Alanine Aminotransferase (ALT) Test Principle: Coupled Enzymatic Assay Methodology and Applications

The alanine aminotransferase (ALT) test is one of the most widely performed liver function assays in clinical laboratories worldwide. This article explains the experimental principle of the ALT test in detail, including the IFCC-recommended coupled enzymatic reaction, NADH-dependent spectrophotometric detection at 340 nm, sample requirements, testing methods, and clinical applications. Understanding the alanine aminotransferase test principle is essential for laboratory professionals, clinicians, and researchers seeking accurate and reproducible enzyme activity measurements.

Technical Overview

Alanine aminotransferase (ALT, EC 2.6.1.2), historically referred to as serum glutamic-pyruvic transaminase (SGPT or GPT), is a cytosolic enzyme that catalyzes the reversible transfer of an amino group from L-alanine to alpha-ketoglutarate (2-oxoglutarate). ALT is present at high concentrations in the cytoplasm of liver hepatocytes, with lower amounts found in the kidney, heart, and skeletal muscle. When hepatocytes are injured or destroyed, ALT is rapidly released into the bloodstream, making serum ALT activity one of the most sensitive and specific biochemical markers of hepatocellular damage.

The modern ALT assay is based on a two-step coupled enzymatic reaction originally refined and standardized by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). The principle is as follows:

• Primary reaction (catalyzed by ALT): L-alanine + 2-oxoglutarate is converted to pyruvate + L-glutamate. The high concentration of L-alanine in the reagent drives the equilibrium toward pyruvate formation.
• Indicator reaction (catalyzed by lactate dehydrogenase, LDH): pyruvate + NADH + H+ is converted to L-lactate + NAD+.
• Because reduced nicotinamide adenine dinucleotide (NADH) strongly absorbs ultraviolet light at 340 nm while its oxidized form (NAD+) does not, the rate of decrease in absorbance at 340 nm is directly proportional to the catalytic concentration of ALT in the sample.

The analyzer monitors the absorbance change continuously during the linear phase of the reaction, typically after a lag phase of 60 to 90 seconds that allows endogenous pyruvate in the patient sample to be consumed by LDH before measurement begins. Many reagent formulations also include pyridoxal-5'-phosphate (P-5'-P), the essential coenzyme of ALT, to ensure full activation of the apoenzyme form and to improve comparability of results, particularly in samples from patients with marginal vitamin B6 status.

Testing Samples

Serum

Serum is the preferred specimen for the ALT test. Venous blood is collected into a plain tube without anticoagulant, allowed to clot for 20 to 30 minutes at room temperature, and then centrifuged to separate the serum. Hemolyzed samples should be avoided because erythrocytes contain ALT at concentrations several times higher than plasma, and hemolysis also releases endogenous pyruvate and LDH that can interfere with the coupled reaction and photometric reading.

Plasma

Plasma collected with lithium heparin or EDTA anticoagulants is acceptable on most automated chemistry analyzers. Anticoagulants such as citrate should generally be avoided because they dilute the sample volume and may affect enzyme activity measurement. Laboratories must verify specimen acceptability according to the instructions of the reagent manufacturer.

Sample Stability and Storage

ALT is relatively stable compared with many other serum enzymes. Whole blood should be processed promptly, and separated serum or plasma should be analyzed within 24 hours if kept at room temperature. The sample remains stable for approximately 3 to 7 days at 2 to 8 degrees Celsius and for several months when frozen at minus 20 degrees Celsius or below. Repeated freeze-thaw cycles should be avoided.

Interference Considerations

• Hemolysis: may cause falsely elevated results due to release of erythrocyte ALT and pyruvate.
• Icterus: high bilirubin levels can interfere with absorbance readings in the UV range; bichromatic analysis with a secondary wavelength helps correct this.
• Lipemia: turbidity scatters light and distorts absorbance measurement; ultracentrifugation or blanking may be required.
• Drugs and substances: certain medications and high concentrations of reducing agents may affect the NADH-dependent reaction.

Testing Items

The core testing item is the catalytic activity concentration of alanine aminotransferase in serum or plasma, expressed in units per liter (U/L) or microkatal per liter (ukat/L), where 1 U/L equals approximately 0.0167 ukat/L. Typical adult reference intervals are approximately 7 to 40 U/L, although exact limits vary by laboratory, method, sex, and population. The test is frequently reported together with aspartate aminotransferase (AST), alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), and bilirubin as part of a comprehensive liver panel. The AST to ALT ratio (De Ritis ratio) provides additional diagnostic information, with a ratio above 2 suggesting alcoholic liver disease.

Testing Methods

Continuous Monitoring (Kinetic UV) Method - IFCC Reference Method

The IFCC 2002 reference method is the international standard for ALT measurement and is implemented on virtually all automated clinical chemistry analyzers. The optimized reaction conditions include incubation at 37 degrees Celsius in Tris buffer at pH 7.15, L-alanine at 500 mmol/L, 2-oxoglutarate at 15 mmol/L, NADH at 0.18 mmol/L, excess LDH, and optional P-5'-P at 0.10 mmol/L. The procedure follows a defined sequence:

• Step 1: Mix sample with the reagent containing L-alanine, NADH, LDH, buffer, and P-5'-P.
• Step 2: Incubate during the lag phase so that endogenous pyruvate is consumed by LDH and the reaction reaches steady state.
• Step 3: Initiate the measuring reaction by adding 2-oxoglutarate, the substrate that starts ALT catalysis.
• Step 4: Record the decrease in absorbance at 340 nm every 15 to 30 seconds over the linear phase, commonly for 120 to 180 seconds.
• Step 5: Calculate the mean absorbance change per minute and convert it into enzyme activity.

