Alanine Aminotransferase (ALT) Activity Assay: Principles, Methods, and Applications

The alanine aminotransferase (ALT) activity assay is one of the most widely performed enzymatic tests in clinical chemistry, serving as a sensitive and specific biomarker of hepatocellular injury. This technical article reviews the biochemical principles, sample requirements, testing items, analytical methods, and practical applications of ALT activity measurement. Whether used for routine liver function panels, drug safety monitoring, or preclinical research, a clear understanding of ALT assay methodology is essential for accurate interpretation of results.

Technical Overview

Alanine aminotransferase (ALT, EC 2.6.1.2), formerly known as serum glutamic pyruvic transaminase (SGPT) or glutamate pyruvate transaminase (GPT), is a pyridoxal-5'-phosphate-dependent enzyme that catalyzes the reversible transfer of an amino group from L-alanine to alpha-ketoglutarate, producing pyruvate and L-glutamate. ALT is present at its highest concentration in the cytosol of hepatocytes, with lower levels found in the kidney, heart, and skeletal muscle.

When hepatocytes are damaged or destroyed, ALT is rapidly released into the bloodstream, making serum ALT activity a highly sensitive and relatively liver-specific indicator of hepatocellular injury. Enzyme activity is conventionally expressed in international units per liter (U/L), where one unit corresponds to the conversion of one micromole of substrate per minute under defined conditions. Typical adult reference intervals are approximately 9–50 U/L for males and 7–35 U/L for females when measured by the IFCC method at 37 °C, although laboratories should establish or verify their own ranges.

The clinical interpretation of ALT results is often enhanced by parallel measurement of aspartate aminotransferase (AST). The AST/ALT ratio (De Ritis ratio) provides diagnostic clues: a ratio above 2 suggests alcoholic liver disease or cirrhosis, while a ratio below 1 is typical of viral hepatitis and non-alcoholic fatty liver disease.

Testing Samples

Serum

Serum is the preferred sample type for routine ALT determination. Venous blood is collected into a plain tube, allowed to clot for 20–30 minutes at room temperature, and centrifuged to obtain a clear, hemolysis-free serum specimen. ALT is reasonably stable in serum, remaining clinically valid for approximately 24–48 hours at room temperature and for several days at 2–8 °C. For longer storage, samples should be frozen at −20 °C or below, and repeated freeze–thaw cycles should be avoided.

Plasma

Heparinized plasma is acceptable on most automated analyzers and produces results comparable to serum. EDTA plasma may also be used, but samples must be separated from cells promptly. Hemolyzed specimens should be rejected whenever possible, since erythrocyte contents and released NADH-interfering substances can artificially elevate results. Marked lipemia can interfere with photometric readings at 340 nm and may require ultracentrifugation or sample dilution.

Tissue Homogenates and Cell Lysates

In preclinical and cell biology studies, ALT activity is measured in liver tissue homogenates, primary hepatocyte cultures, or cell culture supernatants. Tissues are typically homogenized in cold isotonic or Tris-based buffer, centrifuged to remove debris, and the resulting supernatant is assayed. Results are commonly reported as specific activity (U/mg protein) after protein quantification by the BCA, Lowry, or Bradford method, enabling normalization across samples.

Other Biological Samples

ALT activity can occasionally be determined in other matrices, including cerebrospinal fluid, peritoneal or pleural effusions, bile, and fermentation broths in industrial microbiology. Matrix-specific validation, including linearity, precision, and recovery studies, is required before reporting results from non-standard sample types.

Testing Items

Core testing items for the ALT activity assay include:

• ALT catalytic activity in serum or plasma (U/L), the primary clinical test item.
• ALT specific activity in tissue homogenates or cell lysates (U/mg protein), used in research and toxicology.
• AST activity and the calculated AST/ALT ratio for differential diagnosis.
• Pyridoxal-5'-phosphate-stimulated ALT activity, to detect underestimation caused by vitamin B6 depletion in patients with severe liver disease or renal failure.
• Associated liver panel items frequently requested together with ALT, including AST, alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), total bilirubin, albumin, and total protein.

Testing Methods

IFCC Kinetic UV Method (Reference Method)

The internationally recommended procedure is the IFCC (International Federation of Clinical Chemistry and Laboratory Medicine) reference method performed at 37 °C. The assay couples the ALT reaction to the lactate dehydrogenase (LDH) indicator reaction. First, ALT converts L-alanine and alpha-ketoglutarate to pyruvate and L-glutamate. Pyruvate is then reduced by NADH in the presence of LDH to form lactate and NAD+. The rate of NADH oxidation, monitored as a decrease in absorbance at 340 nm, is directly proportional to ALT activity.

