Knowing Liver Function Tests (LFT)

Liver function tests

Liver function tests
Intervention
ICD-10-PCS	K-70 to K-77
MeSH	D008111
MedlinePlus	003436

Liver function tests (LFTs or LFs) are groups of blood tests that give information about the state of a patient'sliver. These tests include prothrombin time (PT/INR), aPTT, albumin, bilirubin (direct and indirect), and others. Liver transaminases (AST or SGOT and ALT or SGPT) are useful biomarkers of liver injury in a patient with some degree of intact liver function. 

Most liver diseases cause only mild symptoms initially, but these diseases must be detected early. Hepatic (liver) involvement in some diseases can be of crucial importance. This testing is performed on a patient's blood sample.

 Some tests are associated with functionality (e.g., albumin), some with cellular integrity (e.g.,transaminase), and some with conditions linked to the biliary tract (gamma-glutamyl transferase and alkaline phosphatase). 

Several biochemical tests are useful in the evaluation and management of patients with hepatic dysfunction. These tests can be used to detect the presence of liver disease, distinguish among different types of liver disorders, gauge the extent of known liver damage, and follow the response to treatment. 

Some or all of these measurements are also carried out (usually about twice a year for routine cases) on those individuals taking certain medications, such as anticonvulsants, to ensure the medications are not damaging the person's liver.

Contents

   [hide] 

• 1 Standard liver panel
  • 1.1 Albumin
  • 1.2 Aspartate transaminase
  • 1.3 Transaminases
  • 1.4 Alkaline phosphatase
  • 1.5 Total bilirubin
  • 1.6 Direct bilirubin
  • 1.7 Congenital bilirubin disorders
  • 1.8 High bilirubin in neonates
  • 1.9 Gamma glutamyl transpeptidase
  • 1.10 INR
• 2 Other tests
  • 2.1 5' Nucleotidase
  • 2.2 Coagulation test
  • 2.3 Serum glucose
  • 2.4 Lactate dehydrogenase
• 3 See also
• 4 References
• 5 External links

Standard liver panel[edit]

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Although example reference ranges are given, these will vary depending on age, gender, ethnicity, method of analysis, and units of measurement. Individual results should always be interpreted using the reference range provided by the laboratory that performed the test.

Albumin[edit]

Reference range

3.5 to 5.3 g/dL

Albumin is a protein made specifically by the liver, and can be measured cheaply and easily. It is the main constituent of total protein (the remaining from globulins). 

Albumin levels are decreased in chronic liver disease, such as cirrhosis. It is also decreased in Nephrotic_syndrome, where it is lost through the urine. The consequence of low albumin can be edema since the intravascular oncotic pressure becomes lower than the extravascular space. 

An alternative to albumin measurement is prealbumin, which is better at detecting acute changes (half-life of albumin and prealbumin is about 2 weeks and about 2 days, respectively).

Aspartate transaminase[edit]

Reference range

6-40 IU/L[5]

AST, also called serum glutamic oxaloacetic transaminase or aspartate aminotransferase, is similar to ALT in that it is another enzyme associated with liver parenchymal cells. It is raised in acute liver damage, but is also present in red blood cells, and cardiac and skeletal muscle, so is not specific to the liver. 

The ratio of AST to ALT is sometimes useful in differentiating between causes of liver damage.^[6][7] Elevated AST levels are not specific for liver damage, and AST has also been used as a cardiac marker. When the AST is higher than ALT, a muscle source of these enzymes should be considered. 

For example, muscle inflammation due to dermatomyositis may cause AST>ALT. This is a good reminder that AST and ALT are not good measures of liver function because they do not reliably reflect the synthetic ability of the liver and they may come from tissues other than liver (such as muscle).

Transaminases

Main article: Elevated transaminases

AST/ALT elevations instead of ALP elevations favor liver cell necrosis as a mechanism over cholestasis. When AST and ALT are both over 1000 IU/L, the differential can include acetaminophen toxicity, shock, or fulminant liver failure. When AST and ALT are greater than three times normal but not greater than 1000 IU/L, the differential can include alcohol toxicity, viral hepatitis, drug-induced level, liver cancer, sepsis, Wilson's disease, post-transplant rejection of liver, autoimmune hepatitis, and steatohepatitis (nonalcoholic). AST/ALT levels elevated minorly may be due to rhabdomyolysis, among many possibilities.

Alkaline phosphatase

Reference range

30 to 120 IU/L[8]

Main article: Elevated alkaline phosphatase

Alkaline phosphatase (ALP) is an enzyme in the cells lining the biliary ducts of the liver. ALP levels in plasma rise with large bile duct obstruction, intrahepatic cholestasis, or infiltrative diseases of the liver. 

ALP is also present in bone and placental tissue, so it is higher in growing children (as their bones are being remodelled) and elderly patients with Paget's disease. In the third trimester of pregnancy, ALP is about two to three times higher.

Total bilirubin

Reference range

0.1–1.0 mg/dL

Measurement of total bilirubin includes both unconjugated and conjugated bilirubin. Unconjugated bilirubin is a breakdown product of heme (a part of hemoglobin in red blood cells). It is very hydrophobic and is mainly transported bound to albumin circulating in the blood. 

