CHI3L1 and central nervous system cancer: When age was added to the model, neither plasma IL-6 or YKL-40 could differentiate GBM from gliomas of lower grade at a given age (IL-6, GBM vs. OII-III p = 0.81, IL-6, GBM vs. AII-III p = 0.88; YKL-40, GBM vs. OII-III p = 0.058; YKL-40, GBM vs. AII-III p = 0.29).