The purpose of testing liver function is to detect any damage to the liver, determine the cause, assess prognosis, and differentiate jaundice. Therefore, the clinical application of liver function tests is of significant value in diagnosis, treatment, and prognosis. There are many types of liver function tests, numbering in the hundreds, but each test can only investigate a specific function of the liver, and no single test can comprehensively reflect all liver functions. Thus, to obtain a more accurate conclusion, multiple tests should be conducted, and repeated examinations are necessary when needed. Additionally, the evaluation of liver function tests must consider clinical manifestations comprehensively to avoid one-sidedness and subjectivity. It is also important to conserve human, material, and financial resources; selectively conducting necessary liver function tests is crucial.
Now, we will discuss commonly used liver function tests that have clinical significance.
1. Investigating Hepatocyte Damage (1) Enzymes Indicating Hepatocyte Damage When the hepatocyte membrane is damaged, the cytoplasmic substances that enter the serum increase. Generally, the activity of cell damage indicating enzymes and serum iron levels can be measured to confirm this. Commonly used cell indicating enzymes in hepatology include alanine aminotransferase (ALT, GPT), aspartate aminotransferase (AST, GOT), adenosine deaminase (ADA), cholinesterase (ChE), and lactate dehydrogenase (LDH).
1. Alanine aminotransferase and aspartate aminotransferase; to date, ALT is the most sensitive and has the greatest practical value in clinical use. Through the measurement of ALT, mild hepatitis can be detected early in patients with latent infections after excluding other factors. Furthermore, the measurement of ALT is significant for tracking the recovery of acute hepatitis, understanding the activity level of chronic hepatitis, evaluating the effectiveness of drug treatment, and screening blood donors in conjunction with the determination of hepatitis B surface antigen (HBsAg). When hepatocytes are damaged, the increase in ALT is greater than that of AST. In cases of viral hepatitis, measuring the AST/ALT ratio is even more meaningful. Under normal circumstances, the ratio is around 1.15. In the early stages of acute hepatitis, most ALT and part of the AST are released, causing the AST/ALT ratio to drop to around 0.56. In alcoholic hepatitis and cirrhosis, the AST/ALT ratio is often >2, and it may also increase in cholestasis and liver cancer. AST has two isoenzymes: one is ASTs, which accounts for 60% and is found in the cytoplasm; the other is ASTm, which accounts for 40% and is found in the mitochondria. In mild liver damage, only ASTs increase, while in cases of substantial hepatocyte necrosis, serum ASTm also increases significantly, and the ASTm/ASTs ratio also markedly increases. 2. Adenosine deaminase (ADA): It can reflect residual lesions in acute liver disease or chronic liver damage better than ALT. The positive rate of ADA is higher than that of ALT in chronic liver disease, especially in cirrhosis. In biliary diseases, ADA does not increase. Some hospitals routinely test ALT and ADA as markers of liver damage. 3. Serum cholinesterase: ChE is synthesized by hepatocytes, with normal values based on the indicator method pH being 0.4-0.8. In liver abscesses, ChE enzyme activity decreases significantly and parallels the degree of lesions. In fatty liver, it increases. 4. Lactate dehydrogenase: The measurement of LDH lacks specificity for liver disease, but LDH has five isoenzymes, and an increase in LDH5 suggests liver disease, sometimes with normal ALT while LDH5 has already increased. (2) Measurement of Total Serum Bile Acids (CCA) Fasting and 2-hour postprandial measurements of total serum bile acids, especially the latter, are more sensitive for diagnosing chronic liver disease than measuring serum bilirubin, AST, alkaline phosphatase (AKP), or gamma-glutamyl transferase (γ-GT). The normal value for fasting and postprandial total serum bile acids is 0-15mmol/L, and in chronic liver disease, the fasting value remains normal while the postprandial value can rise to 22-600mmol/L. (3) Measurement of Serum Iron When hepatocytes are severely damaged, the iron content in the cytoplasm increases and enters the serum. The increase in blood iron levels and the degree generally correlate with the severity of liver damage and the iron content. In hemochromatosis, blood iron also increases, indicating hepatocyte damage, but unlike other liver diseases, iron-binding capacity decreases. Normal serum iron values are 80-150μg/dl. 2. Detecting Excretory and Secretory Dysfunction (1) Hyperbilirubinemia Hyperbilirubinemia can occur at various stages of bilirubin metabolism, due to excessive production, abnormal conjugation, or dysfunction in secretion and transport, seen in intrahepatic cholestasis or extrahepatic obstructive jaundice. (2) Cholestasis Indicating Enzymes (including isoenzymes) Useful enzymes for diagnosis include: AKP, γ-GT, 5′-nucleotidase (5′-NT), and leucine aminopeptidase (LAP), with AKP and γ-GT being more commonly used. These enzymes are present in higher concentrations in the epithelial layer of intrahepatic bile ducts. When the epithelial layer is damaged and pressure in the bile ducts increases, these enzymes increase in serum. 