Unexplained isolated elevation in serum aspartate aminotransferase: think macro!

Distribution of serum concentrations reported for macroenzyme aspartate aminotransferase (macro-AST)

Background

The presence of macroenzyme (M) is often the explanation of an isolated elevation of aspartate aminotransferase (AST). Where M is identified, it is reasonable for the clinician to ask where an individual patient's result fits in with known concentrations of M. In this context, we conducted a survey of literature to examine the distribution of reported serum concentrations of macro-AST. We also analyzed the distribution data to examine whether elevations were consistent with simple alteration of circulatory half-life (t_1/2) of M relative to normal AST.

Methods

Distributions of M were compiled from the literature. These distributions were compared to predictions based on fixed changes in t_1/2 applied to the reference interval for AST.

Results

There was a bimodal distribution of literature values for M (n =51), comprised roughly of populations A (M <200 U/L; 60% of total) and B (M >200 U/L; 40% of total). The two distributions were reasonably well characterized by a simple projection to the right of the reference interval for AST according to increased t_1/2 (A: t_1/2 =3.3 days; B: t_1/2 =19.8 days) relative to AST (t_1/2 =0.7 days).

Conclusions

Knowledge of distributions for M may be useful in discussion with clinicians regarding significance of M for individual patients. Distributions for M were consistent with the simplest explanation for elevated AST due strictly to an extended circulatory lifetime for M. Caveats to analysis, however, include selection within literature data mainly for patients with various co-morbidities.

[Isolated aspartate aminotransferase elevation in a young, healthy person. Case report]

The authors present diagnostic methods used in a young healthy person who had isolated aspartate aminotransferase elevation. Polyethylene glycol precipitation test, aspartate aminotransferase serum electrophoresis and immunofixation were performed for measuring the macro-aspartate aminotransferase. It was found that aspartate aminotransferase activity in the patient was almost completely eliminated after precipitation of immunoglobulins with polyethylene glycol. In addition, aspartate aminotransferase migrated in the control samples to the anode while in the patient towards the cathode. Finally, a wider and more intense staining band was visible in the region of immunoglobulin A in the patient sample on the immunofixation gel as compared to the control sample. The authors conclude that that increased aspartate aminotransferase activity was due to macro formation. The elevated level of immunoglobulin A and selective increase of polyclonal immunoglobulin A (κ and λ light chains) indicated that the macro format was created by immunoglobulin A bound to aspartate aminotransferase.

Prevalence and long-term course of macro-aspartate aminotransferase in children

OBJECTIVES

To investigate the prevalence, association with clinical conditions, and long-term course of macro-aspartate aminotransferase (macro-AST).

STUDY DESIGN

Forty-four children with an isolated elevation of serum AST were screened for macro-AST with electrophoresis and % polyethylene glycol (PEG) precipitable activity (PPA).

RESULTS

All children were healthy, except they had elevated AST values. Seventeen children (38.6%) were macro-AST-positive. They had higher AST values than the 27 children who were macro-AST-negative (P = .001). Values <67.1% PPA and >82.2% PPA were associated with a very low probability of being macro-AST-positive and macro-AST-negative, respectively. Thirty-eight children underwent clinical and laboratory follow-up (mean, 4.7 +/- 3.8; range, 1-16 years). All remained symptom-free. AST levels decreased significantly only in children who were macro-AST-negative (P = .006). Macroenzyme persisted in 6 of the 9 children who were macro-AST-positive after 6.0 +/- 4.1 years.

CONCLUSIONS

Macro-AST was present in more than one-third of children with an isolated increase of AST levels. The lack of pathological correlates in a long period argues for the benign nature of this phenomenon in childhood. We suggest that our %PPA thresholds can be used as a screening test and that electrophoresis be reserved for confirming positive screen test results and cases in which %PPA levels are of intermediate discriminant accuracy.