Low ALT (Alanine Aminotransferase) is defined as a condition where blood ALT levels fall below the reference value of <41 U/L. This is a rare occurrence and is typically not associated with any significant health problem. However, abnormally low ALT levels can indicate more serious underlying conditions such as liver failure, chronic kidney disease, or vitamin B6 deficiency. Therefore, careful monitoring and investigation of persistently low ALT levels are important.