The impact of sample storage time on estimates of association in biomarker discovery studies

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Abstract
Using serum, plasma or tumor tissue specimens from biobanks for biomarker discovery studies is attractive as samples are often readily available. However, storage over longer periods of time can alter concentrations of proteins in those specimens. We therefore assessed the bias in estimates of association from case-control studies conducted using banked specimens when maker levels changed over time for single markers and also for multiple correlated markers in simulations. Data from a small laboratory experiment using serum samples guided the choices of simulation parameters for various functions of changes of biomarkers over time. In the laboratory experiment levels of two serum markers measured at sample collection and again in the same samples after approximately ten years in storage increased by 15%. For a 15% increase in marker levels over ten years, odds ratios (ORs) of association were significantly underestimated, with a relative bias of -10%, while for a 15% decrease in marker levels over time ORs were too high, with a relative bias of 20%. Biases in estimates of parameters of association need to be considered in sample size calculations for studies to replicate markers identified in exploratory analyses.

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