The liver is an important metabolic organ of the human body, and its main functions are 1. metabolic functions: such as the assimilation, storage and alienation of sugar, fat and protein; nucleic acid metabolism, activation and storage of vitamins; inactivation and excretion of hormones; production of bilirubin and bile acids; metabolism of iron, copper and other metals; 2. excretion functions: such as the excretion of bilirubin and certain dyes; 3. detoxification functions: such as the oxidation, reduction, hydrolysis and binding of various compounds; 4. 4. coagulation and fibrinolytic factors, the production of fibrinolytic inhibitory factors and the removal of active coagulation factors; 5. immune functions of the liver: general liver tests and examinations can be divided into two categories: examinations of liver damage, such as the release of liver enzymes (often mistakenly referred to as “liver function” tests) and real liver tests for the above functions. Liver function tests are valuable in diagnosing liver disease, estimating the severity of the disease, determining the prognosis and following up on the outcome. However, it should be noted that the liver of normal people has considerable compensatory capacity, and some liver damage cannot be revealed by laboratory tests; none of the indicators can accurately reflect the overall liver function alone; there are many liver function tests, which need to be selected in conjunction with clinical practice; the specificity and sensitivity of liver function tests are not high, so they must be combined with medical history and physical examination.
I. Test of protein metabolism
Reflects the synthesis function of liver parenchyma cells. It mainly includes serum albumin, prealbumin, prothrombin time, lipoprotein, cholinesterase, phospholipid cholesterol acyltransferase.
(i) Serum albumin
Synthesized only in the liver, it maintains plasma colloid osmotic pressure, endogenous nutrient source and carrier of certain substances.
The rate of synthesis is 100-200 mg per kg body weight per day, and its half-life in the body is long (about 17-21 days), with a degradation rate of about 4% per day. The decrease of albumin after liver damage often shows up only 1 week after the disease, so serum albumin is not a good indicator of acute liver disease. In clinical use, hypoalbuminemia is not specific for liver disease and can produce a decrease in serum albumin when there is too little intake or impaired digestion and absorption, too much protein breakdown (infection, fever, cancer, etc.), and loss from abnormal pathways (e.g., protein-losing gastrointestinal disease, combined nephropathy). In addition, if a patient has ascites, edema, or intravascular albumin entering the extravascular pool, this can lead to a decrease in serum albumin levels. Therefore, the decrease in serum albumin in patients with liver disease reflects not only the decrease in the synthesis function of the liver itself, but also the decrease in body uptake, ascites (expansion of the extravascular pool), intestinal stasis (protein loss from the intestine), and co-infection (increased catabolism). If the serum albumin level drops to 25g/L, ascites may appear, and below 20g/L, the clinical prognosis is poor.
(ii) Prealbumin
It is synthesized by the liver and has a half-life of only 1.7 days. Its serum level decreases early and changes more obviously in liver disease, and with the change of disease, prealbumin rapidly returns to normal, while in patients with severe hepatitis it often remains at low values, and is a good indicator for follow-up of acute hepatitis and severe liver disease.
(ii) Coagulation factors
Coagulation factors are almost always synthesized in the liver, and the half-life of coagulation factors is much shorter than that of albumin, especially vitamin K-dependent factors (II, VII, IX, X), such as the half-life of factor VII is only 1.5 – 6 hours, so in the early stage of liver impairment, the albumin test is completely normal, but vitamin K-dependent coagulation factors have a significant decrease, so in the early stage of liver disease, the coagulation factor test can be used as a screening The coagulation factor test can be used as a screening test in the early stages of liver disease.
The following screening tests are commonly performed in the setting of liver disorders.
1. Prothrombin time (PT) measurement It reflects plasma factor II, V, VII, and X. Prolonged PT is a characteristic of cirrhosis in the decompensated phase, and is also an important laboratory test to diagnose cholestasis and whether the liver synthesizes vitamin K-dependent factors II, VII, and X. In fulminant hepatitis, if PT is prolonged and both fibrinogen and platelets are reduced, DIC can be diagnosed. in acute and chronic liver disease, PT prolongation of more than 4-5 seconds and non-response to vitamin K injection suggest extensive liver parenchymal damage and indicate poor long-term prognosis. In cases of cirrhosis and portal hypertension, PT can be used to predict the risk of portal shunt surgery. In cholestasis, impaired absorption of fat-soluble vitamins leads to prolonged PT. Within 24 hours after parenteral administration of vitamin K, PT improves by at least 30%, so the cause of prolonged PT can be identified.
