Endocrine Abstracts

Background: Soluble alpha klotho (sαKL) is a circulating protein that has been linked to the growth hormone (GH) axis. We previously showed its association to disease activity in patients with acromegaly, with considerable robustness towards biological confounders. However, there is scarce data in literature regarding the analytical performance of the assay, and pre-analytical stability of sαKL in blood samples.

Objective: We aimed to evaluate analytical performance and pre-analytical stability of sαKL in blood samples, following the guidelines from the Clinical & Laboratory Standards Institute.

Methods: SαKL concentrations were measured by an ELISA (Immuno-Biological Laboratories, Hamburg, Germany). We compared different incubation times (1 h vs. overnight), analyzed precision by 10 repeated measurements within a plate and linearity by serial dilutions (no dilution, 1:2, 1:4, 1:8 and 1:16). We also tested stability of sαKL under different storage conditions: 1) room temperature before (whole blood, 1, 24, 48 or 120 h) or after centrifugation (serum, 15 min, 24, 48 or 72 h); 2) freeze/thaw cycles (0-4 cycles); 3) long-term storage at -20^oC (baseline compared to 20 and 31 months). Finally, we compared sαKL concentrations in serum and EDTA samples collected in parallel from the same individuals (n=18).

Results: Short incubation times were associated with a significant drift of concentrations obtained over the plate (>25%), an effect not seen after extending the first incubation to overnight (<5%). sαKL measurements by this assay presented with low intra- and inter-assay coefficients of variation (%CV) (<10% for both). Dilution linearity was good at concentrations below 3,000 pg/ml (recovery rate (RR) (%mean (range)): 96 (92-107). Measured sαKL concentrations were not significantly affected by storage for up to 120 hours at room temperature (CV (%mean (range)) (whole blood, before centrifugation: 2.9 (1.5-5.1) or serum, after centrifugation: 8.9 (1.8-18.9)), or by up to 4 freeze/thaw cycles (CV: 4.9 (1.4-10.1), RR: 99.1 (87.3-105.8)). Serum sαKL also exhibits excellent long-term storage stability for more than 2 years at -20^oC (CV: 8.6 (2.7-17.5), RR: 89.1 (76.5-111.7)). Furthermore, sαKL concentrations (pg/ml) (median (interquartile range) in serum did not differ from those seen in EDTA (404.4 (341.8-462.6) vs. 428.6 (384.8-514.8), P=0.27).

Conclusion: After extending incubation times to overnight, the sαKL assay exhibits good performance characteristics. We suggest to dilute samples at least 1:4, especially in patients with GH excess. sαKL is a biomarker with considerable preanalytical stability at different storage conditions. This facilitates its use as a GH responsive biomarker in studies and clinical practice.