LIVER FUNCTION TESTS

Transcription

1 LIVER FUNCTION TESTS A- Metabolic Functions of the Liver: 1. The liver plays a major role in carbohydrate, lipid and protein homeostasis, with the processes of glycolysis, the Krebs cycle, gluconeogenesis, glycogen synthesis and glycogenolysis, lipogenesis, ketogenesis, amino acid synthesis and degradation, and protein synthesis; all taking place in the hepatocytes. 2. Hepatocytes also metabolize and detoxify endogenous (haem) and exogenous products (drugs), which are then excreted via the biliary tree. B- Importance and Classification of Liver Function Tests: Importance: Liver function test are valuable for: 1. Diagnosis of liver disease. 2. Categorize the liver disease into its appropriate category. 3. Following the progress of liver disease. Classification: Liver function tests can be classified according to many categories: A- According to its function of the test: 1. Basic Metabolic function tests: include estimation of serum levels of liver enzymes e.g. sgpt (ALT) and sgot (AST). 2. Excretory function tests: include estimation of serum levels of total and direct bilirubin. 3. Synthetic function tests: include estimation of serum levels of albumin and cholesterol. B- According to the type of variables: 1. Hepatic anion transport tests: as bilirubin excretion and plasma bile acid conjugation. 2. Plasma protein abnormalities: they include a) Determination of plasma albumin: (see section No. 3). 1 P a g e

2 Clinical Chemistry of Liver b) Determination Coagulation factors: Through estimation of: Vit. K (fat soluble vitamin): where the decrease in the level of this vitamin (as in cholestasis) leads to increase the tendency of the patient to bleeding and failure of lipids absorption. Prothrombin time: due to deficiency of one or more coagulation factors as in hepato cellular damage. c) Plasma Ig: they not have large clinical significance but important in chronic cases as in liver cirrhosis IgA, IgM and IgG d) Ceruloplasmin: it is metalo-protein formed if Cu bind with protein and act as α1 protease inhibitor. 3. Plasma enzyme tests: a) Estimation of the soluble enzymes released from cytoplasm and mitochondria. b) Estimation of the enzymes released from membrane where they are bind to it. They include: o γ-glutamyl transferase (GGT). o Alkaline phosphatase (ALP). c) Certain enzymes that their synthesis is impaired when hepato cellular function is greatly impaired e.g. cholinesterase enzyme 4. Metabolic capacity tests: Galactose clearance. Caffeine clearance. C- According to type of liver disease: 1. Tests of hepatic fibrosis: Collagen: that deposited in space on basement membrane. N-terminal peptide. 2. Liver cirrhosis induced by alcoholic drink: estimation of desialated transferin. 3. Cholestasis tests: e.g. Estimation of total and direct bilirubin. 2 P a g e

3 Clinical Chemistry of Liver 4. Liver tumors: e.g. Estimation of alpha-fetoprotein as liver tumor marker. Some of routine liver function test group: Test Plasma albumin. Plasma total and direct bilirubin. Plasma enzyme activities ALT/AST. Alkaline phosphatase and GGT. Function Protein synthesis. Hepatic anion transport. Hepatocellular integrity. Presence of Cholestasis. Liver Function Tests: Albumin The liver also makes albumin, an essential protein that circulates in blood. Albumin levels are low in people with severe chronic liver disease, because the liver does not make normal amounts of albumin. However, albumin levels also may fall in a variety of medical conditions. A low albumin level is often temporary, so it is not a reliable way to diagnose liver disease. Liver Function Tests: Bilirubin Bilirubin is a waste product from the breakdown of red blood cells. The liver processes bilirubin so it can be excreted in stool. Bilirubin flows through the liver's bile ducts, dissolved in bile. Bilirubin blood levels may be elevated in people with impaired bile flow. This can occur in severe liver disease, gallbladder disease, or other bile system conditions. Very high bilirubin levels cause jaundice, in which the skin and whites of the eyes turn yellow. Bilirubin can be a useful liver function test in people with a known bile flow problem. An elevated bilirubin may also be present in people with a type of anemia, called hemolytic anemia. 3 P a g e