2, activated partial thromboplastin time measurement (APTT) In severe liver disease, factor Ⅸ, X, D, Ⅺ synthesis is reduced, resulting in prolonged APTT; vitamin K deficiency, factor Ⅸ, X can not be activated, APTT can also be prolonged.
3, prothrombin coagulation time (TT) measurement TT prolonged mainly in response to reduced plasma fibrinogen content or structural abnormalities and the presence of FDP, factors VII, IX, X also have an impact. In cirrhosis or acute fulminant liver failure combined with DIC, TT is a commonly used test.
4.Hepatic prothrombin kinase test (HPT) HPT can reflect the combined activity of factors II, VII and X. The test is highly sensitive, but its sensitivity is too high, so it has poor correlation with prognosis.
5.Anti-thrombin III (AT-III) assay AT-III is mainly synthesized in the liver, and 70% to 80% of thrombin is inactivated by it, which forms a 1:1 covalent complex with thrombin and inhibits thrombin. AT-III activity is significantly reduced in severe liver disease, and is more significantly reduced when combined with DIC.
(iv) Lipoproteins
Alpha-lipoprotein is often significantly reduced in acute liver damage. In the early stage of acute viral hepatitis, most of the serum α-lipoproteins disappear, and slowly increase after the 60th day of the disease in patients with severe liver disease.
(v) Cholinesterase (CHE) and lecithin cholesterol acyltransferase (LCAT)
CHE is synthesized by the liver and its decreasing level in cirrhosis parallels that of albumin and can be used as an indicator of prognosis. However, in hepatic abscess, CHE is significantly decreased and the etiology is unknown. LCAT is also synthesized and secreted by the liver, which can reflect liver reserve function and is more sensitive. In the case of hepatocellular damage, the degree of decrease in serum LCAT parallels the severity of liver damage.
(vi) Blood ammonia measurement
Normal blood contains a small amount of free ammonia, most of which is synthesized in the liver through the ornithine cycle. When liver function is severely impaired (80% of liver tissue is destroyed), ammonia cannot be detoxified and aggregates in the central nervous system, causing hepatic encephalopathy. Patients with cirrhosis, hepatocellular carcinoma, fulminant hepatic failure, post-portal shunt and post-TIPS who show symptoms of neuropsychiatric disorders should have their blood ammonia measured immediately to consider the diagnosis of hepatic encephalopathy. However, blood ammonia can be normal in patients with acute hepatic encephalopathy and must be determined comprehensively. Uremia, upper gastrointestinal hemorrhage, shock patients venous blood ammonia can also be greater than the normal value (less than 45umol/l).
Second, the test of lipid metabolism
The liver is the main organ for cholesterol synthesis and can combine cholesterol from plasma lipoprotein molecules and fatty acids from lecithin molecules into cholesterol esters. Endogenous cholesterol is synthesized by the liver in 80% of cases. When hepatocytes are damaged, cholesterol synthesis is reduced, resulting in a decrease in cholesterol esters due to a decrease or lack of LCAT.
Under normal conditions, 70% of cholesterol is ester and 30% is free, with a ratio of 3:1. In hepatocellular disease, the percentage of serum cholesterol esters decreases (often less than 70%), and the more severe the hepatocellular damage, the more pronounced the decrease in cholesterol esters. In acute hepatic necrosis, serum cholesteryl ester levels may be reduced to very low levels or even disappear, a sign of a dangerous prognosis.
In cholestatic jaundice, serum cholesterol levels are increased, especially in malignant biliary obstruction. In chronic biliary obstruction, the increase is greater than in acute biliary obstruction. In cholestasis, the elevated serum cholesterol is mainly non-esterified cholesterol, unless it is complicated by hepatocellular damage, the cholesterol ester ratio is generally normal.
