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Complete Blood Count (CBC)

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • If the elastic band was on your arm a long time while the blood sample was taken.
  • Taking medicines that can cause low platelet levels. Some examples of the many medicines that cause low platelet levels include steroids, some antibiotics, thiazide diuretics, chemotherapy medicines, quinidine, and meprobamate.
  • A very high white blood cell count or high levels of a type of fat (triglycerides). These can cause falsely high hemoglobin values.
  • Having an enlarged spleen camera.gif, which may cause a low platelet count (thrombocytopenia) or a low white blood cell count. An enlarged spleen may be caused by certain types of cancer.
  • Pregnancy, which normally causes a low RBC value and less often a high WBC value.

What To Think About

  • The white blood count can change by as much as 2,000 WBCs per microliter (mcL) from exercise, stress, or smoking.
  • Children normally have higher WBC (leukocyte) counts than adults.
  • Other red blood cell tests that may be done include:
    • Erythrocyte sedimentation rate (ESR). An ESR test measures how quickly red blood cells (erythrocytes) settle in a test tube. When inflammation in the body is present (such as from an infection or cancer), red blood cells may settle more slowly than normal. An ESR may help find certain inflammatory diseases when CBC results are normal. To learn more, see the topic Sedimentation Rate.
    • Reticulocyte count. This test counts the number of immature red blood cells (reticulocytes) in a blood sample. Generally, only a few reticulocytes are present in the blood in relation to mature red blood cells. But recent bleeding or mature red blood cells being destroyed can cause a lot of new reticulocytes made. This test can help find some types of anemia and check how well treatment is working. To learn more, see the topic Reticulocyte Count.
  • Hematocrit measurements can be very different depending on the method and type of machine used to do the test.


  1. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerJoseph O'Donnell, MD - Hematology, Oncology
Current as ofAugust 6, 2012

WebMD Medical Reference from Healthwise

Last Updated: August 06, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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