1. Alkaline phosphatase: An increase in serum AKP generally suggests liver-biliary diseases or bone lesions associated with cholestasis. In cholestasis, γ-GT, 5′-NT, and LAP also increase, but not in bone diseases. Besides cholestasis, AKP also increases in intrahepatic inflammation and liver cancer, with the degree of increase being cholestasis > liver cancer > hepatocellular damage. There are seven isoenzymes of AKP in normal human serum; in extrahepatic cholestasis, the AKP isoenzyme type VII increases, while in intrahepatic cholestasis, only the AKP type II activity increases. In malignant obstruction and metastatic cancer, the increase in AKP is most significant, while in hepatocyte and hemolytic jaundice, AKP is generally normal or mildly elevated; thus, measuring AKP can also help distinguish jaundice. 2. Gamma-glutamyl transferase: γ-GT is an enzyme involved in protein metabolism, found in the cytoplasm of hepatocytes and bile duct epithelial cells. The clinical significance of measuring serum γ-GT is generally consistent with that of AKP, but it is less specific than AKP. Its main values are: ① determining whether the increase in serum AKP comes from the liver or bone, as γ-GT remains normal in bone diseases; ② in acute hepatitis, γ-GT returns to normal more slowly than ALT; if it remains elevated, it suggests a transition to chronic liver disease; ③ if γ-GT is persistently elevated in chronic hepatitis, it indicates hepatocyte necrosis; ④ in obstructive jaundice, γ-GT often increases significantly, being higher in malignant obstruction than in benign obstruction. γ-GT has four isoenzymes, with γ-GT3 being the highest in normal individuals, while γ-GT1 increases in hepatocellular lesions. In primary liver cancer, γ-GT2 isoenzyme increases, with reported positive rates of 27-54% abroad, and up to 90% domestically. 3. Leucine aminopeptidase: LAP is a protease, abundant in the liver, and its clinical significance is similar to that of AKP; LAP activity significantly increases in both intrahepatic and extrahepatic cholestasis, especially in malignant cholestasis, where its activity continues to increase with disease progression. 4. 5′-nucleotidase: 5′-NT is primarily found in bile canaliculi and sinusoidal spaces within the liver. The clinical significance of increased serum 5′-NT activity is similar to that of AKP and LAP. Its activity increases in obstructive jaundice and liver cancer, while it is normal or mildly elevated in hepatocellular jaundice. 5. 5′-nucleotidase phosphodiesterase isoenzyme (5′-NPD-V): This enzyme activity often increases in primary liver cancer, with a positive rate of 80%; in metastatic liver cancer, the positive rate is 80-98%, while in hepatitis and cirrhosis, the positive rate is less than 10%. This isoenzyme can complement the deficiencies of α-FP, especially in α-FP negative cases, achieving a combined positive rate for liver cancer diagnosis of up to 94%. (3) Increased Serum Copper Copper absorbed from food is primarily excreted through the liver into bile. Intrahepatic and extrahepatic cholestasis can lead to increased serum copper due to impaired bile flow. Normal serum copper values are 90-140μg/dl. (4) Clearance Tests Hepatocyte damage reduces excretory function. Clinically, sodium bromosulfophthalein (BSP) and indocyanine green (ICG) are commonly used for clearance tests. BSP retention test: In normal individuals, after intravenous injection of 5mg BSP per kilogram of body weight, the amount of BSP retained in serum after 45 minutes should be less than 5% of the injected amount. The amount of this dye retained in the blood correlates with the degree of liver function damage. If it exceeds 6%, it indicates possible hepatocyte abnormalities; at 10%, damage is certain; 20-40% indicates mild damage; 50-80% indicates moderate damage; and over 90% indicates severe hepatocyte damage. This test is very sensitive and can often show BSP retention in the blood before other liver function tests become abnormal. It is used to assess liver function in patients with fewer symptoms and to determine the presence of toxic liver damage. However, this test is complex to perform and can cause allergic shock reactions, even leading to death; thus, it is gradually being abandoned. ICG test: The clinical significance of this test is generally similar to that of the BSP test, with the advantage of being unaffected by factors such as fever or certain medications, having almost no toxicity, and not being metabolized or processed through extrahepatic pathways, thus not causing allergic reactions. Due to these characteristics, the ICG test has been used to replace the BSP test. An intravenous injection of 0.5mg per kilogram of body weight is given, and blood is drawn after 15 minutes for colorimetric and quantitative determination of ICG concentration. The average ICG retention in normal individuals is 7.84±4.34%; in chronic hepatitis, it is 15.1%; and in cirrhosis, it is 29.2%. ICG can be used for population screening for liver disease, and for patients with obvious symptoms, continuous ICG clearance tests are valuable for observing the course of the disease.
