The liver is a vital organ of the body, which performs multiple functions, including filtration and purification of the blood, the transformation of absorbed substances, and storage. A liver test is a blood test to assess this liver function. Carried out by means of a blood sample, it makes it possible to measure various markers of liver activity, in particular its enzymes. Several signs may indicate the need for a liver test, such as, for example, jaundice, nausea, or even vomiting blood. Finally, these liver tests constitute a non-invasive means of checking for the presence of liver pathology and also allow the severity and progression of liver disease to be measured, as well as its response to treatment.

The liver fulfills multiple functions in the body: it filters and purifies the blood, transforms and stores substances absorbed by the digestive tract, and in particular drugs. In addition, it is at the origin of the manufacture of bile, a biological liquid that promotes digestion and more specifically that of lipids, as well as the production of most proteins. Thus, the liver makes most of the proteins found in the blood. Among them, albumin, which plays a role in the regulation of blood volume as well as in the distribution of fluids in the body.

The purpose of liver function tests is to check the functioning of the liver and in particular that of its main enzymes. Enzymes are substances produced by living cells and which promote chemical reactions, such as for example digestion (thus, enzymes act as catalysts, accelerators in chemical changes).

More commonly called “hepatic assessment”, these biological examinations make it possible to highlight damage or disease of the liver. In practice, several parameters are measured on the same blood sample.

The main blood markers evaluated will be:

• ALAT: Alanine Aminotransferase. It is an enzyme present mainly in liver cells. Physiologically, its values ​​in the blood are low;
• ASAT: Aspartate Aminotransferase. This enzyme, located mainly in the cells of the heart and the liver, is released into the bloodstream in the event of damage to the heart, liver or muscle cells;
• Alkaline phosphatase (PAL). Very high concentrations of PAL may indicate that the bile ducts are more or less blocked or in an inflamed situation, or that the liver is infiltrated by a tumor;
• Bilirubin. It is a bile pigment that comes from the breakdown of heme from hemoglobin;
• Albumin. The most abundant protein in the blood, it keeps water from escaping from the blood vessels, it nourishes the tissues, it carries hormones, vitamins, drugs and ions throughout the body. It is very sensitive to any liver damage;
• Total proteins. Their measurement reflects nutritional status and makes it possible to detect certain pathologies of the liver or kidney, in particular;
• Gamma-glutamyl transferase (GGT). This enzyme participates in the metabolism of amino acids;
• Lactate dehydrogenase (LDH). This hydrogen transfer enzyme participates in the metabolism of carbohydrates in different organs and tissues;
• 5′-Nucleotidase. This enzyme is present in cell membranes (mainly hepatic);
• The level of pro-thrombin. It is indicative of the state of coagulation.

Furthermore, the presence of bilirubin in the urine can also be tested. 

In fact, the level of some of these substances makes it possible to measure the presence and the degree of inflammation of the liver (and in particular ALTs and ASATs). As for other substances, their level indicates, for example, whether the liver normally performs its functions of protein production and bile secretion (especially albumin and bilirubin). Note: it happens that values ​​higher than normal are not correlated with damage related to the liver but are actually caused by other disorders.

• A liver test is prescribed when there are possible signs of liver damage;
• On the other hand, it is also indicated in the diagnosis of viral hepatitis, as well as for their monitoring;
• A patient who has a family history of liver damage will also realize this;
• This assessment is also recommended in the event of excessive alcohol consumption, or in the event of taking medications that can damage the liver;
• In addition, during drug poisoning, it will also be necessary to quickly check the condition of the liver.

The main clinical signs that require liver workup are as follows:

• Jaundice, or jaundice;
• Dark urine and faint or discolored stools;
• Nausea, vomiting and diarrhea;
• Loss of appetite;
• Vomiting blood;
• Bloody stools;
• Abdominal pain;
• Slimming;
• Fatigue.

The liver test is, finally, useful in several situations:

• To detect inflammation, lesions or even liver dysfunction;
• To assess the severity of liver injury;
• To follow the evolution of hepatic diseases, but also to monitor a patient’s response to treatment;
• To clarify the diagnosis.

Finally, in a patient in good health and not presenting any risk factors, a routine dosage of hepatic tests is not justified. On the other hand, an annual check is indicated in various situations:

• Obesity, diabetes, dyslipidemia;
• Risk behavior: intravenous drug use, unsafe sex, alcohol abuse, frequent travel to developing countries;
• Taking hepatotoxic drugs;
• History of autoimmune diseases;
• Familial predisposition to liver disease.

A blood test is taken on an empty stomach. It is important to observe a twelve hour fast before the sample. This blood sample, often taken from the bend of the elbow by venipuncture, will be analyzed in a medical biology laboratory.

The hepatic assessment consists in carrying out the biological analysis of various parameters of the liver, by means of routine assay methods making it possible to evaluate the levels of enzymes and to measure the derivatives of hemoglobin as well as those of lipids and lipoproteins. 

Finally, as indicated in a reference document by the University Hospitals of Geneva:

• Generally, if a liver test is performed, ALT and alkaline phosphatase should be given as a priority;
• If these are pathological, it will then be necessary to supplement the dosage with ASAT, GGT and bilirubin.

The reference values ​​of the hepatic tests can vary according to the geographical origin, the sex, the age of the individuals. In addition, variations may also exist depending on the techniques used by the laboratories. 

For liver function tests, the following reference values ​​have been established.

Enzymes :

• ALAT (TGP): Male: <45 IU / L, Female: <34 IU / L;
• ASAT (TGO): Men: <35 UI / L, Female: <35 UI / L;
• Gamma GT (GGT): Male: <55 IU / L, Female: <38 IU / L;
• LDH: Male: <248 IU / L; Female: <248 IU / L;
• Alkaline phosphatases: 30 to 100 IU / L (Men and Women).

Hemoglobin derivatives:

• Total bilirubin: <17 µmol / L (old unit: <10 mg / L);
• Conjugated bilirubin: 0 µmol / L (old unit: <0 mg / L);
• Unconjugated bilirubin: <17 µmol / L (old unit: <10 mg / L)

Glycoprotein :

• PT (prothrombin level): 80 – 100%
• Quick time: 12 – 13 seconds.

 The increase in transaminases (ASAT and ALAT) reflects cellular damage, particularly in the liver, heart, kidney or muscle. A strong elevation of ALT is seen in acute viral hepatitis. Finally, it will be essential to consult your doctor who will interpret the assessments, can guide the diagnosis, prescribe other examinations if necessary (such as imaging and, in a minority of cases, diagnostic biopsy). He can also decide on a therapeutic strategy, and refer his patient to a specialist. In the event of liver disease (hepatopathy), abstinence from alcohol and medication or other hepatotoxic substances, as well as vaccination against hepatitis A and B, are recommended. To conclude, it should also be noted that approximately 1 to 9% of the general population presents an asymptomatic elevation of liver tests. In addition, there are also extrahepatic causes that may explain an elevation of these tests.