4 Sources of Bilirubin: Hemoglobin; 1- BILIRUBIN METABOLISM (Excretory and Anion Transport Hepatic Function) o RBC breakdown; 90% of RBC breakdown occurs within reticulo- endothelial system (RES) cells (mainly in spleen). o Ineffective erythropoiesis (in bone marrow). Other haem containing proteins e.g. myoglobin and cytochrome P450 (mainly in liver). 70 to 80% of daily bilirubin production is derived from the breakdown of senescent red blood cells, while the remainder is derived from ineffective erythropoiesis and the breakdown of other haem-containing proteins. Total daily bilirubin production is 450 up to 550 µmol/day. Formation of Bilirubin: Hemoglobin is broken down to globin and haem. Globin (a protein) is broken down to its constituent amino acids. Haem (a 4 ring structure containing Fe 2+ at its center) is broken down (via biliverdin) to carbon monoxide, iron and bilirubin. Biliverdin gives the green colour sometimes seen in a resolving bruise. The bilirubin at this stage is termed un-conjugated bilirubin because it has not yet been processed by conjugation in the liver. 4 P a g e

5 Clinical Chemistry of Liver Un-Conjugated Bilirubin (Hemo-Bilirubin, Indirect Bilirubin): A hydrophobic molecule. Strongly bound (high affinity) to hydrophobic sites on albumin. Does not appear in urine. Free un-conjugated bilirubin normally <3 µmol/l. Can be displaced from binding sites on albumin by drugs (salicylates, sulphonamides). Free un-conjugated bilirubin is neurotoxic. o At high concentrations it deposits in cell membranes (esp. basal ganglia) causing kernicterus. Uptake by the Liver: The un-conjugated bilirubin - albumin complex is carried in the plasma to the hepatic sinusoids, enters the space of Disse and dissociates at the hepatocyte membrane. The un-conjugated bilirubin is taken up by the hepatocytes by a carriermediated process. Within the hepatocyte the un-conjugated bilirubin is bound to ligandin. 5 P a g e

6 Conjugation by the Liver: Clinical Chemistry of Liver The bilirubin is then conjugated with glucuronic acid by UDP-glucuronyl transferase (UDPGT-I) to bilirubin mono-glucuronide (BMG) and by UDPGT- II to bilirubin diglucuronide (BDG). Conjugated bilirubin is more water soluble and can be excreted in bile or urine. Under normal circumstances there is no conjugated bilirubin present in plasma. Excretion into Bile: Conjugated bilirubin is transported out of the liver cells into the bile canaliculi by an energy-dependant carrier-mediated process which is sensitive to cell injury. This canalicular excretion step rather than conjugation is thought to be the ratelimiting step in bilirubin metabolism. Bile flows through the canaliculi, into the bile ducts, and finally into the duodenum. Reticuloendothelial cells Spleen - Bone Marrow Old RBCs Biliverdin Bilirubin Iron Amino acids Heme Hb Globin Systemic circulation Bilirubin Carried on albumin Liver Albumin Bilirubin Conjugation with Glucuronic acid Kidney Reabsorption Urobilin, brownish to Urine Intestine Free Bilirubin Urobilinogens, colorless Urobilin, brownish to feces Dr. Kakul Husain Firoz 5 6 P a g e

