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Blood Tests

Blood tests to evaluate heart disease

Children may have blood tests done to help the doctor evaluate their illness, or to help monitor their health after surgery.

Illustration of child having a blood test

These tests may include the following:

  • Complete blood count. A measurement of size, number, and maturity of different blood cells in a specific volume of blood. Red blood cells are important because they carry oxygen through the bloodstream to the organs and cells of the body. Having too few red blood cells can make a child feel tired, and having too many red blood cells may also indicate a problem. For example, children with cyanotic congenital heart disease may have high numbers of red blood cells in the bloodstream, in an effort by the body to provide enough oxygen to the organs. White blood cells multiply when inflammation or infection is present.

  • Electrolytes. Minerals in the bloodstream such as sodium, potassium, calcium, and magnesium (that are important for the proper function of organs) may be measured. Electrolytes may be out of balance when a child is taking diuretics.

  • Total protein and albumin. These tests can help evaluate a child's nutritional status.

  • Prothrombin time (PT), partial thromboplastin time (PTT), and international normalized ratio (INR). Tests done to evaluate the effectiveness of anticoagulant drugs (also known as blood thinners) taken for various heart problems, including artificial valve replacement and irregular heart rhythms.

  • Blood gas. A blood sample taken from an artery that measures the amounts of oxygen and carbon dioxide in the bloodstream, as well as the acidity or pH of the blood. A related test, usually done prior to the blood gas, is pulse oximetry. This painless, noninvasive test measures the amount of oxygen in the blood through a small, red-lighted sensor placed on a child's finger, toe, or earlobe.

  • Genetic blood tests. Tests such as DNA, extended chromosome banding, and fluorescent in situ hybridization (FISH) may be used to detect chromosomal abnormalities associated with congenital heart defects. These lab tests must be sent to a special genetics lab and often take days or weeks before results are available.

Another type of test that may be used to measure the amount of oxygen in the blood is pulse oximetry.

Illustration of pulse oximetry using finger probe
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To obtain this measurement, a small sensor (similar to an adhesive bandage) is taped onto a finger or toe. When the machine is on, a small red light can be seen in the sensor. The sensor is painless and the red light does not get hot.

Depending on the results of the blood tests, additional tests or procedures may be scheduled to gather further diagnostic information.