Abnormal liver function tests: a guide to interpretation

KEY POINTS

• Evaluating a patient with abnormal liver function tests should take into account simple clinical characteristics such as dietary habits, use of drugs or herbal remedies, exposure to environmental toxins, history of blood transfusion or other risk factors for parenteral exposure, and recent travel to areas endemic for enterically transmitted hepatitis viruses
• Because the liver tends to respond uniformly to injury, the type of alteration in liver function tests seldom helps you to make a diagnosis. Therefore, you often need to carry out specific diagnostic testing
• A simple, first line screening for the most frequent conditions associated with abnormal liver function tests (viruses, autoimmune and metabolic diseases) will help you to identify the cause in most patients

CLINICAL TIPS

• You may find abnormal liver function tests in up to 17.5% of the asymptomatic general population†, although in 58% of these patients they are not adequately followed up
• Evaluate both the clinical context in which abnormal liver function tests are found and the degree of alteration (mild versus severe)
• Suggest liver ultrasonography to evaluate signs of advanced liver disease, fatty infiltration of the liver, and obstacles to bile flow
• Liver biopsy is the only way of accurately staging liver fibrosis, although it is sometimes unnecessary when obvious clinical or instrumental signs of cirrhosis are evident
• Transient elastography - an instrumental technique able to assess hepatic stiffness - may be proposed in patients with a definite cause of liver disease so as to non-invasively evaluate the stage of the disease
• Although some biochemical scores can help to stage liver disease non-invasively and more sophisticated biochemical markers of liver fibrosis are available, they have not been validated in clinical practice and their practical role has not yet been defined

†This is, in part, because the “normal ranges” for aminotransferases reported by most laboratories are adjusted for gender and age, despite evidence that these factors have an effect (for example, ranges tend to be lower in women).



LIVER FUNCTION TESTS
Abnormal liver function tests can suggest:

• Hepatocellular injury
• Impaired bile flow
• Damaged bile ducts

Abnormal liver function tests can sometimes be associated with extrahepatic diseases and physiological conditions (for example, elevated alkaline phosphatase in pregnancy).

A normal liver function test does not necessarily exclude liver disease. Blood levels of the following enzymes usually reflect the integrity of hepatocytes or cholestasis (not “liver function” in isolation):

• Alanine and aspartate aminotransferases (ALT/ASP)
• γ-glutamyltransferase (GGT)
• Alkaline phosphatase (ALP)

The following tests may provide useful information on how well the liver is functioning:

• Serum albumin
• Bilirubin
• Prothrombin time

However, none of these is specific for liver disease.


Aminotransferases

• Alanine aminotransferase (ALT) is considered more specific than aspartate aminotransferase (AST) for hepatocellular injury because it is predominantly represented in the liver, with low concentrations in other tissues. 
• AST is also expressed in the heart, skeletal muscle, and red blood cells. 
• AST and ALT are both intracellular enzymes, so their presence in the bloodstream can be an indicator of hepatocellular injury (not necessarily hepatocellular death).

Alkaline phosphatase

• Both liver and bone diseases can cause a pathological elevation of serum alkaline phosphatase (ALP), although increased ALP may originate from the placenta, kidney, intestine, and leukocytes. 
• Increased serum ALP occurs in some physiological conditions, such as adolescence (bone origin) and pregnancy (placenta origin).

γ-glutamyltransferase

• The enzyme γ-glutamyltransferase (GGT) is present in renal tubules, the liver, pancreas, and intestine. 
• Although intracellular GGT levels are higher in organs other than the liver, GGT activity in the serum comes mainly from the liver. 
• GGT is contained in the microsomes, and drugs such as anticonvulsants and oral contraceptives can induce its activity.

Both ALP and GGT are synthesized and released by cells lining the bile ducts, and they can be released into the bloodstream by cholestasis and the accumulation of bile salts. For this reason, ALP and GGT can act as markers of cholestasis.


