SPDL1 and non-small cell lung carcinoma: The observed discrepancies among studies evaluating the diagnostic, prognostic, or predictive significance of sPD-1 and sPD-L1 in NSCLC may, at least partly, be explained by their significant heterogeneity with regard to several key pre-analytical and analytical methodological parameters, mainly including differences in the type of biological sample (plasma vs. serum) analyzed, the assays employed (i.e., use of different kits), and the thresholds used as cut-offs.