Caffeine plasma clearance rate: Wang Tiancai and others observed that in 10 normal individuals, the fasting plasma caffeine concentration was 1.5±0.8μmol/L, while in 9 patients with cirrhosis, it was 11.1±10.5μmol/L, and in 8 patients with other liver diseases, it was 3.1μmol/L. This data indicates that in cirrhotic patients, the fasting plasma concentration of caffeine is significantly higher than in healthy individuals. In liver disease patients, the disappearance and clearance rates of caffeine are significantly reduced compared to normal individuals. 3. Differentiating Jaundice (1) Bilirubin Metabolism Tests 1. Total serum bilirubin: Various liver diseases, hemolysis, and bypass bilirubinemia can cause an increase in total serum bilirubin, with normal individuals having <17.1μmol/L, and indirect bilirubin ranging from 1.71-13.7μmol/L. In hemolytic jaundice, both total and indirect bilirubin increase; in obstructive jaundice, both total and direct bilirubin increase; in hepatocellular jaundice, both total and indirect bilirubin increase. 2. Serum 1-minute bilirubin: This primarily reflects the amount of direct bilirubin, with normal direct bilirubin in serum being 0.51-3.4μmol/L. The measurement of serum 1-minute bilirubin is mainly valuable for diagnosing unconjugated bilirubinemia; if total bilirubin >20.5μmol/L and 1-minute bilirubin/total bilirubin <20%, it suggests serum unconjugated bilirubin is negative, indicating unconjugated bilirubinemia. 3. Urinary bilirubin test: This can serve as the simplest screening test for liver disease; in jaundice, urinary bilirubin is negative, suggesting unconjugated bilirubinemia. 4. Urinary urobilinogen test: Normal individuals have only a small amount of urobilinogen in urine; in hemolytic jaundice, it increases significantly (but there is no bilirubin in urine); in hepatocellular jaundice, urobilinogen excretion increases, while in complete obstructive jaundice, there is no urobilinogen in urine, and in the latter two types of jaundice, urinary bilirubin is positive or strongly positive. (2) Enzyme Tests In any liver disease, serum AKP can increase, but significant increases are only seen in intrahepatic and extrahepatic cholestasis. Some literature reports that if serum AKP values exceed 2.5 times normal and ALT is less than 10 times normal, 90% of cases are cholestasis; conversely, 90% are hepatitis. The measurement of γ-GT has clinical significance similar to that of AKP. Other enzymes used to reflect cholestasis include LAP and 5′-NT, as previously mentioned. (3) Blood Lipids and Lipoproteins In intrahepatic and extrahepatic cholestasis, plasma cholesterol and phospholipids significantly increase, and an abnormal lipoprotein called lipoprotein X (Lp-X) appears, which is a sensitive indicator of cholestasis, with a positive rate of 90-99% and specificity close to 100%. 4. Assessing Prognosis (1) Protein Metabolism Tests 1. Serum albumin: It is synthesized by the liver, and its level is a good indicator for estimating prognosis. 2. Serum prealbumin: Also synthesized by the liver, due to its half-life of only 1.9 days in the body, it reflects recent liver damage and its extent more sensitively than albumin. 3. Plasma amino acid profile: In severe acute and chronic liver disease, plasma branched-chain amino acids (BCAA) decrease, aromatic amino acids (AAA) increase, and the BCAA/AAA ratio decreases. Measuring BCAA and AAA not only helps assess the prognosis of liver disease but also guides treatment. 4. Measurement of blood ammonia: Ammonia comes from the breakdown of amino acids and can also be produced from ammonium salts and nitrogen-containing substances in the intestines through bacterial action. Ammonia is a toxic substance, most of which is detoxified by the liver and converted into urea, which is then excreted by the kidneys. The normal value for blood ammonia, according to the Nessler reagent colorimetric method, is 10-60μg%; blood ammonia gradually increases from the pre-coma stage to the coma stage in hepatic encephalopathy, indicating poor prognosis. 