LIVER FUNCTION TESTS are serum assays done to evaluate the general health or detect problems with a patient's liver or biliary system. These are the most commonly performed tests:
- bilirubin (total and direct)
- liver enzymes (aspartate aminotransferase [AST], alanine aminotransferase [ALT], alkaline phosphatase [ALP], and gamma-glutamyl transpeptidase [GGT])
- prothrombin time (PT), to evaluate the liver's ability to synthesize blood clotting factors II, V, VII, IX, and X
- serum albumin level, to evaluate the liver's ability to synthesize this plasma protein.
Who gets tested? Liver function tests are used to monitor liver function in someone with known liver disease, such as cirrhosis, or to investigate the cause of specific signs and symptoms, such as jaundice or liver enlargement. Testing may be ordered before a patient starts therapy with a drug that's metabolized through the liver, such as a statin. And someone with risk factors for hepatitis, such as foreign travel or I.V. drug use, should have testing if he develops such signs and symptoms as malaise, nausea, abdominal pain, dark-colored urine, light-colored stools, or diarrhea.
Preparing the patient. Generally, the patient should be N.P.O. for 4 hours before you draw blood for bilirubin and ALP levels and be N.P.O. for 8 hours and abstain from alcohol for 24 hours before a GGT level. Otherwise, no special preparation is needed.
Getting the specimen. For bilirubin, liver enzymes, and albumin level tests, follow facility policy, such as drawing the specimen in a tube without additives. Use a tube containing the anticoagulant sodium citrate to draw the PT specimen; the ratio of sodium citrate to blood is important, so fill it according to manufacturer specifications. If you collect all the specimens at one time, draw the PT specimen last to avoid contaminating the other specimen with sodium citrate, which can interfere with ALP results.
After drawing the blood, avoid shaking the tubes excessively; just gently invert them several times. Protect them from light, place them in an appropriate biohazard bag, and immediately send them to the lab. (Reasons for precautions when handling the samples appear in Normal values and factors that affect results.)
Interpreting the results. Bilirubin is primarily a by-product of hemoglobin degradation and hemolysis. Increased bilirubin production due to hemolytic anemia or decreased elimination in the stool can raise blood levels and lead to jaundice. Your patient's bilirubin level may be elevated with liver damage or slightly elevated after he receives a blood transfusion.
- Total bilirubin includes both direct and indirect bilirubin.
- Direct or conjugated bilirubin reflects the liver's conversion of bilirubin to a water-soluble state. An elevated direct bilirubin level indicates a disorder within the liver itself.
- Indirect or unconjugated bilirubin hasn't been made water-soluble. The level doesn't appear on the lab report; it's calculated by subtracting the direct bilirubin reading from the total. Hemolysis or inherited liver disease can increase indirect bilirubin.
The liver enzymes can help isolate the source of liver problems. Liver cell damage such as that caused by cirrhosis increases ALT and AST. Cholestasis (lack of bile flow) due to bile duct obstruction by gallstones or caused by a disease that impairs bile formation in the liver raises ALP levels. Normally, ALP levels vary with age and sex, and nonhepatic sources such as bone disease, pregnancy, and childhood growth can cause elevations.
Most liver diseases raise GGT levels, so GGT testing can help determine if increased ALP levels are due to a liver problem. Other causes of elevated GGT levels are cholangitis, obstructive jaundice, cholecystitis, and alcohol consumption in someone with chronic alcohol abuse.
The liver synthesizes most blood clotting factors, so a prolonged PT may be a sign of a diseased liver.
A low albumin level is a sign of liver failure. But it can also occur with chronic renal failure, malnutrition, and illnesses that cause protein catabolism, so serum albumin isn't as sensitive as PT to assess the liver's synthetic ability.
What follow-up care does the patient require? Follow-up studies for a patient with abnormal liver function studies may include additional or serial lab tests, abdominal ultrasound, computed tomography scan, magnetic resonance imaging, or endoscopy. The patient also may be referred to a hepatologist and undergo liver biopsy.
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