EPIDEMIOLOGY AND EVALUATION
It is difficult to estimate the prevalence of altered liver function tests in the general population. The prevalence of abnormal liver function tests can depend on the degree of abnormality that is considered significant and on the methods used to assess it.

Altered liver function tests are increasingly encountered in clinical practice. This is mainly because measurement of aminotransferase and hepatic markers of cholestasis (ALP and GGT) is included in the screening of blood donors and hospitalisation due to non-hepatic conditions.

Measurement of liver function tests as part of screening procedures, such as periodic “health check ups” or insurance physicals, can lead to problems where asymptomatic people are found to have a mildly altered liver biochemistry. You should therefore be cautious about ordering tests in apparently healthy patients, and discuss the possibility of false positive results with the patient as part of obtaining their consent to the test. However, if a test shows abnormal liver function, you should follow it up as outlined in this module as some liver diseases (such as hepatitis C) can present with minimal symptoms and fluctuating aminotransferase results.

In these patients, identifying the cause of biochemical abnormalities can be challenging. Furthermore, with diagnostic tests being readily available and with the costs associated with serological, radiological, and pathological evaluations, you should individualise the decision as to whether and how to proceed with further testing, and also follow a series of pragmatic steps:

1. Evaluate the clinical context

• The first step is to evaluate the clinical context in which abnormal tests are found. In particular, whether the observed biochemical abnormality is associated with a history suggestive of liver disease and/or symptoms and signs of liver disease, or with the presence of extrahepatic conditions that may involve the liver. 
• This information can help to establish a probability of a specific diagnosis, as well as reduce the number of tests and the amount of time needed to make a diagnosis.

2. Identify the type of injury

• The second step is to identify whether the injury is predominantly hepatocellular or cholestatic. 
• Consider the degree of elevation of liver function tests and the rate and characteristics of the increase or decrease over time.
• Although in clinical practice the pattern of altered liver function tests may sometimes display a mixed picture (usually markedly elevated aminotransferases with mild elevation of the markers of cholestasis), and therefore the patient will need to be fully investigated, the above simplification may help you to direct the diagnostic work up and focus on certain diseases.

3. Identify the disease 

• The third step is to identify the disease responsible for the biochemical abnormality. 
• It is important to take into account the prevalence of specific illnesses in the population and the circumstances in which the alteration has been found. 
• Identifying the cause of the altered enzyme levels often needs specific biochemical tests and virological and immunological assays, and sometimes genetic testing.

4. Determine the severity of the liver injury

• The fourth step is to determine the severity of liver injury and how useful the final diagnosis is in terms of treatment and prognosis. 
• This can be done invasively via a liver biopsy or by non-invasive biochemical tests, clinical scores, or using transient elastography.

RISK FACTORS
The following factors in the patient’s history can help you to make a diagnosis:

Age

• Younger patients are more likely to have diseases that usually manifest early in life (such as Wilson’s disease).

Sex

• The female to male ratio of autoimmune liver diseases is 4:1.

Ethnicity

• Hereditary haemochromatosis is more common in people of northern European descent, while it is rare in Africans.

History of travel

• In patients who have travelled to developing countries, you should think of viral hepatitis or other tropical diseases.

Use of alcohol, drugs, and herbal remedies

• Alcoholic liver disease is one of the most common causes of altered liver function tests in Western countries. 
• But diagnosing alcohol abuse may be difficult because many patients under-report their alcohol use. 
• Furthermore, the limit of safe alcohol use in patients with other diseases involving the liver, such as non-alcoholic fatty liver disease, is not known.
• A list of drugs, over the counter preparations, and herbal remedies taken by the patient should be obtained. 
• Herbal remedies are often overlooked because they are considered harmless. 
• However, they may frequently be associated with hepatotoxicity.
• The altered tests should be repeated after appropriate withdrawal of alcohol and medications.

Blood transfusion and major surgery

• A history of blood transfusions should trigger to check serology for hepatitis B and C virus infections.

