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The History of ALT

Alanine aminotransferase was originally known as serum glutamic-pyruvic transaminase, or SGPT, before laboratory naming conventions shifted toward more systematic enzyme names decades later. Its clinical story really begins in 1955, when researchers Arthur Karmen, Felix Wroblewski, and John LaDue at New York University discovered that transaminase enzymes leaked into the bloodstream in large amounts after a heart attack, publishing landmark findings that transformed how doctors detected tissue damage. Their initial focus was on a related enzyme's role in diagnosing heart injury, but it quickly became clear that ALT specifically tracked liver damage far more precisely, cementing it as one of the very first blood tests capable of detecting a failing liver before a patient showed any outward symptoms. Within a few years, transaminase testing had become a routine part of hospital laboratory panels worldwide.

What ALT Actually Is

ALT is an enzyme concentrated overwhelmingly in liver cells, where it helps the body metabolize amino acids by transferring a chemical group from alanine onto another molecule, producing byproducts used in energy production and glucose regulation. Under normal conditions, only small amounts circulate in the blood, but when liver cells are damaged or die — from infection, alcohol, medication toxicity, or fatty liver disease — ALT leaks out in much larger quantities, making it one of the most sensitive available markers of liver cell injury. Unlike some other liver-related enzymes, ALT is found in comparatively small amounts elsewhere in the body, which is exactly what makes it such a specific and trusted signal that a problem originates in the liver itself rather than in muscle, heart, or other tissue.

How ALT Is Measured

Modern labs measure ALT using a kinetic enzymatic assay that tracks a related chemical reaction rather than ALT itself directly: a coupled reaction converts one of ALT's byproducts using a second enzyme, consuming a light-absorbing compound called NADH in the process, and the rate at which that compound disappears — tracked by a spectrophotometer — is directly proportional to how much ALT is present. This method was standardized by the International Federation of Clinical Chemistry to ensure results are comparable across different machines and laboratories worldwide, a major improvement over the inconsistent early techniques used in the 1950s and 1960s, when results from one hospital often couldn't be reliably compared to another.

Scientists Behind the Science

Arthur Karmen developed the specific spectrophotometric technique that made rapid, precise transaminase measurement possible, and early results were even reported in "Karmen units" named in his honor before standardized international units took over. Felix Wroblewski and John LaDue's parallel research established the broader clinical value of measuring transaminase enzymes in the blood, work that directly launched an entirely new category of diagnostic blood testing. Their combined findings, published in the mid-1950s in leading medical journals, didn't just introduce a single new test — they established the underlying principle, still used today, that damaged organs release detectable enzymes into the bloodstream, a concept that now underlies dozens of other diagnostic blood tests used throughout modern medicine and hospital laboratories everywhere.