Abstract
Osteoporosis is a disease of the bone characterized by a disruption in the processes of bone remodelling and metabolic turnover, resulting in an increased likelihood of fracture. Antiresorptive treatments, such as bisphosphonate therapies, are designed to correct the imbalance in turnover that causes bone loss. Therapy may be monitored by infrequent bone mineral density (BMD) measurements (bone quality) and/or more frequent, non-invasive laboratory testing (typically by the measurement of serum or urine markers of bone resorption). Measurement of bone ‘biomarkers’ has been shown to be useful in predicting treatment response and for assessing compliance. 1
In patients with severe osteoporosis, BMD may remain low despite the use of antiresorptive therapies and thus fracture rates remain high. In addition, many individuals cannot tolerate bisphosphonates. Continuing research for therapies led to the development and use of anabolic therapies such as recombinant human PTH (hPTH) (e.g. teriparatide), which act primarily on osteoblasts to stimulate bone formation, increase BMD and reduce fracture risk. 2 The management of patients receiving this anabolic therapy has been shown to be enhanced by the measurement of the bone formation marker serum total N-terminal propeptide of type I collagen (P1NP). This biomarker can demonstrate early effectiveness of, and compliance with, this complex and expensive treatment and is monitored regularly in these patients in our metabolic bone clinic. 3,4
A recent evaluation of an assay for P1NP using the Roche Elecsys 2010 automated analyser confirmed analytical reproducibility and clinical usefulness for monitoring changes in bone turnover in individuals on PTH therapy. 5 Of note they reported that P1NP was stable even at room temperature. Previously we had applied recommendations (according to the manufacturer's kit insert) that serum for P1NP analysis should be stored frozen or, if transported directly to the laboratory for analysis, should arrive the same day if sent at room temperature, or within two days if refrigerated. We wished to confirm the stability of P1NP in serum.
P1NP was measured in 20 different patient samples with a range of P1NP concentrations and subjected to a variety of preanalytical conditions. Samples were measured using the Roche Cobas e411 assay, which uses the same P1NP electrochemiluminescence immunoassay as in the aforementioned study. Samples were stored at room temperature (24°C) and measurement after storage for 120 h was compared with analysis at time 0 h. Results of these analyses revealed no significant change in measured P1NP concentration (t-test, P = 0.2047) under these varying storage conditions (Figure 1).
Effect of exposure of serum to room temperature upon total N-terminal propeptide type 1 collagen (P1NP) concentration. Serum was initially analysed for P1NP before being left in the laboratory at a mean temperature of approximately 24°C for up to 120 h (5 days) and re-analysed. There were no significant changes in P1NP concentration (r = 0.959, mean coefficient of variation (CV) 2.8%, t-test P = 0.2047). Analysis was carried out on the Roche Cobas e411 (between-batch CV 3.7%)
In a further experiment, aliquots of 10 of the previous samples were stored at 4°C after the initial measurement and stored for one, three and four weeks at 4°C before reanalysis. Again there were no significant changes in P1NP in these samples over time under these storage conditions (t-test, P = 0.1018).
Our data confirm that P1NP is stable in serum for at least five days at room temperature and for an extended period of at least four weeks at 4°C. This information facilitates sample collection and measurement procedures for P1NP analysis; of importance given that most samples are sent away to specialist centres, removing the need for a courier service and/or transport on ice.
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