Abnormal levels of one or all parts of a CBC may indicate a variety of conditions, as shown below:

• Red blood cell tests. High levels of RBC, hematocrit or hemoglobin indicate a lack of fluid in the body. This may be caused by smoking, congenital heart disease (cardiac abnormality, defect or malformation that is present from birth), dehydration or kidney disease. Low levels are usually an indicator of anemia, which could be caused by blood loss, malnutrition or leukemia.
  
  
• White blood cell tests. High WBC levels may indicate a heart attack, infection or an inflammatory disease such as rheumatoid arthritis. Severe emotional stress or tissue damage, such as burns, can also cause an elevated WBC. Low levels can be caused by kidney or liver disease, radiation exposure or the presence of a toxic substance in the body. The WBC differential can also help to determine the cause of high WBC levels. For instance, neutrophil increases usually indicate a heart attack or infection, lymphocyte increases can be a sign of leukemia and increased eosinophils can be a sign of an allergic reaction.
  
  
• Platelet tests. Platelet levels may be increased by severe bleeding, infection, strenuous exercise, pregnancy and other factors. Low levels are usually caused by an infection, cancer or a lack of either folic acid or vitamin B-12.