Detection of macro-creatine kinase and macroamylase by polyethylene glycol precipitation and ultrafiltration methods

Abstract

Background

Macroenzymes may cause elevations in serum enzyme activity. Macroenzymes are not common; however their detection is important because they cause diagnostic confusion and therapeutic errors.

Methods

We analyzed 2 of the most prevalent macroenzymes in the literature, macro-creatine kinase (macro-CK) and macroamylase, using 2 methods for detection, polyethylene glycol (PEG) precipitation and ultrafiltration (UF). Enzyme measurements were made using a Roche Modular Analytics P analyzer. Imprecision was assessed using quality control material. We evaluated 125 samples from apparently healthy subjects to establish reference intervals. For macro-CK comparison, 94 samples with activities > 200 U/l were analyzed with both PEG precipitation and UF and compared to electrophoresis. PEG precipitation and UF were compared for macroamylase detection using 130 samples with amylase activities > 110 U/l.

Results

UF was more precise and demonstrated narrower reference intervals for both analytes. PEG precipitation and UF were able to detect true cases of macro-CK with overall agreement with electrophoresis of 79.8% and 80.9%, respectively. Both methods detected the same number of ‘positive’ macroamylase samples; however PEG precipitation resulted in a greater number of ‘indeterminate’ cases.

Conclusion

This is the first report where UF has been shown useful for the detection of both macro-CK and macroamylase.

Introduction

Macroenzymes typically have higher molecular masses than the corresponding enzymes that are normally found in serum [1], [2]. The prevalence of macroenzymes is not well defined. Macroenzymes are generally considered rare, affecting only 1.5–2.5% of the population [3]. The most common types of macroenzymes documented in the literature are macro-creatine kinase (macro-CK) and macroamylase, which was the first macroenzyme identified in 1964 [4]. The prevalence of macro-CK was reported to be 0.5–2.3% and 3.7% for type 1 and 2 respectively [5]. Macroamylasemia is reported to occur in 2.5% of hyperamylasemic patients and 1% of the general population [6]. Several different methods have been employed to detect macroenzymes such as: electrophoresis, gel-filtration chromatography (GFC), and polyethylene glycol (PEG) precipitation [4], [5], [6], [7], [8], [9], [10], [11], [12]. Ultrafiltration (UF) is another method that has been used for the detection of macroprolactin [13], [14], [15], [16], [17]. A literature search failed to identify any publication using UF to detect macroenzymes. The method of choice for detecting macro-CK is electrophoresis which has previously been compared to PEG precipitation [7]. However, electrophoresis is not recommended for use in the detection of macroamylase because of the large variability in migration patterns [6]. PEG precipitation has been recommended as a good alternative to GFC for the detection of macroamylase [4], [9], [11], [12]. In the present study, we determined the performance characteristics of PEG precipitation and UF for the detection of macro-CK and macroamylase. Both methods are easy to perform and can accommodate a large number of samples for clinical studies. Our goal was to validate the usefulness of PEG precipitation and UF by comparing the 2 methods to electrophoresis for macro-CK detection. We then analyzed the use of both methods for the detection of macroamylase. Specifically, we wanted to validate the use of UF for the detection of macro-CK and macroamylase.