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Modified Reference Intervals for ALT and AST


  • A manufacturer-dictated change in assay procedure has required us to change our reference intervals for ALT and AST in all relevant species.
  • The change in procedure has resulted in lower values for both upper and lower reference limits for ALT and AST in all relevant species.
  • Results obtained with the new procedure are, on average, 5-6% lower for ALT (dogs and cats combined) and 6-7% lower for AST (dogs, cats, horses, cattle, camelid combined) than those obtained with the old procedure.
  • Results obtained with this new procedure for ALT and AST for all other species will be, on average, 5-7% lower.
  • The change in assay procedure, results and our reference intervals will take effect as of May 1st 2012.

New Reference Intervals for ALT and AST

As a consequence of reference standardization of the reagents, the Clinical Pathology Laboratory has adjusted our reference intervals for AST and ALT. The change in assay procedure results in a decrease in the values for both AST and ALT.

We have summarized the changes in reference intervals and have also provided guidelines as to how results in an individual patient will change if sequentially monitoring results before and after the change in assay procedure (see tables below). These new reference intervals will be effective as of May 1st, 2012 and will be updated in our computer system, so your reports will reflect the change on this date.

For more information on these changes, please contact the laboratory.

ALT AST Reference Table 1

Changes expected with sequential monitoring of patients or species without established reference intervals

When comparing results from the same patient sequentially, a decrease in ALT and AST results may be due to changes in the underlying disease or the change in assay procedure. To provide guidance on the degree of change to expect with the change in assay procedure alone, we determined the average and range of the change observed between the two assay procedures for ALT and AST (see table below). At the medical decision limit, which is the upper limit of the reference interval, the average bias can be used as a guideline for the expected change in results due to assay procedure. Note, that there is a proportional bias, i.e. the difference between the two assays increases as the enzyme concentration increases (the largest differences between the two assay procedures are seen at enzyme concentrations that are well above the upper reference limit).

ALT AST Reference Table 2

* Difference reflects new value minus the old value in absolute numbers (U/L) and as a percentage (%) of the old value.