The chemistry panel, also called the metabolic panel or serum multichannel automated chemistry (SMAC), consists of a variety of tests that document primarily the function of the liver and kidneys. In some hospitals, the test includes electrolytes and a lipid profile, but there is some variation between laboratories in the list of tests included.
Serum glucose refers to the amount of sugar in the blood. The level may be elevated or decreased in diabetics but is usually normal in children with orthopedic and rheumatic diseases. It may go up in children being treated with steroids. If the blood sample has been allowed to sit for too long before being processed, the value will go down.
Blood urea nitrogen (BUN) is a measure of kidney function. It is elevated in children with impaired kidney function. It often goes toward the top of normal to slightly over in children receiving nonsteroidal anti-inflammatory drugs (NSAIDs) because these drugs decrease the rate at which the kidneys cleanse the blood. These slight elevations are normal and should not be a cause of concern.
Creatinine (Cr) level is also a measure of kidney function, but it is not affected by mild changes in the GFR the way BUN is. Because children normally have lower creatinine levels than adults, the BUN/creatinine ratio may become elevated. This is not something to be concerned about if the underlying values are normal. One concern is that many laboratories do not report age-adjusted normal values. A creatinine of 1.2 mg/dl is normal for an older adult but very abnormal for a child under the age of ten. If the laboratory is not using age-adjusted normal values, the value of 1.2 mg/dl will not be indicated as abnormal in the child's results.
Sodium (Na), potassium (K), calcium (Ca), chloride (Cl), carbon dioxide (CO2), and phosphorous (P) are electrolytes. Their levels may indicate abnormal kidney function or, under certain circumstances, other metabolic abnormalities. These should be normal in children with orthopedic and rheumatic conditions unless there is significant internal organ involvement. Like blood glucose and ESR, electrolyte levels are very sensitive to proper handling of the blood specimen and may be unreliable if the sample was mishandled.
Low sodium levels may be the result of fluid retention, brain injury, or medications. Children with very low sodium levels may have problems including weakness and muscle cramps, but values above 130 are rarely troublesome. High levels of sodium suggest dehydration or kidney disease.
Like the sodium level, the potassium level (K) is normally regulated by the kidneys. Low levels of potassium also cause weakness and muscle cramps. High levels may interfere with heart rhythm, and very high levels are dangerous. Low sodium levels with high potassium levels can occur in diabetics and in people who have poorly functioning adrenal glands.
Serum calcium measures the level of calcium in the blood. It may be abnormal in children with disease of the parathyroid gland, which secretes a hormone to regulate calcium levels. Sarcoidosis is a rheumatic disease that may cause elevated calcium levels. The level may also be abnormal in children with kidney disease, excessive vitamin D, or metabolic abnormalities. These problems may result in bone and joint abnormalities or pain if they persist for an extended period. Low calcium may be associated with severe muscle cramps. Rickets may cause problems in children who do not get enough vitamin D and is associated with a low serum calcium level and joint pains; however, rickets is extremely rare in the United States because most dairy products contain supplemental vitamin D.
Most often the serum chloride (Cl) level is normal. A very low chloride level can occur in someone who is sick and vomiting a lot. If you are dealing with a teenager who chronically has low chloride levels, you need to consider the possibility of bulimia (throwing up after eating to avoid gaining weight). Teenagers won't always admit to this unless confronted with the evidence. If an increased amylase level accompanies the low chloride, it strongly suggests the possibility of bulimia. The amylase comes from irritation of the salivary glands by the persistent vomiting.
Carbon dioxide levels in the blood go up and down as part of the mechanism that controls acid-base balance. If the CO2 level is significantly off, a thorough metabolic workup should be done by the physician. However, the CO2 level is another of the tests that may produce an abnormal reading if the blood specimen was allowed to sit for too long before it was processed.
Phosphorous is closely related to calcium metabolism. Children with kidney problems or calcium metabolism problems may have too high or too low a phosphorous level. Otherwise, the phosphorous level is rarely abnormal. Children who are using too much antacid may develop low phosphorous levels (hypophosphatemia). The antacids contain chemicals that bind the phosphorous in the intestines and prevent it from being absorbed.
Albumin is a serum protein manufactured by the liver that serves as both a building block and a carrier molecule. If the level is low, it may be the result either of decreased production from liver disease or poor nutritional intake or of increased loss through the gastrointestinal system or the kidney. In either case, it needs to be investigated. Systemic lupus erythematosus and amyloidosis are among many diseases that may cause increased loss of albumin through the kidney. This is easily detected by urine tests for protein.