DIAGNOSIS
Abnormal liver function tests may show two (sometimes overlapping) patterns:

• Hepatocellular predominant
• increases mainly in aminotransferases (AST and ALT) with or without a lesser increase in hepatic markers of cholestasis
• Cholestatic predominant
• increases mainly in markers of cholestasis (ALP and GGT) with or without a lesser increase in aminotransferases.

Some diseases may show a mixed picture, although there is often a predominant hepatocellular or cholestatic pattern.

Hepatocellular predominant
The degree of abnormality of aminotransferases may be useful for narrowing the differential diagnosis. This is especially true when high levels of aminotransferases are observed, and should prompt further evaluation without delay.

Because minimal or mild abnormality in aminotransferases is the most common biochemical alteration, the test should be repeated first to rule out laboratory error, although a second normal result does not necessarily exclude the presence of disease. For example, patients infected by hepatitis C virus characteristically show a fluctuating pattern from normal to mildly abnormal aminotransferases. In these patients, simply repeating the test without further investigation may not reveal an important disease. You should therefore start a first line, clinical screening for the most prevalent causes of chronic liver disease and repeat the test (see figure).

Figure: Tests for viral hepatitis [source]

This will identify the responsible agent in many patients. Extrahepatic causes of abnormal aminotransferases (especially in patients with an isolated raised AST) should be ruled out by considering the clinical context of abnormal enzyme levels.

Although there is no universal definition of mild, moderate, and severe abnormality of aminotransferases, the degree of abnormality can practically considered to be:

• Mild when the enzyme level is less than five times the upper reference value
• Severe when AST and ALT are greater than 10 to 15 times the upper reference value.

Although not the rule, chronic liver diseases are often characterised by a mild abnormality of aminotransferases, whereas a severe abnormality is usually encountered in patients with acute hepatic injury.

Table 1: Causes of mild and severe abnormality of aminotransferases
Mild (up to five times the upper reference value)	Severe (10 to 15 times the upper reference value)
More common	More common
Alcohol	Acute viral hepatitis (A-E)
Chronic viral hepatitis (B and C)	Ischaemic hepatitis
Haemochromatosis	Medications and toxins
Autoimmune hepatitis	Autoimmune hepatitis
Steatosis and steatohepatitis
Less common	Less common
Medications and toxins	Wilson’s disease
Wilson’s disease	Acute bile duct obstruction
α1-antitrypsin deficiency	Acute viral hepatitis (cytomegalovirus and Epstein-Barr virus)

Mild abnormality
Alcoholic liver disease

This is one of the most common reasons for mildly abnormal aminotransferases in the Western world. The quantity and duration of alcohol use are important for establishing a diagnosis. A clue to the diagnosis is that the AST/ALT ratio in these patients is typically 2:1 or more.15

When a patient’s history is not reliable, increased GGT with normal ALP and macrocytosis make alcohol induced liver damage more likely.16
Chronic viral hepatitis

Chronic viral hepatitis is one of the most common causes of mild abnormality of aminotransferases worldwide. Infection by hepatitis C and B viruses has a high prevalence, with 170 million and 350 million infected people in the world respectively. Positive antibodies to hepatitis C virus in a patient with a history of risk factors for infection (blood transfusion, exposure to dirty needles, or intravenous drug use) and mildly abnormal aminotransferases strongly suggest current hepatitis C virus infection, although patients may lack or under-report risk factors for infection. Detection of hepatitis C virus RNA in serum by polymerase chain reaction technique is used to confirm active viral infection and replication. A negative polymerase chain reaction in patients positive for anti-hepatitis C virus antibodies needs to be repeated in three months to avoid false negatives. If aminotransferases abnormality persists despite negative viraemia, you must rule out other causes of liver injury.17

Hepatitis B virus infection can be acquired during adulthood by sexual transmission and injecting drugs, or vertically at birth. The presence of hepatitis B virus surface antigen (HBsAg), hepatitis B virus e antigen (HBeAg) and antibody (anti-HBeAb), and hepatitis B virus core antibodies (IgG and IgM anti-HBc) are the principal markers that help define the status of people infected with hepatitis B virus (see table 2).