What are the normal liver function test (LFT) values and how should they be interpreted in a healthy adult without liver disease?

Normal Liver Function Test Values and Interpretation

In healthy adults without liver disease, normal ALT values are 29-33 IU/L in men and 19-25 IU/L in women, though it's critical to understand that normal LFTs do not exclude liver disease, as liver disease develops silently and tests may remain normal until complications of liver failure or portal hypertension develop. 1, 2

Standard LFT Panel Components

A comprehensive initial liver function test panel should include the following markers 1, 2:

• Bilirubin (total, direct, and indirect) - measures bile metabolism and excretion 1
• Albumin - reflects hepatic synthetic function 1, 2
• ALT (Alanine Aminotransferase) - most specific marker for hepatocellular injury with normal values of 29-33 IU/L in men and 19-25 IU/L in women 1, 2
• ALP (Alkaline Phosphatase) - elevated in cholestatic liver disease 1, 2
• GGT (Gamma-Glutamyl Transpeptidase) - confirms hepatic origin of elevated ALP 1, 2
• Full blood count if not performed within the previous 12 months 1

Normal Reference Values

The key normal values for healthy adults are 1, 2:

• ALT: 29-33 IU/L (men), 19-25 IU/L (women)
• AST: Similar range to ALT, though less liver-specific
• AST:ALT ratio: <1 in healthy individuals (ratio >1 suggests advanced fibrosis/cirrhosis even when both values are within normal range) 1

Critical Interpretation Principles

Pattern Recognition Over Absolute Values

The pattern of abnormality matters more than individual values 2:

• Hepatocellular pattern: Predominant elevation of aminotransferases (ALT/AST) >> ALP 1, 2
• Cholestatic pattern: Predominant elevation of ALP with or without elevated bilirubin 1, 2

The Paradox of "Normal" LFTs

This is the most important clinical pitfall: Approximately 30% of routine LFTs show at least one abnormal value outside the reference range, yet only 3.9% of patients with abnormal ALT/AST are diagnosed with significant liver disease within 5 years 2. Conversely, and more dangerously, normal LFTs do not exclude significant liver disease 2, 3:

• 50% of patients with chronic hepatitis C who are both anti-HCV and PCR positive have completely normal transaminase values 3
• Many patients with significant liver fibrosis may have liver enzymes in the normal reference range 3
• Patients continue to present with undiagnosed end-stage liver disease that might have been preventable by earlier diagnosis 3

Synthetic Function Assessment

True liver function (as opposed to hepatocellular injury markers) requires measurement of 2:

• Albumin - reflects synthetic capacity
• INR/Prothrombin time - reflects coagulation factor synthesis
• Platelet count - may indicate portal hypertension

Fibrosis Assessment in "Normal" LFTs

Even when LFTs appear normal, the AST:ALT ratio retains utility 1:

• AST:ALT ratio >1 indicates advanced fibrosis/cirrhosis even if both values are within the normal reference interval 1
• This ratio should be calculated reflexively when any abnormality is detected 1

Common Pitfalls to Avoid

Do Not Assume Normal LFTs Exclude Disease

The most dangerous assumption is that normal liver tests mean no liver disease 2, 3:

• Liver disease develops silently with no signs or symptoms 1, 2
• Tests may be normal until complications of liver failure or portal hypertension develop 2
• A patient with hepatitis C with normal ALT can progress to end-stage liver disease if undiagnosed 3

Recognize High Intraindividual Variability

More than 30% of adults with elevated AST, ALT, or bilirubin levels would be reclassified as normal if retested 4:

• 38% of adults with initially elevated bilirubin had normal levels at repeat testing (mean 17.5 days apart) 4
• 36% and 31% for AST and ALT respectively 4
• Clinicians should retest asymptomatic individuals with abnormal results before extensive workup 4

Consider Extrahepatic Causes

Elevated transaminases don't always indicate liver disease 5:

• Strenuous exercise can cause muscle damage-induced transaminitis 5
• AST is present in heart, skeletal muscle, kidneys, brain, and red blood cells 1, 2
• ALT is more specific for liver injury due to low concentrations in other tissues 1, 2

Test Based on Risk Factors, Not LFT Results

For high-risk populations (injecting drug users, migrants from high-prevalence areas, prisoners), test for viral hepatitis regardless of LFT values 3:

• Routine liver tests to screen for chronic HCV infection are of limited value because about 50% of infected patients have normal transaminases 3
• Do not wait for LFTs to become abnormal before testing high-risk patients 3

When to Check LFTs in Healthy Adults

Despite normal baseline health, LFTs should be considered in 1:

• Non-specific symptoms: Fatigue, nausea, or anorexia (75% of patients with autoimmune hepatitis have these symptoms) 1
• High-risk groups: Even when asymptomatic, based on risk factors rather than symptoms 3
• Medication monitoring: For drugs with known hepatotoxic potential 1