Third, the test of carbohydrate metabolism
The liver is an important organ for regulating glucose metabolism. The normal liver maintains normal blood glucose levels through glycogen synthesis, glycogenolysis, glycolysis, and gluconeogenic reactions. The normal liver stores enough glycogen to maintain the blood glucose level for about 24 h. After 24 h, the maintenance of blood glucose level in fasting state is completely dependent on glycogen isomerization, that is, the resynthesis of glucose from precursors such as lactate, pyruvate and amino acids, which is stimulated by hyperglycemia and adrenaline and inhibited by insulin. In liver disorders, glucose metabolism mechanisms are unstable or failing, which can produce hypoglycemia or glucose intolerance. Mild hypoglycemia occurs in about 50% of patients with uncomplicated acute viral hepatitis with a blood glucose concentration of 45-60 mg/dl, but in most cases, hypoglycemia is not clinically significant. In contrast, in cases of hepatic necrosis and Reye’s syndrome, it may be significant and life-threatening. In patients with apparent acute liver disease, hepatocellular carcinoma, alcohol consumption or exposure to organophosphorus poisoning, the possibility of hypoglycemia needs to be considered in the differential diagnosis if there are changes in mental status. On the other hand, hyperglycemia and glucose intolerance are mostly seen in chronic liver disease and cirrhosis. This is mainly due to the fact that in liver disease, the number of insulin receptors in peripheral blood mononuclear cells and their affinity are reduced and there is an extensive receptor defect. It is also due to the portal collateral circulation, where insulin is shunted from the liver and its effect on the liver is diminished.
IV. Tests of bilirubin metabolism
The bilirubin decomposed into various heme-containing components is called indirect bilirubin (UCB), which runs in the blood circulation with albumin as a carrier and is taken up by hepatocytes in the hepatic sinusoids, with which glucuronic acid in the endoplasmic reticulum combines to form direct bilirubin (CB), which reaches the intestinal cavity with bile and is reduced by bacteria to bilirubinogen, a small portion of which returns to the liver via the portal vein and enters the enterohepatic circulation; a portion of bilirubinogen passes through the kidneys and It is excreted in urine.
The daily production of serum bilirubin in normal people is less than 50 mg, while the liver processes up to 1500 mg of bilirubin daily. Because the reserve capacity of the liver to process bilirubin is very large, serum bilirubin measurement is not a sensitive test of liver function and is mainly used clinically for (1). Diagnosis and classification of jaundice: total bilirubin (TSB) generally does not exceed 85umol/l in hemolytic jaundice, and the ratio of direct bilirubin to total bilirubin is less than 20% in hemolytic jaundice, greater than 40% in general hepatocellular jaundice, and often more than 60% in cholestatic jaundice.
(2). Reflect the degree of liver injury and determine the prognosis: In chronic liver disease, persistent and significant hyperbilirubinemia suggests a serious prognosis. Serum bilirubin >340umol/l is a high degree of jaundice, and if TSB >500umol/l often indicates the presence of severe hepatic substantive lesions, or with hemolysis or renal failure .
(3). To judge the efficacy and guide treatment. Serum bilirubin measurement helps to judge the effect of treatment after biliary obstruction, and helps to judge the response to treatment and progress of primary biliary cirrhosis; it helps to judge the degree of liver damage during chemotherapy for hepatocellular carcinoma.
(4). Certain extrahepatic factors affect the results of serum bilirubin measurement: e.g. estrogen administration, cholangiography, oral contraceptives, pregnancy, sepsis, etc.
Obstructive lipoprotein-X assay: abnormal lipoproteins that appear in the blood in cases of cholestasis and obstructive jaundice, and whose production is related to lecithin reflux in the bile. LP-X is positive in more than 80% of cases of cholestasis by liver tissue biopsy.
