Leukopenia

Leukopenia (also known as leukocytopenia, or leucopenia, from Greek λευκό-white and πενία-deficiency) is a decrease in the number of white blood cells (leukocytes) found in the blood, which places individuals at increased risk of infection.

Neutropenia is a sub-type of leukopenia that refers to a decrease in the number of circulating neutrophil granulocytes, the most abundant white blood cells. The terms leukopenia and neutropenia may occasionally be used interchangeably, as the neutrophil count is the most important indicator of infection risk.

Causes
Low white cell counts maybe due to a recent inflection such as a cold/flu. It can also be associated with chemotherapy, radiation therapy, myelofibrosis and aplastic anemia (failure of white and red cell creation, along with poor platelet production). In addition, many common medications can cause leukopenia (see below). HIV and AIDS are also a threat to white cells.

Other causes of low white blood cell count include: Influenza, systemic lupus erythematosus, Hodgkin's lymphoma, some types of cancer, typhoid, malaria, tuberculosis, dengue, Rickettsial infections, enlargement of the spleen, folate deficiencies, psittacosis and sepsis. Many other causes exist, such as a deficiency in certain minerals such as copper and zinc.

Pseudoleukopenia can develop upon the onset of infection. The leukocytes (predominately neutrophils, responding to injury first) are marginalized in the blood vessels so that they can scan for the site of infection. This means that even though there is increased WBC production, it will appear as though it is low from a blood sample, since the blood sample is of core blood and does not include the marginalized leukocytes.

Medications causing leukopenia
Some medications can have an impact on the number and function of white blood cells. Medications which can cause leukopenia include clozapine, an antipsychotic medication with a rare adverse effect leading to the total absence of all granulocytes (neutrophils, basophils, eosinophils). Other medications include immunosuppressive drugs, such as sirolimus, mycophenolate mofetil, tacrolimus, and cyclosporine. Interferons used to treat multiple sclerosis, like Rebif, Avonex, and Betaseron, can also cause leukopenia. The antidepressant and smoking addiction treatment drug Wellbutrin (Bupropion HCL) can also cause leukopenia with long-term use. Minocycline, a commonly prescribed antibiotic, is another drug known to cause leukopenia.

There are also reports of leukopenia caused by Depakote (divalproex sodium or valproic acid), a drug used for epilepsy (seizures), mania (with bipolar disorder) and migraine. Decreased white blood cell count (leukopenia) may be present in cases of arsenic toxicity.

The anticonvulsant drug, Lamotrigine, has been associated with a decrease in white blood cell count.

Diagnosis
Leukopenia can be identified with a complete blood count.

Below are blood reference ranges for various types leucocytes/WBCs. The 2.5 percentile (right limits in intervals in image, showing 95% prediction intervals) is a common limit for defining leukocytosis.