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Liver Blood Test
Introduction to liver blood test
An initial step in detecting liver damage is a simple blood test to determine the presence of certain liver enzymes in the blood. Under normal circumstances, these enzymes reside within the cells of the liver. But when the liver is injured for any reason, these enzymes are spilled into the blood stream. Enzymes are proteins that are present throughout the body, each with a unique function. Enzymes help to speed up (catalyze) routine and necessary chemical reactions in the body.
Among the most sensitive and widely used of these liver enzymes are the aminotransferases. They include aspartate aminotransferase (AST or SGOT) and alanine aminotransferase (ALT or SGPT). These enzymes are normally contained within liver cells. If the liver is injured, the liver cells spill the enzymes into blood, raising the enzyme levels in the blood and signaling the liver damage.
What are the aminotransferases?
The aminotransferases catalyze chemical reactions in the cells in which an amino group (amino acids are building blocks of proteins) is transferred from a donor molecule to a recipient molecule. Hence, the names "aminotransferases."
Medical terms can sometimes be confusing, as is the case with these enzymes.
Another name for aminotransferase is transaminase.
The enzyme aspartate aminotransferase (AST) is also known as serum glutamic oxaloacetic transaminase (SGOT); and
alanine aminotransferase (ALT) is also known as serum glutamic pyruvic transaminase (SGPT).
To put matters briefly, AST = SGOT and ALT = SGPT.
Normally, where are the aminotransferases?
AST (SGOT) is normally found in a diversity of tissues including liver, heart, muscle, kidney, and brain. It is released into serum when any one of these tissues is damaged. For example, its level in serum rises with heart attacks and with muscle disorders. It is therefore, not a highly specific indicator of liver injury.
ALT (SGPT) is, by contrast, normally found largely in the liver. This is not to say that it is exclusively located in liver, but that is where it is most concentrated. It is released into the bloodstream as the result of liver injury. It therefore serves as a fairly specific indicator of liver status.
What are normal levels of AST and ALT?
The normal range of values for AST (SGOT) is from 5 to 40 units per liter of serum (the liquid part of the blood).
The normal range of values for ALT (SGPT) is from 7 to 56 units per liter of serum.
What do elevated AST and ALT mean?
AST (SGOT) and ALT (SGPT) are sensitive indicators of liver damage or injury from different types of disease. But it must be emphasized that higher-than-normal levels of these liver enzymes should not be automatically equated with liver disease. They may mean liver problems or they may not. For example, elevations of these enzymes can occur with muscle damage. The interpretation of elevated AST and ALT levels depends upon the entire clinical evaluation of a patient, and so it is best done by doctors experienced in evaluating liver disease.
The precise levels of these enzymes do not correlate well with the extent of liver damage or the prognosis (outlook). Thus, the exact levels of AST (SGOT) and ALT (SGPT) cannot be used to determine the degree of liver disease or predict the future. For example, patients with acute viral hepatitis A may develop very high AST and ALT levels (sometimes in the thousands of units/liter range). But most patients with acute viral hepatitis A recover fully without residual liver disease. For a contrasting example, patients with chronic hepatitis C infection typically have only a little elevation in their AST and ALT levels. Some of these patients may have quietly developed chronic liver disease such as chronic hepatitis and cirrhosis (advanced scarring of the liver).
It is, therefore, worth mentioning that these liver enzymes do not give an indication of the function of the liver. Sometimes they are mistakenly referred to as “liver function tests” or LFTs, but it is a misnomer commonly used.
What liver diseases cause abnormal aminotransferase levels?
The highest levels of AST and ALT are found with disorders that cause the death of numerous liver cells (extensive hepatic necrosis). This occurs in such conditions as:
acute viral hepatitis A or B,
pronounced liver damage inflicted by toxins as from an overdose of acetaminophen (brand-name Tylenol), and
prolonged collapse of the circulatory system (shock) when the liver is deprived of fresh blood bringing oxygen and nutrients.
AST and ALT serum levels in these situations can range anywhere from ten times the upper limits of normal to thousands of units/liter.
Mild to moderate elevations of the liver enzymes are commonplace. They are often unexpectedly encountered on routine blood screening tests in otherwise healthy individuals. The AST and ALT levels in such cases are usually between twice the upper limits of normal and several hundred units/liter.
One of the most common cause of mild to moderate elevations of these liver enzymes is fatty liver. In the United States, the most frequent cause of fatty liver is alcohol abuse. Other causes of fatty liver include diabetes mellitus and obesity. Chronic hepatitis C is also becoming an important cause of mild to moderate liver enzyme elevations.
What medications cause abnormal aminotransferase levels?
A host of medications can cause abnormal liver enzymes levels.
Pain relief medications such as:
ibuprofen (Advil, Motrin),
naproxen (Naprosyn, Naprelan, Anaprox, Aleve),
diclofenac (Voltaren, Cataflam, Voltaren-XR)), and
Anti-seizure medications such as:
valproic acid (Depakote, Depakote ER, Depakene, Depacon),
carbamazepine (Tegretol, Tegretol XR, Equertro), and
Antibiotics such as:
tetracyclines, [for example, tetracycline (Achromycin)]
isoniazid (INH) (Nydrazid, Laniazid)
trimethoprim (Trimpex; Proloprim, Primsol)
nitrofurantoin (Macrodantin; Furadantin; Macrobid),
fluconazole (Diflucan ) and some other anti-fungals, etc.
Cholesterol lowering drugs such as:
lovastatin (Mevacor, Altocor),
simvastatin (Zocor), and
Cardiovascular drugs such as:
quinidine (Quinaglute, Quinidex), etc.
Anti-depressant drugs of the tricyclic type
With drug-induced liver enzyme abnormalities, the enzymes usually normalize weeks to months after stopping the medications.
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