7 Clinical Chemistry of Liver 7 P a g e

8 BILIRUBIN: Bilirubin is usually measured by the Jendrassik Grof modification of the van den Bergh reaction: conjugated bilirubin (direct-reacting) (water soluble). unconjugated bilirubin (indirect-reacting) (water insoluble). Normal serum total bilirubin <17 µmol/l (all unconjugated). Normal urine contains no unconjugated bilirubin, since it is albumin-bound and not filtered. Bilirubin only appears in urine when conjugated bilirubin is elevated in plasma. Increased amounts of unconjugated bilirubin are found in plasma in: TWO o Increased bilirubin production. o Decreased uptake or conjugation of bilirubin. o in generalised hepatocellular dysfunction. o in specific rare inherited syndromes (Gilbert s and Criggler-Najjar syndromes). Increased amounts of conjugated bilirubin are found in the plasma (and urine) in: Decreased excretion of bilirubin: o in obstructive liver disease. LIVER DISORDERS AND JAUNDICE Jaundice or icterus is the yellow appearance of skin and sclerae due to the presence of an excessive amount of bilirubin (jaundice becomes clinically visible when serum bilirubin is > 40 µmol/l). The liver has a large reserve capacity- jaundice only appears with severe impairment of liver function. CLASSIFICATION OF JAUNDICE ACCORDING TO ITS CAUSE: A. Prehepatic: Excess bilirubin production. B. Intrahepatic: 8 P a g e

9 Decreased conjugation of bilirubin by liver cells. Decreased excretion of bilirubin into bile canaliculi. C. Posthepatic: Biliary obstruction. A. PREHEPATIC JAUNDICE (Increased Production of Bilirubin). 1- Haemolytic Disorders: Abnormal haemoglobins (e.g. sickle cell anaemia). RBC membrane defects (e.g. hereditary spherocytosis). RBC enzyme defects (e.g. G6PD deficiency). Autoimmune haemolytic anaemias. Blood group incompatibility; o foetal / maternal (newborns) o transfusion. Bacterial toxins causing haemolysis. Malaria. 2- Ineffective Erythropoiesis: Megaloblastic anaemias. Results of laboratory tests in cases of excess bilirubin production: Increased un-conjugated bilirubin (rate of bilirubin production exceeds maximum rate of conjugation). Haemolytic jaundice is never deep (except in newborns). Normal conjugated bilirubin (no obstruction). Urine bilirubin negative. Urine urobilinogen increased (increased flux through pathway). Other liver function tests normal. Haematological tests: o increased reticulocyte count. o tests to identify cause of haemolysis (RBC morphology, abnormal haemoglobin, Coombs test, etc.). 9 P a g e

10 B. INTRAHEPATIC JAUNDICE (Decreased Handling of Bilirubin by the Liver) Specific Inherited Defects In Bilirubin Metabolism (Rare) 1- Defect In Uptake / Conjugation Of Bilirubin: A- Gilbert s Syndrome: Inherited disorder - decreased expression of UDPGT I due to a TA insertion in the TATA box. Mildly increased levels of unconjugated bilirubin (levels up to 85 µmol/l), made worse by viral infections and fasting. B- Criggler-Najjar Syndromes : Criggler-Najjar type I syndrome : Inherited disorder - severe mutation of UDPGT I gene leading to absent UDPGT I activity. Severe disorder with very high levels of unconjugated bilirubin. Criggler-Najjar type II syndrome (Arias syndrome): Inherited disorder - milder mutation of UDPGT I gene leading to reduced UDPGT I activity. Milder than type I. 2- Defect In Excretion Of Bilirubin Into Bile Canaliculi: Dubin-Johnson and Rotor s Syndromes : Benign inherited disorders. Increased levels of conjugated bilirubin in serum and urine. Due to impaired excretion. 3. POSTHEPATIC JAUNDICE Causes of extrahepatic obstruction ( cholestasis ): In the lumen (gallstones, worms). In the wall (bile duct carcinoma, stricture, atresia). Outside the wall (carcinoma of head of pancreas, lymph node enlargement at 10 P a g e