V. Bile acid metabolism test
Bile acids are the main organic anion synthesized from cholesterol in the liver and can reflect the excretory function of the liver more specifically. 18-24 g of bile acids are circulated in the body for enterohepatic circulation every day, and the fasting serum bile acids increase significantly in viral hepatitis and extrahepatic biliary obstruction. In patients with active chronic hepatitis, elevated serum bile acids levels often precede elevated transaminases, and relapse is likely if serum bile acids remain elevated even if liver histology improves. In cholestatic liver disease, especially primary biliary cirrhosis and primary sclerosing cholangitis, serum bile acids are often significantly elevated. Measurement of serum bile acid/goose deoxycholic acid ratio: This helps to identify the cause of liver disease. The ratio is 0.5-1.0 in normal subjects, decreases to 0.1-0.5 in cirrhosis, and increases to 0.96-3.6 in extrahepatic obstruction. The ratio in patients with viral hepatitis and liver tumors often overlaps with that in healthy subjects and patients with cirrhosis, and often decreases in patients receiving liver transplants with acute rejection reactions, and may appear before standard liver function tests become abnormal.
Serum enzymatic tests
(A) Transaminases
These include alanine aminotransferase (ALT) and aspartate aminotransferase (AST).
The amount of aminotransferases in the liver is about 100 times higher than that in the blood, and as long as 1% of the liver cells are damaged, it is enough to increase the enzyme activity in the serum by a factor of 1. The concentration of transaminases in hepatocytes is 1000-5000 times higher than in serum, and 2 out of 15,000 hepatocytes are destroyed to increase transaminases. It is still considered as the “gold standard” for hepatocyte damage.
ALT is found mainly in the cytoplasm of hepatocytes and to a lesser extent in mitochondria; it is present in small amounts in kidney, myocardium and skeletal muscle; AST is highest in myocardium, followed by liver, skeletal muscle, kidney and red blood cells. In the liver, 80% is in the mitochondria and 20% in the cytosol.
The significance of elevated transaminases.
1, acute viral hepatitis: elevation is relatively early, with ALT being the most important, peaking at 1~2 weeks
2, chronic viral hepatitis: mild to moderate elevation, lasting for months to years, or fluctuate
3, cirrhosis: depends on the degree of hepatocyte necrosis and liver fibrosis
Post-hepatitis: mild to moderate elevation in the active phase; normal or slightly higher in the resting phase
Alcoholic: lower or normal
Cholestatic: higher, parallel to bilirubin level
4 .Acute severe hepatitis: enzyme bile separation, poor prognosis
5 .Sub-acute severe hepatitis: elevated ALT, or elevated followed by sudden enzyme bile separation
6.Liver ischemia: rapid increase and decrease within 1 day, with AST as rapid
7, drug-related liver damage: can recover after discontinuation of drugs
8.Other: biliary tract lesion (especially obstruction), cardiac disease, muscle disease, and pancreatic, lung, kidney, hyperthyroidism, etc.
About 20% of elevated transaminases cannot be found for a while, and should be examined for hemochromatosis, Wilson’s disease or α1-antitrypsin deficiency liver disease; and some non-hepatic diseases, such as celiac disease, Addison’s disease, anorexia nervosa, myositis or muscle damage after excessive exercise.
Elevated transaminases only reflect hepatocyte damage, not the number of functioning hepatocytes.
AST/ALT ratio: Because of the different distribution in hepatocytes, the magnitude of elevation varies from lesion to lesion.
1, alcoholic liver damage >2 (mitochondrial damage, AST release into the blood; ALT activity is reduced, associated with VitB6 deficiency)
2, acute liver damage overwhelmingly 1
3, cirrhosis can be >1, even up to 2
4, increased in acute Wilson’s disease, fulminant Wilson’s disease >4
(II) Adenosine dehydrogenase (ADA)
In acute hepatitis recovery, the positive rate of ADA elevation is higher than that of transaminases in cirrhosis. ADA is normal in obstructive jaundice, which helps to identify jaundice.
(iii) Lactate dehydrogenase (LDH)
Lack of specificity for liver disease. There are 5 isoenzymes, LDH is elevated in cardiac disease, LDH1>LDH2; LDH5 is elevated in liver disease, LDH5>LDH4.