Whenever the serum albumin level is low, the body must compensate by increasing other elements in the blood to maintain an appropriate osmotic balance. Most often the body increases the level of cholesterol in the blood to accomplish this. High cholesterol and low albumin are commonly found in children with kidney disease. Changes in diet are not likely to have any effect on these problems if the kidney or liver diseases that cause the problem are not corrected. If the body is not able to maintain an appropriate osmotic balance, water tends to leak out of the blood vessels, producing swelling. Painful swollen feet in children may be a sign of kidney malfunction and low albumin levels; sometimes the first clue is that socks are leaving deep marks on the feet or calves.
The chemistry panel also includes total protein, total globulin, and the albumin-globulin ratio. Globulins are larger proteins in the blood that include a number of acute phase reactants as well as immunoglobulin molecules. The level of acute phase reactants goes up in people who are ill, and so the globulin level goes up in people who are ill. At the same time, the albumin level often goes down when people are sick. The combination of factors often leads to a decreased albumin-globulin ratio. Low globulin levels also may be an indication of low immunoglobulin levels, associated with a variety of problems.
Bilirubin is commonly included in the chemistry panel. Most often it is reported as total bilirubin and direct bilirubin. The liver produces bilirubin from breakdown products in the blood. Direct bilirubin has been completely processed by the liver, while the total figure includes bilirubin that has not been so processed. Liver disease is the most common cause of significantly elevated bilirubin levels. If the bilirubin level is too high the child will look jaundiced, or yellow. In children with rheumatic or musculoskeletal disease, the most common cause of a significantly increased bilirubin level is hemolysis (the breakdown of red cells). This can occur after internal bleeding or if there is increased breakdown of red cells by antibodies. The latter complication, autoimmune hemo-lytic anemia, is often seen in children with SLE. Because red cells also contain an enzyme called AST (see next section), physicians may initially be confused into thinking that children with elevated bilirubin and AST levels have a problem with the liver, when in fact the liver is fine but too many red cells are being broken down.
SGOT/AST is an enzyme contained in muscle cells, liver cells, and red blood cells. Damage to any of these can result in increased amounts of AST being present in the blood. If the AST is elevated without obvious explanation, it is important to be sure that muscle enzyme testing (CK and aldolase) is done to look for muscle diseases, as well as a reticulocyte count to look for hemolytic anemia. Normal CBC and chemistry panels do not include the proper tests to look for these problems, and an elevated AST may be the only hint. But in a child with hemolysis due to excessive red cell destruction, the bilirubin level will also go up. This does not happen with muscle disease. If the elevated AST is the result of problems with the liver, there should be a significant elevation of the ALT as well (see next section).
SGPT/ALT is another enzyme that is primarily found in the liver. Thus, if the AST level is elevated but the ALT is not, it suggests that the source of the AST is outside the liver. Mild elevations of ALT can occur from disease outside the liver if the AST level is also elevated. There are many different liver diseases that may result in an elevation of the ALT level. The ALT level may also rise if the liver is being irritated by medications. Methotrexate is known to irritate the liver in some children, but NSAIDs and many other drugs may also cause liver irritation in some children. This is why it is important to monitor these tests routinely in children on medication.
It should always be remembered that some rheumatic diseases may cause damage to the liver, and some liver diseases may cause bone and joint discomfort. This is especially true in patients with infectious hepatitis. All children with significantly elevated liver enzyme levels need careful evaluation.
The level of LDH, lactic acid dehydrogenase, another enzyme measured in the blood, goes up with damage to many types of cells, including those in the brain, heart, lungs, liver, blood, muscle, or spleen. When there is increased cell turnover, the LDH level can be very high. Many physicians believe that an LDH level over 1,000 means that the child must have a tumor, but children with rheumatic diseases, such as systemic-onset JA or dermatomyositis, and other children with systemic illness may have very high LDH levels (well over 1,000 units). If a child is thought to have pauciarticular JA and the LDH is very high, something is wrong. These children should be evaluated for leukemia or bone tumors.
Alkaline phosphatase is an enzyme that is associated with bone growth. Because children have actively growing bones, the normal alkaline phosphatase value for children is much higher than the normal value for adults. If the laboratory is not reporting age-adjusted values, a normal value in a child may be reported as abnormal. In an adult, an elevated alkaline phosphatase may indicate gallbladder disease, but this is a rare problem in children. (Note that children with sickle cell disease can get gallbladder problems.) Occasionally, children have values of alkaline phosphatase many times the expected normal value without any apparent illness.
Uric acid is a breakdown product of DNA. Thus, increased uric acid means that either there is increased breakdown of cells or the kidneys are not properly clearing the uric acid, possibly both. Children sick with hemolytic uremic syndrome have high uric acid levels. So do children with leukemia. Sometimes children with elevated uric acid levels and joint pain are referred for possible gout. These children should be carefully evaluated for other problems. Gout is essentially unheard of in childhood except in the setting of kidney disease or cancer chemotherapy, where the drugs are causing many tumor cells to die very quickly and the kidneys cannot handle the load.
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