Liver Function Test (LFTs)
As a primary care clinician, I see liver enzymes as part of routine lab work upwards of 30 times per day. Sometimes they are normal, sometimes they are abnormal. But each time, I must take a close look at the patient’s LFTs and determine if any action needs to be taken. As part of today’s article, we will look at the LFT test normal range, help you gain comfortability with the approach to liver function test interpretation from all standpoints, and make sure that LFTs are explained thoroughly.
Liver function tests can be confusing for students and we get quite a bit of requests for cases to review liver enzymes or questions about a specific type of liver enzyme. As a new clinician, it was confusing for me, especially because I was never sure when I should be worried.
Basic Interpretation of LFT’s
How high can the AST and ALT go? What does an abnormal PT/INR in a patient with liver disease mean? Today, we will start with the basic interpretation and give some guidelines as to what it means when levels increase.
Elevated Liver Enzymes… The First Sign Something Is Going Wrong
So why do we care if liver enzymes are elevated, after all, does this tell us specifically tell us WHY or WHAT is causing the abnormality? In many cases, elevated liver enzymes can be one of the first signs that something is starting to go wrong. In addition, once we know that something is wrong, we can trend liver enzymes to determine if things are getting better or worse. In the case of the MELD score and the Child-Pugh score, this is exactly the case and allows us to better risk stratify our patients.
What are the common liver enzymes?
We will start with the most commonly known and associated, AST and ALT. These are the most commonly seen enzymes, as they are noted in a complete metabolic panel.
When elevated, these can indicate liver injury or liver inflammation. They are usually higher at baseline in men and can actually be falsely elevated by erythromycin and Lasix. ALT is generally more specific for the liver because AST can be released from the liver, heart, bones, kidney, brain, and more. (Friedman, 2020)
Chronic Elevation of AST/ALT
If we see chronically elevated AST and ALT, there are numerous causes that might contribute. In our practice, we see:
- NAFLD (non-alcoholic fatty liver disease)
- Alcohol use
- Excess Tylenol Use
and many more. The differential is very broad.
Normal vs. Pathological Values
Depending on the lab, “normal” values may be different, but it is generally accepted that under 40 IU is a good metric as normal. In cases where the liver enzymes are extremely elevated, up to 8 or 10 times above normal, we consider diagnoses such as:
- Ischemic hepatitis
- Viral hepatitis
- Advanced cirrhosis
- Tylenol overdose
and many more (Friedman, 2020).
What do lab tests tell us?
As part of a complete metabolic panel, the alkaline phosphatase (ALK PHOS) is also measured. This enzyme is also associated with the liver, but is far less specific than AST and ALT. It can also be related to the bone or intestinal tract. It is also classically elevated in adolescents who are growing due to turnover of bone.
Elevated ALK PHOS - Cholestasis
An elevated ALK PHOS indicates cholestasis. Interestingly, levels may also double during pregnancy. This is non-specific and can be related to the following:
- liver disease
- viral illness
- chronic cirrhosis
- or CHF.
The normal range is roughly between 30-100; again depending on the lab.
Elevated Bilirubin - Cholestasis
Elevated bilirubin can also be an indicator of cholestasis. In cases where bilirubin is elevated, we must recall that there is a very common benign cause of elevated Bilirubin, which is known as Gilbert’s disease. In the absence of any other elevated liver enzymes or symptoms, this is one of the primary diagnoses to consider. An isolated elevated bilirubin can be related to the above. Depending on the lab, a normal bilirubin could be from 0.3 to 1.2, but it varies.
Lactate Dehydrogenase (LDH)
Another enzyme that is very non-specific and not typically always associated with the liver is that of lactate dehydrogenase. Although you may have a hard time discerning the organ that is causing LDH to be elevated; it does have one very unique pearl.
In certain cases, when other liver enzymes are elevated and the origin is unclear, an ALT to LDH ratio < 1.5 indicates ischemic hepatitis. At the same time, a ratio of ALT to LDH > 1.5 is suggestive of viral hepatitis. (Friedman, 2020). Who would have known? Not me. One range of normal LDH was 140-280.
Another sometimes helpful lab test is the GGT or gamma glutamyl-transferase, an enzyme that is also non-specific and related to many other tissues. It can be elevated with diseases of the liver, biliary tract, or pancreas.
Elevated AST/ALT; ALK PHOS and NORMAL GGT
In the case where a patient has elevated liver enzymes as well as ALK PHOS, GGT is not elevated in a patient with bone disease. This could potentially be a helpful way for clinicians to narrow down the differential.
Isolated Elevation of GGT
GGT is also helpful for alcohol use. In the absence of any other significant issues, an isolated elevated GGT can be an indicator of significant alcohol use.
Albumin is an additional test that is sometimes talked about but never really seems to be discussed as part of patient overall health. In many cases, normal albumin is seen and does not really help us too much. It can be normal in:
- viral hepatitis
- drug induced hepatitis
- obstructive jaundice.
A low albumin level can be related to cirrhosis or decreased nutritional status.
A high albumin level can be due to decreased total volume or high protein intake.
Using Liver Tests to Predict Patient Outcome
As disease progresses, unfortunately liver enzymes change and may become more severely altered. In some cases, intrinsic tests of the liver may start to be abnormal as well. We may start to see the patient’s PT/INR changing.
Increased PT/INR; MELD and Child-Pugh Scores
An increased PT can also indicate vitamin K deficiency. In many cases, there are tests that are used to help determine the prognosis or mortality in cirrhosis or chronic liver disease. The MELD Score takes into account both elevated bilirubin as well as PT/INR and if they are elevated, predicts worse mortality risk for patients. The Child Pugh score is very similar. It should be noted that according to Shah (2020), the INR is a poor predictor of overall bleeding risk.
I hope that today’s review has been helpful in regards to a basic overview of the liver enzymes that can help you clue in to a patient’s diagnosis. We’ve kept it basic for a reason! You need to understand the foundation before you can delve into the myriad of other conditions that can influence the elevation of liver enzymes. As always, study hard fellow Medgeeks.
Friedman, L. (2020). Liver Biochemical Tests that detect injury to Hepatocytes. UpToDate, date of access 18-20 March 2020.
Friedman, L. (2020). Enzymatic measures of Cholelithiasis. UpToDate. Date of access 18-20 March 2020.
Shah, N and Intagliata, N (2020). Hemostatic abnormalities in patients with liver disease. UptoDate, Date of access 18-20 March 2020.
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