Validating Sampling Methods for Measurement of Circulating Matrix Metalloproteinase-9 and Tissue Inhibitor of Metalloproteinase-1

To the Editor: W e read with great interest the article by Mroczko et al published in this journal, in which they conclude that serum matrix metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinase1 (TIMP-1) measurements are diagnostic and prognostic in patients with pancreatic cancer. We have some comments concerning the appropriate sample for determining circulating MMP-9 and TIMP-1 ignored by some clinicians, which may lead to misinterpretation of data or wrong expectations. Recently, studies have shown that serum MMP-9 and TIMP-1 levels do not represent the true circulating concentrations and that the mode of procedure for preanalytical blood collection could impact the absolute values of target MMP and TIMP serum levels. The reason behind this is that platelets and leukocytes contain abundant MMPs and TIMPs, and inappropriate blood collection and processing procedures can lead to high analyte concentrations and highly nonspecific background results in serum owing to their release from platelets and leukocytes. For example, studies by Gerlach and coworkers suggested that the circulating levels of MMP-9 should be assessed in citrate or heparin plasma samples, but not in serum samples because of artificially higher MMP-9 levels in serum and because they do not correlate with the MMP-9 levels in plasma samples. Olson et al also showed that MMP-9 concentrations were higher in serum than in plasma and that MMP-9 levels in serum were more strongly associated with peripheral neutrophil count compared with MMP-9 levels in plasma. Thus, these studies support the view that the choice of sampling methods affects the measurements of circulating MMP-9 levels. Therefore, inappropriate specimen collection methods can result in misleading effects on MMPs and TIMPs measurement. The possible interfering factors have to be considered, before the diagnostic prognostic validity of MMPs and TIMPs as potential biomarkers can be explored in clinical trials. In the light of the recent knowledge, we propose that interested clinicians should pay attention to sampling methods and use plasma as the preferred clinical specimen instead of serum for MMP-9 and TIMP-1 determination limiting the preanalytical problems.