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BLOOD TESTS: Why your child needs all those sticksby Frank R. Sinatra, M.D. See: Pediatric Lab Values Blood tests are feared by children, dreaded by parents, and disliked by nurses. Despite what you might think, doctors don't like blood tests very much either. Unfortunately, many important aspects of liver function can only be evaluated by periodic blood testing. The liver is a complex organ with several major activities, including synthesis of proteins, such as albumin and the proteins necessary for normal blood clotting, and the removal of potential toxins, such as bilirubin and numerous drugs. The liver is also important in the maintenance of normal blood sugar (glucose) concentrations. Blood tests, therefore, are used to evaluate the ability of the liver to make albumin (serum albumin) and clotting factors (prothrombin time and partial thromboplastin time), to remove potential toxins (serum bilirubin) and to maintain normal blood sugar concentrations (serum glucose). In addition, measurement of certain proteins released into the circulation by injured liver cells or bile duct cells can be used to identify or monitor liver injury. These proteins are called "liver enzymes" and include AST (or SGOT), ALT (or SGPT), alkaline phosphatase and GGT-P. Other blood tests commonly used in the management of children with chronic liver disease include the complete blood count (CBC), serum electrolytes (sodium, potassium, chloride, and bicarbonate) and "kidney function tests" (BUN and creatinine). Blood tests are also used to measure the level of certain drugs (such as cyclosporine and tacrolimus) and vitamins (such as vitamins A, E, and D). Tests of Synthetic FunctionSerum Albumin. Albumin is the major blood protein made by the liver. It is important for maintaining normal blood volume. When serum albumin levels are low due to the liver’s inability to make sufficient albumin or because of abnormal loss in the kidney or intestines, fluid may leak out of the blood vessels and produce abnormal tissue fluid or "edema." When excess fluid collects in the abdominal cavity it is known as "ascites." Albumin is also important in the blood transport of many drugs, hormones, and other chemicals. Blood Clotting Studies. Most of the proteins necessary for normal blood clotting are produced by the liver. With decreased liver function, the production of these proteins decreases and bleeding may occur. The results of blood clotting studies are usually expressed in terms of seconds, or how long it takes the blood to clot under certain conditions. The longer it takes the blood to clot, the higher the prothrombin time (PT) or partial thromboplastin time (PTT). Results may also be expressed relative to a normal control (INR). In this case, a normal value is close to 1.0. A patient in whom the clotting time was twice as long as the control would therefore have an INR of 2.0; a patient who clotted in half the control time would have an INR of 0.5. Tests of DetoxificationSerum Bilirubin. Bilirubin is the yellow pigment produced by the breakdown of red blood cells and removed from the circulation by the liver. Doctors are usually interested in knowing the values for two forms of bilirubin: the unconjugated ("indirect") form that has not been changed by the liver and the conjugated ("direct") form that has been chemically changed or "conjugated" by the liver. When liver function decreases, the blood level of bilirubin may rise sufficiently high to cause yellow discoloration of the eyes and skin known as "jaundice." Tests of Liver InjuryLiver Enzymes (AST, ALT, Alkaline Phosphatase, and GGT-P). Liver injury of many types often results in the release of normal liver proteins (enzymes) into the blood. Measurements of the various enzymes can be used to detect such problems as hepatitis, drug toxicity, and transplant rejection. Although not specific, the height and the particular pattern of increase of the various enzymes will often provide a clue as to the type and severity of liver injury. Liver enzymes are also used to follow the progress of chronic liver disorders. Other Blood Tests Commonly Obtained in Children with Liver DiseaseIn addition to the so-called "liver function tests," doctors often need to closely follow tests of "electrolyte balance," kidney function, and blood cell status. Electrolytes include sodium, potassium, chloride, and bicarbonate and are very important in maintaining normal function of the heart, brain, and skeletal muscles. These values can change rapidly in response to changes in intake, unusual losses (such as vomiting or diarrhea) or kidney abnormalities. The doctor may need to measure electrolyte levels several times a day in a sick patient in order to provide appropriate intravenous fluid therapy. BUN and creatinine measurements are used to monitor kidney function with increasing levels indicating a worsening of kidney function. The complete blood count CBC) allows the doctor to evaluate the red blood cells (hemoglobin and hematocrit), white blood cells (white blood count) and platelet count. This provides important information concerning bone marrow function, blood loss, infection, and bleeding due to decreased platelets. Finally, measurement of antibiotic and cyclosporine levels allow the doctor to adjust drug doses in order to maintain adequate blood levels and to avoid potentially toxic levels. It is important to remember that blood tests must be evaluated in the context of the medical history and physical examination, and that isolated blood test results may be misleading. Also, pediatric normal values are often different than "normal values" listed by laboratories dealing primarily with adults. Your doctor will be able to interpret the test results in the context of your child’s individual case. Someday we may be able to use a "Star Trek Dr. McCoy"
device to measure blood chemistries without drawing blood. Until then we
are stuck with the sticks. Dr. Sinatra is Professor of Pediatrics, University of Southern
California School of Medicine and Head, Pediatric Gastroenterology and
Nutrition at Women's and Children's Hospital - Los Angeles County + USC
Medical Center. Reviewed on 02/23/2004
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