Aminotransferase levels in clinical practice - what is normal?

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No universal consensus exists as to what the upper limit of normal for alanine aminotransferase (ALT) should be. The purpose of this review is to provide insight on the wide variation in the upper limit of normal for ALT, and to highlight key elements resulting in these differences. We also wish to point to potential solutions for standardizing the upper limit of normal. There are considerable differences in the values for what constitutes a 'normal' ALT serum concentration set by laboratories, and reference values may vary two-fold between different institutions (from 30 to 60 U/l). Key reasons for this include using markedly different reference populations to establish the bounds of normal, and differences in analyzers and assays used to quantify ALT levels. A number of studies have attempted to re-evaluate the upper limit of normal and propose new cut-offs that would allow better detection of liver disease. The upper limit of normal for ALT continues to vary from one laboratory to another. Despite ongoing efforts, there is still an unmet demand to standardize ALT reference ranges, which is critical for clinical practice. More rigorous inclusion criteria for reference population studies and harmonization in analyzer differences is required to standardize ALT reference ranges universally.