New Definitions for Healthy Ranges of Alanine Aminotransferase, a Blood Test of Liver Function

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The summary below is from the full report titled "Updated Definitions of Healthy Ranges for Serum Alanine Aminotransferase Levels." It is in the 2 July 2002 issue of Annals of Internal Medicine (volume 137, pages 1-9). The authors are D Prati, E Taioli, A Zanella, E Della Torre, S Butelli, E Del Vecchio, L Vianello, F Zanuso, F Mozzi, S Milani, D Conte, M Colombo, and G Sirchia.

What is the problem and what is known about it so far?

Alanine aminotransferase (ALT) is a blood test that doctors often use to evaluate patients for liver disease. High levels of ALT in a patient's blood suggest that he or she has liver disease. However, the test is not perfect. Many patients with liver disease have normal ALT levels. Infection with hepatitis C virus (HCV) and fatty deposits in the liver are two common reasons why people have abnormal ALT levels. The current "normal" ranges of ALT were defined before we knew about or could test for these conditions. As a result, some of the people considered "normal" (free of liver disease) when these ranges were established actually had HCV infection or fatty liver disease. The true normal values for ALT may be lower than the levels that laboratories currently consider normal. Knowing what ALT levels most accurately identify people with and without liver disease would be helpful.

Why did the researchers do this particular study?

To define new normal ranges for ALT among a group of people known not to have HCV infection or a fatty liver.

Who was studied?

6835 blood donors in Italy who had negative results on tests for HCV infection and 209 who were found to have HCV infection.

How was the study done?

The researchers measured ALT levels in all of the blood donors. They also collected information related to each person's risk for liver disease, including body size and other factors related to fatty liver. They then measured the ALT levels in patients who were unlikely to have liver disease, including HCV infection and fatty liver. Last, they applied these new ranges to the patients known to have HCV infection to see how many would have abnormal ALT levels according to the new definition of normal.

What did the researchers find?

The new upper limit of normal range for ALT (30 U/L for men and 19 U/L for women) were substantially lower than the levels that laboratories currently consider to be the upper range of normal (40 U/L for men and 30 U/L for women). Using these new definitions of normal, the researchers could more accurately identify the patients with HCV infection than they could using the old ranges.

What were the limitations of the study?

The researchers were unable to test whether the new ALT ranges could more accurately identify patients with fatty liver than the old ALT ranges.

What are the implications of the study?

Laboratories should consider revising the upper limits of normal for ALT to improve the accuracy of this test in identifying patients with liver disease.