11 porta hepatis). Clinical and biochemical features of obstruction (intrahepatic or extrahepatic): Jaundice, dark urine, pale stools. The jaundice may be very deep, despite a generally well patient. Increased conjugated bilirubin in serum and urine. Types of Gallstones: 1- Cholesterol stones. Common gallstone. Predisposing factors:obesity, increasing age, female sex, diabetes, hyperlipidaemia. Chronic cholecystitis. Relative deficiency of tri- and dihydroxy bile salts leading to "lithogenic bile". o Oral chenodeoxycholic acid has been used therapeutically. 2- Pigment stones. Excess bilirubin production in haemolytic anaemias. Presentation: Intermittent passage of gallstone: Severe right upper quadrant abdominal pain (biliary colic). Intermittent jaundice and bilirubinuria. Intermittent elevations of ALP, GGT. Prolonged painless obstructive jaundice. Acute cholecystitis. CLASSIFICATION OF JAUNDICE ACCORDING TO TYPE OF BILIRUBIN A- Unconjugated Hyperbilirubinaemia Types Physiological jaundice Jaundice develops on days 3-5 (never on day 1). Levels of bilirubin may reach 150 µmol/l, all unconjugated. Usually phototherapy is all the treatment that is required. Blood group incompatibility ("haemolytic disease of the newborn ): Jaundice develops on day 1. Rhesus or ABO systems commonly responsible, others rarely. A few foetal RBCs cross into maternal circulation, mother makes antibodies if 11 P a g e

12 blood groups are incompatible. These IgGs then pass into foetal circulation, causing haemolysis of foetal RBCs. Excessive bilirubin production is not a problem for the foetus, since it crosses placenta and is conjugated, excreted by mother's liver. However, bilirubin starts rising rapidly after birth. Severe haemolysis in utero causes foetal anaemia and heart failure ("hydrops foetalis"). Inherited haemolytic disorders (as in adults). Features: - Lethargy, hypotonia, poor feeding, spasticity and death. Treatment Phototherapy Light at nm causes formation in skin of isomers of unconjugated bilirubin which are more soluble, excreted in urine. B- CONJUGATED HYPERBILIRUBINAEMIA: Types Neonatal hepatitis (Jaundice): o Many infective agents can cause hepatitis in neonates (e.g. rubella, CMV, herpes simplex, toxoplasmosis). o In utero, bilirubin is transported across the placenta, conjugated and excreted by the mother's liver. o After birth the neonate must excrete the bilirubin. o Factors contributing to neonatal jaundice:** Increased relative RBC mass. Shorter RBC lifespan. Immature liver, especially in premature neonates. Takes 2-4 weeks to develop fully. Many infective agents can cause hepatitis in neonates (e.g. rubella, CMV, herpes simplex, toxoplasmosis). 12 P a g e

13 2- BILE ACID (= BILE SALT) METABOLISM Source of Bile Acids: Bile acids are soluble metabolites of cholesterol. Formation of Bile Acids: The products of cholesterol metabolism are chenodeoxycholic acid and cholic acid, known as primary bile acids because of their hepatic origin. The rate limiting and regulated step in their formation is 7-α-hydroxylation of cholesterol. Prior to secretion into the bile canaliculi, the primary bile acids are conjugated to glycine or taurine (this increases their solubility). Conjugated bile acids are transported out of the liver cells into the bile canaliculi by an energy-dependant carrier-mediated process (which is independent of the bilirubin transport mechanism). In the GIT, bacterial enzymes deconjugate and α dehydroxylate the primary bile acids and convert them to the secondary bile acids lithocholic acid and deoxycholic acid. Most of the bile acids in the GIT are reabsorbed into the portal circulation (75% in the ileum and 10% in the colon), taken up by the liver again and reexcreted (enterohepatic circulation). Re-uptake of bile acids by the liver is highly efficient, but sensitive to liver damage. 13 P a g e

14 PROTHROMBIN TEST Besides its functions in metabolism, the liver makes proteins that are essential to normal blood clotting. True liver function tests check the liver's ability to make these proteins. They include: Prothrombin time (PT): A test of the time it takes for a blood sample to clot, under specific conditions in a lab; if low levels of clotting factors are present, the prothrombin time is longer. International normalized ratio (INR): Not really a test, but a standardized way for all labs to report PT, so their results can be compared accurately with each other. PT and INR rise in people with severe liver disease because the liver fails to make normal amounts of certain clotting factors. An elevated PT can have many other causes besides liver disease, however. PT is often checked together with PTT (partial thromboplastin time), which is not a liver function test. If PT and/or PTT are elevated, a problem with bleeding or clotting may be present. *************** 14 P a g e