Addition of high-concentration hydrophobic drugs (certain antibiotics, diuretics) and high free fatty acids can cause elevated unconjugated bilirubin. Heme can also come from myoglobin, found mostly in muscle, cytochromes, found mostly in mitochondria, catalase, peroxidase, and nitric oxide synthase. 

The liver is responsible for clearing the blood of unconjugated bilirubin, and about 30% of it is taken up by a normal liver on each pass of the blood through the liver by the following mechanism: bilirubin is taken up into hepatocytes, 'conjugated' (modified to make it water-soluble) by UDP-glucuronyl-transferase, and secreted into the bile by CMOAT (MRP2), which is excreted into the intestine. 

In the intestine, conjugated bilirubin may be metabolized by colonic bacteria, eliminated, or reabsorbed. Metabolism of bilirubin into urobilinogen followed by reabsorption of urobilinogen accounts for the yellow color of urine, as urine contains a downstream product of urobilinogen.

 Further metabolism of urobilinogen into stercobilin while in the bowels accounts for the brown color of stool. Thus, having white or clay-colored stool is an indicator for a blockage in bilirubin processing and thus potential liver dysfunction or cholestasis.

Increased total bilirubin (TBIL) causes jaundice, and can indicate a number of problems:

• 1. Prehepatic: Increased bilirubin production can be due to a number of causes, including hemolytic anemias and internal hemorrhage.
• 2. Hepatic: Problems with the liver are reflected as deficiencies in bilirubin metabolism (e.g., reduced hepatocyte uptake, impaired conjugation of bilirubin, and reduced hepatocyte secretion of bilirubin). Some examples would be cirrhosis and viral hepatitis.
• 3. Posthepatic: Obstruction of the bile ducts is reflected as deficiencies in bilirubin excretion. (Obstruction can be located either within the liver or in the bile duct).
• 
  

Direct bilirubin

Reference range

0.1–0.4 mg/dL

The diagnosis is narrowed down further by evaluating the levels of direct bilirubin.

• If direct (conjugated) bilirubin is normal, then the problem is an excess of unconjugated bilirubin (indirect bilirubin), and the location of the problem is upstream of bilirubin conjugation in the liver. Hemolysis, or internal hemorrhage can be suspected.
• If direct bilirubin is elevated, then the liver is conjugating bilirubin normally, but is not able to excrete it. Bile duct obstruction by gallstones, hepatitis, cirrhosis or cancer should be suspected.
• 
  

Congenital bilirubin disorders

About 5% of the population has Gilbert's syndrome, a mutation (or variation) in the UDP-glucuronyl-transferase promotor that manifests itself as jaundice when the individual is stressed (i.e. starves). 

Autosomal recessive knockouts of UDP-glucuronyl-transferase can lead to Crigler-Najjar syndrome and elevations of unconjugated bilirubin. Defects in CMOAT (MRP2) results in Dubin-Johnson syndrome and elevations of conjugated bilirubin.

High bilirubin in neonates

Neonates are especially vulnerable to high bilirubin levels due to an immature blood-brain barrier that predisposes them to kernicterus/bilirubin encephalopathy, which can result in permanent neurological damage. 

Neonates also have a low amount of functional UDP-glucuronyl-transferase and can have elevated unconjugated bilirubin, since conjugation is limited. So, newborns are often treated with blue light (420-470 nm) to turn the hydrophobic, albumin-binding unconjugated bilirubin into a form that is more hydrophilic and able to be secreted in urine, sparing the neonate's brain.

Gamma glutamyl transpeptidase

Reference range

0 to 42 IU/L[8]

Although reasonably specific to the liver and a more sensitive marker for cholestatic damage than ALP, gamma glutamyl transpeptidase (GGT) may be elevated with even minor, subclinical levels of liver dysfunction. 

It can also be helpful in identifying the cause of an isolated elevation in ALP (GGT is raised in chronic alcohol toxicity).

INR

Prothrombin time (PT) and its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) are measures of the extrinsic pathway ofcoagulation. 

This test is also called "ProTime INR" and "INR PT". They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage, and vitamin K status.

Other tests

Other tests commonly requested alongside LFTs include:

5' Nucleotidase

5' Nucleotidase (5'NTD) is another test specific for cholestasis or damage to the intra- or extrahepatic biliary system, and in some laboratories, is used as a substitute for GGT for ascertaining whether an elevated ALP is of biliary or extrabiliary origin.

Coagulation test

The liver is responsible for the production of coagulation factors. INR measures the speed of a particular pathway of coagulation, comparing it to normal. Increased levels of INR means blood is taking more time than usual to clot. 

The INR increases only if the liver is so damaged that synthesis of vitamin K-dependent coagulation factors has been impaired; it is not a sensitive measure of liver function.

It is very important to normalize the INR before operating on people with liver problems (usually by transfusion with blood plasma containing the deficient factors), as they could bleed excessively.

Serum glucose

The serum glucose test, abbreviated as "BG" or "Glu", measures the liver's ability to produce glucose (gluconeogenesis); it is usually the last function to be lost in the setting of fulminant liver failure.

Lactate dehydrogenase

Lactate dehydrogenase (LDH) is found in many body tissues, including the liver. Elevated levels of LDH may indicate liver damage. LDH isotype-1 (or cardiac) is used for estimating damage to cardiac tissue, although troponin and creatine kinase tests are more preferred.
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