count (leukopenia) most often comes from a low neutrophil count. Causes include:

• Recent viral illness, which can lower counts for a while.
• Medicines, including chemotherapy, some antibiotics, antithyroid drugs and some psychiatric medicines.
• Autoimmune conditions such as lupus, and HIV.
• Low vitamin B12 or folate, or severe undernutrition.
• Liver or spleen disease.
• Bone marrow conditions or cancers that crowd out normal cells.

Some healthy people of African, Middle Eastern or Jewish ancestry naturally run lower neutrophil counts without extra infection risk. This is called benign ethnic neutropenia and is worth mentioning to your clinician if your count is mildly low every time.

Gabriel's optimal range

Typical lab ranges run from about 3.4 to 10.8 thousand per microliter, though each lab sets its own. Gabriel's optimal range is 3.8 to 6.0, based on long-term aging research where mortality was lowest in roughly that band. A count that is in the lab range but above 6 can be a quiet signal of inflammation, smoking or excess weight worth discussing.

What to do with an abnormal result

1. Look at the differential. Which cell type is high or low narrows the cause: neutrophils, lymphocytes, monocytes, eosinophils or basophils.
2. Check the rest of the CBC. Problems in more than one cell line (red cells, white cells, platelets) are more concerning than one mild flag.
3. Note recent illness, exercise, stress and medicines. All of them move the count.
4. Ask about a repeat CBC. Clinicians often recheck the count, and look at a blood smear, to see whether a mild change persists.
5. If you have a report in hand, you can upload it to Gabriel for a plain-words read of every marker, then take your questions to your clinician.

See a clinician promptly if an abnormal count comes with fever above 100.4°F, drenching night sweats, weight loss you can't explain, easy bruising or bleeding, or swollen lymph nodes. If you know your neutrophils are low and you get a fever, call your clinician or seek care the same day.