Estimation of the Stability of Parathyroid Hormone when Stored at −80°C for a Long Period

Abstract

Background and objectives: Stability of parathyroid hormone (PTH) at ؊80°C for long storage periods has never been studied. This can be of importance for the conclusions of studies where blood banks have been constituted. The study's aim was to evaluate stability of PTH when stored as serum or plasma EDTA samples at ؊80°C.Design, setting, participants & measurements: Samples were collected from 16 chronic hemodialysis patients using EDTA and gel-separator tubes. Plasma and serum were aliquoted; one aliquot was assayed with Elecsys and Liaison methods to determine the "baseline" values and another aliquot after 1, 3, 6, and 12 mo. The factors "method," "tubes," "subjects," and "time" were included in a mixed linear model to evaluate their effects on measured PTH values. The prediction interval methodology was used to assess where a future result could be obtained with a defined probability.Results: With the Liaison method, the maximum storage times with either dry or EDTA tubes were estimated to be 9 and 2 mo, respectively. With the Elecsys method, samples could be stored at least 2 yr with acceptable level of degradation.Conclusion: PTH stability at ؊80°C is not infinite. Maximum storage time and acceptance limits (30%) were defined, showing that with one method, samples should be stored for not more than 2 mo, whereas the other could be stored for up to 2 yr. With any PTH assay, the maximum storage time should be evaluated to ascertain that samples will keep their initial reactive profile after prolonged storage periods.

Sampling and storage conditions influencing the measurement of parathyroid hormone in blood samples: a systematic review

“…One study used results from plasma or serum from fresh whole blood as comparator [14], but most used results from serum or plasma which had been separated and frozen prior to analysis. For the following analysis, data have been grouped according to sample type and assay characteristics (i.e., second or third generation).…”

“…Using third generation methods, PTH was reportedly stable for 14 days in serum [43] and for 12 months in serum and plasma kept at −20°C and −80°C [30]. Observed stability may be method dependent: Cavalier et al [14,16] reported that if stored at −20°C or −80°C PTH was more stable in serum than EDTA plasma when measured using the DiaSorin Liaison method (9 vs. 2 months, respectively). In contrast, PTH was stable in both frozen serum and plasma for at least 2 years when analysed using the Roche Elecsys method.…”

“…Most used as comparator a sample which had been frozen at baseline, as opposed to a freshly analysed sample, used by one study only [14], so interpretation of test results could have been confounded by changes in PTH concentration following freezing. Different approaches were taken to define significance of change, e.g., some studies used a prespecified percentage change compared to baseline [16,17,25,29,33,38,44], whilst others used paired statistical analyses [15, 18, 19, 21-24, 26, 28, 30-32, 34, 35, 37, 39, 40, 42, 43].…”

Interpretation of serum parathyroid hormone concentrations in dialysis patients: what do the KDIGO guidelines change for the clinical laboratory?

“…We demonstrated previously that if the primary tube is a gel type serum tube and is centrifuged without opening in the dialysis centre (assuming that the dialysis center has a centrifuge), measurement of serum PTH may be delayed by at least 18 h if the primary tube is maintained at 48C (30). Serum seems also preferable when samples have to be stored for long periods of time at -808C, such as for research studies (31).…”

Considerations in parathyroid hormone testing

“…Our major concerns are mainly the definition of the "zero" point and the cut-off below which one can decide whether there is a degradation of the peptide or not. Regarding the "zero" point, most of the studies on PTH stability have used, as comparator, a sample that Brought to you by | ULg Library Authenticated Download Date | 6/8/15 11:18 AM has been immediately frozen at −20 °C [24][25][26][27][28][29][30][31] or at −80 °C [32][33][34], but few of them used an unfrozen sample, immediately assayed after centrifugation [22,[35][36][37]. The authors that used a frozen sample as T0 did that on purpose to run all the samples in one batch in order to reduce the CV, but it is unclear if this could have some consequences on the final results.…”