A two-step protocol is typically employed: serum is pre-incubated with L-alanine, NADH, LDH, pyridoxal-5'-phosphate, and Tris buffer (pH 7.15) to consume endogenous pyruvate, and the reaction is initiated by adding alpha-ketoglutarate. The change in absorbance per minute is measured during the linear phase, and activity is calculated using the molar absorptivity of NADH (approximately 6.22–6.3 L·mmol⁻¹·cm⁻¹), the path length, and the sample-to-total volume ratio.

Colorimetric Endpoint Method (Reitman–Frankel)

The classical Reitman–Frankel method uses 2,4-dinitrophenylhydrazine (DNPH) to derivatize the pyruvate formed by ALT, generating a colored hydrazone that is measured spectrophotometrically around 505–540 nm. Pyruvate standards are used to construct a calibration curve. This endpoint method is inexpensive and suitable for laboratories without UV-capable analyzers, but it offers lower analytical sensitivity and greater susceptibility to interference than the kinetic UV method.

Dry Chemistry and Point-of-Care Testing

Multi-layered dry chemistry slides and portable analyzers employ the same coupled LDH–NADH principle in a solid-phase format, with reflectance photometry replacing transmission measurement. These systems deliver rapid turnaround times and are useful in blood donor screening stations, ambulatory clinics, and resource-limited settings, provided that method-specific reference intervals are applied.

Quality Control and Method Validation

Calibrators and control materials should be traceable to the IFCC reference measurement procedure. Laboratories should run at least two levels of quality control per analytical batch, monitor performance using Westgard rules or equivalent statistical criteria, and participate in external quality assessment schemes. Key performance characteristics to verify include precision (within-run and between-run coefficient of variation, typically below 5% for the UV method), linear range, analytical sensitivity, and interference from hemolysis, icterus, and lipemia.

Applications

ALT activity testing supports a broad range of clinical, pharmaceutical, and research applications:

• Clinical hepatology: detection and monitoring of acute and chronic hepatitis, non-alcoholic fatty liver disease (NAFLD/NASH), alcoholic liver disease, autoimmune hepatitis, cirrhosis, and ischemic hepatitis.
• Drug safety monitoring: surveillance for drug-induced liver injury (DILI), a critical endpoint in clinical trials and post-marketing pharmacovigilance, where ALT elevation above three times the upper limit of normal combined with bilirubin changes is a recognized safety signal.
• Blood donor screening: ALT testing is mandated in many countries as a surrogate marker for transfusion-transmissible hepatitis and as part of donor health assessment.
• Preclinical toxicology: ALT measurement in experimental animals is a standard component of safety evaluation for new chemical entities, agrochemicals, and industrial compounds.
• Occupational and public health: monitoring workers exposed to hepatotoxic solvents and individuals with metabolic risk factors.
• Basic research: studies of hepatic metabolism, oxidative stress, nitrogen metabolism in plants and microorganisms, and evaluation of hepatoprotective compounds in cell and animal models.

FAQ

Frequently asked questions regarding the alanine aminotransferase activity assay are summarized below:

• Is fasting required before ALT testing? Fasting is not strictly required, but patients should avoid alcohol consumption and strenuous physical exercise for at least 24 hours before sampling, as both can transiently elevate ALT activity.
• What does an elevated ALT result mean? Elevated ALT indicates hepatocellular damage and warrants further evaluation for viral hepatitis, fatty liver disease, alcohol-related injury, medications, or less commonly cardiac and skeletal muscle injury.
• Why is hemolysis a concern? Hemolysis releases intracellular components that interfere with the coupled NADH reaction and can falsely increase ALT activity, so hemolyzed samples should be recollected whenever feasible.
• Can ALT be falsely low? Yes. Vitamin B6 (pyridoxine) deficiency, chronic renal failure, and advanced cirrhosis can reduce measurable ALT activity; supplementation of the assay with pyridoxal-5'-phosphate mitigates this problem.
• How quickly are results available? On automated chemistry analyzers, ALT results are typically reported within 30–60 minutes of sample receipt.
• How often should ALT be monitored? In patients on hepatotoxic therapy or with known liver disease, ALT is commonly checked at baseline and at intervals of one to six months depending on the risk profile of the intervention and regulatory guidance.