Biological variation of serum cholinesterase catalytic concentrations

Measurements of serum cholinesterase (CHE, also called pseudocholinesterase) catalytic concentrations are primarily used as a test of liver function and, less frequently, as an indicator of possible organic phosphorous insecticide poisoning in agriculture or organic chemical industry workers [1]. Preoperative screening of CHE activity has been also advocated to identify individuals bearing genetic causes of enzyme deficiency in whom some muscle relaxant drugs, such as suxamethonium, administrated to aid in endotracheal intubation in surgery, may not be hydrolysed by CHE rapidly enough and cause apnea by a prolonged paralysis of respiratorymuscles [1]. Historically, many methods were proposed to measure CHE, using different acyl(thio)choline esters as substrates [2]. At present, however, all the most popular automated measuring systems use butyrylthiocholine for determining the CHE activity as this substrate provides the best reproducibility. To check the quality of CHEmeasurements it is essential to correctly define analytical performance specifications (APS). In 2014, the Strategic Conference organized by the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) proposed models for establishing APS, recommending that the choice of the most appropriate model for a specific measurand should be based on its biological and clinical characteristics [3]. Allocation of CHE to the correctmodel to derive APS can be however not easy. Although this enzyme surely plays a role in monitoring the clinical conditions mentioned above, the measurand has not a defined role in the decision making of a specific disease and test results are not interpreted through established decision limits, so that outcome-based APS cannot be defined. On the other hand, its exact biological role is unknown, and this might be a limitation for applying the model based on biological variation (BV). As CHE demonstrated rather stable concentrations in healthy individuals, it appears however rational to use the BV model to derive APS [4]. This requires the availability of reliable BVdata. Unfortunately, all the available studies evaluating CHE BV suffered important limitations, among those the collection of serial samples in enrolled individuals at a variable time distance, the derivation of the within-subject BV (CVI) without subtracting analytical variation, the analysis of samples in different batches without considering the between-run analytical variation, or the wrong inclusion of the CHE seasonal variation in CVI estimate. Table 1 summarizes the drawbacks of each published study evaluated on the basis of the compliance with the EFLM critical appraisal checklist quality items (BIVAC-QI) [5]. Maybe for these major limitations, the EFLM BV database (https://biologicalvariation.eu/) does not include CHE in the list of evaluated measurands. Therefore, we decided to perform a study assessing BV components of CHE catalytic concentration by adopting an accurately designed experimental protocol in accordance with previous remmendations for the BV data production [6]. We employed frozen (−80 °C) serum samples from 35 apparently healthy Caucasian volunteers (13 men, 12 pre-, and 10 post-menopausal women; ages 19–62 years) obtained for a previous BV study [7]. CHE activity in serum is stable for several years if stored at temperature lower than −20 °C [1]. All sample donors were free of any evident disease, had no history of chronic disease, and did not receive any medication, including hormonal contraceptives if women, previously shown to decrease CHE [8, 9]. Other criteria for inclusion were that the subjects should be within 80–120% of ideal body weight and maintain their *Corresponding author: Federica Braga, UOC Patologia Clinica, ASST Fatebenefratelli-Sacco, Via GB Grassi 74, 20157, Milan, Italy, Phone: +39 0239042743, Fax: +39 0250319835, E-mail: federica.braga@unimi.it. https://orcid.org/0000-00033562-7180 Mariangela Altilia, Alessia Capoferri andMauro Panteghini, Research Centre for Metrological Traceability in Laboratory Medicine (CIRME), Università Degli Studi di Milano, Milan, Italy Clin Chem Lab Med 2022; 60(8): e177–e180