Calculation of Enzyme Activity

ALT activity in U/L is calculated using the following relationship: activity equals the absorbance change per minute multiplied by the total reaction volume, divided by the product of the molar absorptivity of NADH at 340 nm (approximately 6.3 L per mmol per cm), the light path length (1 cm), and the sample volume, with results multiplied by 1000 to convert to units per liter. Because reagent volumes and instrument optics vary, laboratories rely on method-specific calibration factors and calibrators traceable to the IFCC reference measurement system, such as the certified reference material ERM-AD454.

Reitman-Frankel Colorimetric Method

Before UV kinetic methods became standard, ALT was measured by the classic Reitman-Frankel procedure. In this endpoint method, ALT converts L-alanine and alpha-ketoglutarate to pyruvate during fixed incubation, and the pyruvate formed reacts with 2,4-dinitrophenylhydrazine (DNPH) under alkaline conditions to produce a reddish-brown hydrazone that is read at approximately 505 to 530 nm. Activity is determined against a pyruvate standard curve. Although simple and inexpensive, this method suffers from poorer precision, subjective endpoint timing, toxic hydrazine chemistry, and interference from endogenous keto acids, so it has largely been replaced by the kinetic UV method.

Dry Chemistry and Point-of-Care Methods

Dry-slide and portable analyzers employ the same coupled ALT-LDH-NADH chemistry in multilayer film or cartridge formats, enabling rapid testing in physician offices, blood donation centers, field screening programs, and resource-limited settings. Results correlate well with wet chemistry methods when calibrated appropriately.

Quality Control and Calibration

• Run at least two levels of quality control materials (normal and elevated) each day and after calibration or reagent lot change.
• Verify the reaction is within the linear measuring range; dilute and re-run samples exceeding the upper limit.
• Participate in external quality assessment schemes to ensure inter-laboratory comparability.
• Monitor pre-analytical variables including sample integrity, clotting time, and storage conditions.

Applications

• Clinical diagnosis: detection and grading of acute viral hepatitis, chronic hepatitis B and C, autoimmune hepatitis, non-alcoholic fatty liver disease (NAFLD/NASH), alcoholic liver disease, cirrhosis, and ischemic or toxic hepatic injury.
• Therapeutic drug monitoring: surveillance of hepatotoxic drugs such as isoniazid, methotrexate, statins, acetaminophen overdose, antiepileptics, and herbal or dietary supplements.
• Pharmaceutical research: assessment of drug-induced liver injury (DILI) in preclinical toxicology studies and clinical trials, including Hy's law evaluation.
• Blood transfusion screening: routine ALT testing of donated blood as a surrogate marker of liver inflammation in many countries.
• Public health and occupational health: liver function screening in health checkup programs and epidemiological surveys of liver disease prevalence.
• Veterinary medicine: evaluation of hepatic disease in companion animals and livestock.
• Basic research: studies of hepatocyte damage, oxidative stress, and liver regeneration in cell and animal models.

FAQ

What causes elevated ALT levels?

Elevated serum ALT most commonly reflects hepatocellular injury from viral hepatitis, fatty liver disease, alcohol consumption, medication or herbal toxicity, obesity, or intense physical exercise. Marked elevations above 10 times the upper reference limit suggest acute hepatitis, severe drug injury, or ischemic hepatopathy, while mild persistent elevations often indicate chronic liver conditions requiring further workup.

Why is LDH added to the ALT reagent?

LDH serves as the indicator enzyme. It rapidly converts the pyruvate generated by ALT into lactate while oxidizing NADH to NAD+. This coupling step converts the otherwise invisible transamination reaction into a measurable change in UV absorbance, allowing the reaction rate to be tracked photometrically at 340 nm.

Does hemolysis affect the ALT result?

Yes. Hemolysis releases ALT and pyruvate from erythrocytes into the surrounding plasma, which can produce falsely elevated activity and distort the kinetic reading. Severely hemolyzed specimens should be rejected and recollected whenever possible.

Is fasting required before an ALT test?

Fasting is not strictly required for the enzyme assay itself, but fasting is often recommended because recent meals, especially high-fat or high-carbohydrate intake, can transiently alter liver enzyme levels and related analytes measured in the same panel, such as glucose and lipids.

What is the difference between ALT and AST?

Both are aminotransferases released during cell injury, but ALT is located mainly in the liver cytoplasm and is more liver-specific, whereas AST is found in liver mitochondria as well as heart, skeletal muscle, kidney, and brain. Consequently, isolated ALT elevation points strongly to hepatic origin, while combined elevation with a high AST/ALT ratio may indicate alcoholic liver disease, muscle injury, or cardiac damage.

Why is pyridoxal-5'-phosphate included in some reagent systems?

P-5'-P is the active coenzyme form of vitamin B6 and is required for maximal ALT catalytic activity. Some patients, including those with malnutrition, alcohol dependence, or uremia, have apo-ALT that is not fully saturated with the coenzyme. Supplementing P-5'-P standardizes and increases measured activity, improving the accuracy and comparability of results across populations.