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Complete Blood Count (CBC): at a glance

giving blood blood test
giving blood blood test

The complete blood count (CBC) is a test that evaluates the cells that circulate in blood and helps determine your general health status. It may be used to screen for, diagnose, or monitor any one of a variety of diseases and conditions.

The complete blood count (CBC) is a test that evaluates the cells that circulate in blood. Blood consists of three types of cells suspended in fluid called plasma: white blood cells (WBCs) that protect against infections, red blood cells (RBCs) that carry oxygen throughout the body, and platelets (PLTs) that help control bleeding. These cells are produced and mature primarily in the bone marrow and, under normal circumstances, are released into the bloodstream as needed.

A CBC is typically performed using an automated instrument that measures various parameters, including counts of the cells that are present in a person’s sample of blood. The results of a CBC can provide information about not only the number of cell types but also can give an indication of the physical characteristics of some of the cells.

Significantly abnormal results in one or more of the blood cell types can indicate the presence of one or more conditions. Typically, other tests are performed to help determine the cause of abnormal results. Often, this requires visual confirmation by examining a blood smear under a microscope. A trained laboratorian can evaluate the appearance and physical characteristics of the blood cells, such as size, shape and color, noting any abnormalities that may be present. Any additional information is noted and reported to the healthcare provider. This information gives the health practitioner additional clues as to the cause of abnormal CBC results.

Why get tested?

The CBC helps determine your general health status. It may be used to screen for, diagnose, or monitor any one of a variety of diseases and conditions, such as anemia, infection, inflammation, bleeding disorder or cancer.

When to get tested?

A CBC may be done as part of a routine health exam. It may also be done when you are ill and have signs and symptoms that may be related to a condition that affects blood cells. A CBC may be performed at regular intervals to monitor treatment for an illness or when you are receiving treatment that is known to affect blood cells

What is the sample required?

A blood sample is drawn typically from a vein in your arm. Alternatively, a blood sample may be collected by pricking your fingertip or, for infants, the heel.

Do I need to do anything to prepare for the test?

No, there is no advanced preparation required for a CBC.

If I have abnormal results on my CBC, what other tests might my doctor order?

It depends on the results that are abnormal and the suspected cause as well as your medical history and findings from your physical examination. Your healthcare provider may request that a blood smear examination be done. Other general tests to check your health and to look for possible causes may include a comprehensive metabolic panel (CMP). A few other general examples include:

  • Abnormal results for WBCs may be followed by a culture if an infection is suspected (e.g., blood culture, urine culture, sputum culture). If inflammation is suspected, then a C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) test may be done. 
  • Abnormal RBC results may prompt a reticulocyte count, iron studies or tests for vitamin B12 and folate to help make a diagnosis. 
  • An abnormal platelet count may be followed by tests that further evaluate platelets, such as platelet function tests. Additional tests may be done to check for bleeding disorders such as prothrombin time (PT) or partial thromboplastin time (PTT).

Numerous other tests specific for certain conditions may be needed to establish a diagnosis. Talk to your healthcare provider about the results of your CBC, whether additional tests are necessary, and why.

For answers to more questions like What is being tested? How to prepare for a CBC? or What do the CBC results mean?, as well as information on 300+ other tests visit (US and Canada) or (UK)

Source: AACC Lab Tests Online, republished from Lab Tests Online.

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