(iv) Glutamate dehydrogenase (GDH)
GDH is 1.7 times more active in the central lobules of the liver than in the surrounding lobules, and liver damage mainly occurs in the central lobules of the liver in alcoholic liver disease, so serum GDH activity can be used as a good indicator of alcoholic liver disease.
(V) Alkaline phosphatase (ALP)
It is mostly distributed in the liver, bone, kidney, small intestine and placenta, and can be increased in adolescence and late pregnancy, and can be increased after fatty meal. In the liver, it is mainly located in the blood sinusoidal side and capillary bile duct side of the microvilli, with bile excreted into the intestinal tract, increased when the capillary bile duct internal pressure, obstructive jaundice, bile sludge, etc.; cholecystitis, cholelithiasis, hepatocellular carcinoma, amyloidosis, nodular disease, granulomatous hepatitis, intrahepatic lymphoma and other diseases can be seen elevated.
Significantly elevated ALP (4 times higher than normal) is a characteristic manifestation of cholestasis, and the elevated level often parallels that of serum bilirubin. ALP has six isoenzymes, ALP2 is hepatic ALP, ALP3 is from bone, AP4 is from placenta, ALP5 is small intestinal ALP, ALP1 is elevated in obstructive jaundice, ALP2 is predominant in acute cases, and ALP5 is significantly increased in cirrhosis.
(vi) Serum r glutamyl transpeptidase (GGT)
It is widely distributed, mostly in the liver, followed by the kidney and pancreas, and also in the heart muscle, lung and brain, but not in the bone. In the liver, GGT is located in the capillary bile duct side of hepatocytes and biliary system, and its serum level is increased when hepatic synthesis is increased and bile excretion is poor due to various reasons: 1 in cholestasis and hepatocellular carcinoma, it is significantly increased, and it is parallel to several other biliary indicators; 2 in acute hepatitis, it is moderately increased; 3 in chronic hepatitis and cirrhosis, it is normal when inactive, and it is continuously increased when active; 4 in alcoholic and drug-related hepatitis, it is moderately or significantly increased, and GGT to AKP ratio is often >2.5.
Because this enzyme is not elevated in bone disease, it can compensate for ALP and help determine the source of ALP. γ-GT has several isoenzymes, but reports are inconsistent; the most significant is hepatocellular carcinoma-specific GGT II. Some reports suggest that GGT II can be positive when the tumor foci are very small or even not yet detected clinically.
(VII) Other enzymes
Monoamine oxidase and prolyl hydroxylase are used for the diagnosis of hepatic fibrosis. The activity of leucine aminopeptidase (LAP) is significantly increased mainly in pancreatic cancer, cholangiocarcinoma, hepatocellular carcinoma and various obstructive jaundice.
VII. Inorganic ion examination
In acute hepatitis, hepatocytes undergo degenerative necrosis, which causes the release of stored iron from the liver into the blood and elevates serum iron. In chronic hepatitis relapse and hepatocellular jaundice, serum iron is also elevated, while in obstructive jaundice it is normal or reduced.
The liver is the organ containing the largest amount of copper in human tissues. In cholestasis, metastatic liver cancer, and primary cirrhosis, serum copper and plasma copper cyanogen are elevated at the same time.
VIII. Immunological examination
Plasma globulin concentration increases and the A/G ratio decreases in liver disease. Possible mechanisms include increased antibody production due to decreased clearance of bacterial antigens from portal blood or stimulation by release of antigenic material from damaged hepatocytes. Also in primary biliary cirrhosis, IgM concentrations are elevated. In about 90% of patients with PBC, anti-mitochondrial antibodies are present in their plasma. Anti-nuclear and smooth muscle antibodies may be present in some patients with non-viral chronic active hepatitis.
In the clinical use of the above liver function tests, serum transaminase, alkaline phosphatase, bilirubin, prothrombin time, and albumin are generally used as routine tests, to which other tests are added as needed. For the diagnosis of specific diseases, various markers (hepatitis virus markers, methemoglobin, etc.) are also tested and combined with imaging tests to make a comprehensive analysis and judgment.