A low white blood cell count can be caused by viral infections, medication side effects, nutritional deficiencies and, in some cases, problems such as sepsis, systemic lupus erythematosus or myelodysplasia.

White blood cells are part of the immune system and levels are considered low when they are less than 4000 leukocytes per μL of blood. This condition is also referred to as leukopenia.

A low white blood cell count should be assessed by a hematologist, so that the underlying cause is identified and treated as necessary.

Doctor explaining blood test results to patient in office.

What causes a low white blood cell count?

The main causes of low leukocyte levels are:

1. Viral infections

Some viral infections, like dengue fever, infectious mononucleosis, hepatitis, measles, or HIV, can cause low white blood cell levels due to the destruction of lymphocytes or neutrophils or due a decrease of white blood cell production in the body.

Viral infections can present with symptoms such as fever, nausea, vomiting, and/or diarrhea, although these symptoms will vary depending on the infection and the area of the body it is affecting.

What to do: if you suspect you may have a viral infection, you should see an infectious disease specialist or general practitioner, who may be prescribed medication to relieve symptoms such as fever or nausea and, in some cases, antivirals.

White blood cell counts typically return to normal when treatment is carried out as prescribed by a doctor.

2. Side effects of medications

A low white blood cell count can also be caused by the side effects of medications, like chemotherapy, corticosteroids, and some antibiotics or diuretics. These medication can affect the production of white blood cells in the bone marrow in some cases.

What to do: if you are experiencing side effects of medications, you are advised to follow-up with your prescribing doctor, who may advise suspending, changing, or adjusting the dose of the medication.

All medications should be used as prescribed by a doctor, in the doses and for the duration indicated.

3. Vitamin B12 or folate deficiency

A deficiency in nutrients such as vitamin B12 or folic acid can impair the production of red and white blood cells in the bone marrow.

A vtamin B12 or folate deficiency can lead to symptoms such as fatigue, paleness, tingling, mental confusion or weakness.

What to do: Low white blood cell counts tend to improve as the B12 or folate deficiency is corrected. Therefore, if you suspect you may have a deficiency, you are advised to consult a hematologist or general practitioner, who may indicate the replacement of these nutrients with supplements, injections or dietary changes.

4. Sepsis

Sepsis is a type of systemic infection that can lead to low or high white blood cell counts, as well as rapid breathing, increased heart rate and abnormal body temperatures. Other symptoms may also present depending on the cause and severity of the infection.

What to do: You should proceed to an emergency room if you present with symptoms of sepsis. Treatment depends on the cause of infection and may involve the use of antibiotics and, in some cases, surgery.

5. Systemic lupus erythematosus

A low white blood cell count is a common finding in systemic lupus erythematosus. This is due to the production of abnormal antibodies that can also stimulate the destruction of neutrophils and/or lymphocytes in the blood.

Systemic lupus erythematosus, or lupus for short, can also cause other abnormal red blood cell and/or platelet levels, as well as symptoms like ever, fatigue, weight loss, skin lesions or joint pain. Read more about the lupus of symptoms and how they can prevent.

What to do: If you suspect you may have lupus, you should see a rheumatologist for assessment. A low white blood cell count tends to improve with treatment, which may involve medications such as oral corticosteroids and antimalarials. Some patients may also benefits from maintaining a lupus diet.

6. Myelodysplasia

Although this is a rare condition, myelodysplasia is a bone marrow disorder that can also cause a low white blood cell count, as well as a drop in red blood cell and/or platelet levels. Depending on the blood cells affected, patients can present with symptoms like fatigue, shortness of breath, heart palpitations, bleeding and, in the case of low leukocytes, an increased risk for infections.

What to do: Myelodysplasia should be investigated thoroughly by a hematologist. Treatment depends on the severity of the patient's symptoms, and may involve blood transfusions, immunosuppressive medications or a bone marrow transplant.