Typically, a person infected with severe acute respiratory syndrome (SARS) has a low white blood cell count, also known as leukopenia. Leukopenia occurs when your white blood cell count is under 3,500 white blood cells per microliter of blood. There are five different types of white blood cells: basophils, eosinophils, lymphocytes, monocytes, and neutrophils. A person infected with SARS has low lymphocyte levels [source: Medline Plus].
White blood cells, or leukocytes, are responsible for fighting infections and viruses in the body. There are many reasons why a person may have low white blood cell levels. In the case of SARS, the overwhelming infection uses up white blood cells at a faster rate than the body can produce them. Other common causes of low blood cell counts include cancers, diseases or viruses, congenital disorders, or drugs that damage the bone marrow. Many different medical conditions lead to low white blood cell counts, including HIV/AIDS, hyperthyroidism, lupus, leukemia, rheumatoid arthritis, and vitamin D deficiency. In fact, simply taking antibiotics and diuretics can lower your white blood cell count [source: Mayo Clinic]. Therefore, it's essential that your doctor does a thorough diagnostic assessment to rule out other conditions that lead to low white blood cell counts.
In particular, your doctor will likely order a chest X-ray if he suspects you may have SARS. SARS patients frequently have signs of pneumonia or respiratory distress syndrome on chest X-rays. Your doctor will also consider other blood-related concerns. There is even a specific blood test for SARS, called the PCR test, which determines if you have SARS antibodies in your blood. If you do, you probably are infected with SARS.