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Aldosterone measurement with LiaisonXL automated system: remarks about reference range

Aldosterone, the primary member of the class of mineralocorticoids, is exclusively produced by the zona glomerulosa of the adrenal cortex and mainly acts on the distal renal tubule by increasing sodium reabsorption and potassium excretion, thus controlling sodium and potassium metabolism. Furthermore, its action on sodium reabsorption means that it also contributes to regulating extracellular fluid volume, and therefore systemic blood pressure. Aldosterone measurements, which are used to investigate patients with suspected excess or deficient aldosterone levels, are usually made by manual immunoassays. However, these are affected by all of the constraints related to this technology, particularly the poor inter-laboratory reproducibility that requires each laboratory to establish its own reference range [1–6]. Hypoaldosteronism is a rare condition that is often due to primary adrenal gland failure. Aldosterone hypersecretion is more frequent, and may be due to primary adrenal gland disease (primary aldosteronism) [5], or increased stimulation by the factors normally affecting aldosterone production (secondary aldosteronism). The most widely used methods are direct radioimmunoassays (RIAs) of blood and urine, but there has recently been a move towards automating aldosterone assays in order to save time and reduce the risk of the human errors associated with manual work. The aim of this study was to validate the RIA aldosterone reference ranges used in our laboratory for a new fully automated chemiluminescent technique based on the Liaison XL analyzer (Liaison Aldosterone, DiaSorin Inc., Stillwater, MN, USA). LIAISON® aldosterone assay is a competitive assay that uses sheep monoclonal antibody for capture of the aldosterone molecule. The principle components of the test consist of magnetic particles (solid phase) coated with anti-sheep antibody that bind sheep anti-aldosterone monoclonal antibody. An aldosterone-labeled conjugate containing an isoluminol derivative competes with aldosterone from the calibrators, controls and patient samples. The precision of the system was evaluated in accordance with the Clinical and Laboratory Standards Institute (CLSI) EP5-A2. Within-series repeatability was assessed by testing two internal and three Bio-Rad hypertension controls in 20 replicates in the same run, and analysis of the data showed that it varied from 5.3% to 2.4% in the concentration range of 169–1280 pmol/L. Between-series reproducibility was assessed by testing 20 replicates on different days (one or two runs per day) using two different kit lots; each sample was run in duplicate twice a day for 5 days, and the obtained coefficients of variation (CVs) ranged from 11.4% to 3.6% for aldosterone concentrations between 172 and 1044 pmol/L. The user instructions contain the manufacturer’s reference ranges, but it is recommended that laboratories establish their own. As a rule of thumb, establishing a reference interval requires selecting 120 samples. However, as it is difficult to collect a statistically significant number of samples from people having the characteristics required to be a reference subject for aldosterone, we chose to confirm the pre-defined ranges, because the CLSI guideline C28-A3 states that a laboratory can verify a reference range established by more stringent techniques, elsewhere by collecting as few as 20 samples from suitable subjects. In our laboratory, we use reference ranges determined over a number of years of experience with radioimmunoassays (ALDO-CTK-2, DiaSorin Inc.), therefore we decided to follow this guideline in order to validate our ranges of 83–416 pmol/L (supine) and 138–831 pmol/L (standing). We selected 20 apparently healthy male and female volunteers, aged 21–65 years, with normal blood pressure and fasting glucose levels. The exclusion criteria were a