5. Total serum protein, albumin, globulin, albumin/globulin ratio, protein electrophoresis, and serum immunoglobulins: Normal values for total serum protein are 6-8g/dl, albumin is 4.0-5.5g/dl, globulin is 2-4g/dl, and the albumin/globulin ratio is 1.5-2.5:1. Using paper electrophoresis, plasma proteins can be divided into albumin (54-61%), α1 globulin (4-6%), α2 globulin (7-9%), β globulin (10-13%), and γ globulin (17-22%). In chronic liver disease, γ globulin increases. Through immunoelectrophoresis, globulins can be divided into immunoglobulin A (IgA), immunoglobulin G (IgG), and immunoglobulin M (IgM). In alcoholic liver disease, IgA increases, while in autoimmune liver disease, IgG and IgM significantly increase; in primary biliary cirrhosis, IgM increases significantly, with a positive rate of up to 90%. (2) Enzyme Tests 1. Cholinesterase (ChE): As previously mentioned, serum ChE decreases in liver disease. ChE has six isoenzymes (i.e., ChE1-6 isoenzymes). In cirrhosis and primary liver cancer, a rapidly moving band appears in front of bands 1-2, which may have diagnostic significance. 2. Prothrombin time (PT) measurement: Prothrombin is also synthesized by the liver, and prothrombin time can serve as a good indicator for the prognosis of severe, diffuse liver disease. In acute liver disease, significantly prolonged PT indicates the occurrence of fulminant liver necrosis. When PT activity drops to below 10% of the normal control group, the prognosis is poor.
Serum alkaline phosphatase (ALP or AKP) measurement: The normal value for serum alkaline phosphatase is 30-90 international units/liter by the King method; the measurement of this value is significant for distinguishing obstructive and hepatocellular jaundice, as well as cancers. However, since this enzyme is quite active in bone tissue, conditions such as childhood, pregnancy, fracture healing, osteomalacia, rickets, osteosarcoma, osteoporosis, liver cirrhosis, liver abscess, liver tuberculosis, leukemia, and hyperthyroidism can also significantly elevate ALP, which must be differentiated.
Serum protein measurement, including total protein (TP), albumin (Alb), and globulin (Glb) ratio measurement (A/G). In normal conditions, the average total protein for adults is 60-80 grams/liter; albumin is 35-55 grams/liter, globulin is 20-30 grams/liter, and the A/G ratio is 1.5-2.5:1. When liver disease occurs, such as acute or chronic hepatitis, total protein remains normal or decreases, albumin decreases, globulin (mainly T-globulin) increases, and the A/G ratio decreases or even reverses.
Aspartate aminotransferase (AST)
The laboratory tests for hepatitis B mainly include two aspects: 1. Blood biochemical tests (liver function tests). 2. Virus marker detection (five hepatitis B items). The commonly used liver function tests mainly include total protein (TP), albumin (ALB), and globulin (GLO), which primarily reflect the liver’s synthetic function. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) mainly reflect the condition of hepatocyte damage. Direct bilirubin (DBIL) and total bilirubin (TBIL) mainly reflect the metabolic function of hepatocytes. The five hepatitis B items check for the presence of hepatitis B virus surface antigen (HBsAg), whether there are protective antibodies (anti-HBs), whether the virus is replicating and infectious (anti-HBe), whether the viral replication is suppressed, and core antibodies (anti-HBc) to determine if there has been an infection with the hepatitis B virus. For instance, an increase in transaminases indicates hepatocyte damage, suggesting that liver function is abnormal, which means acute liver disease. For chronic liver disease, it may be in an active phase.
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