Considering the obvious heterogeneity across studies, the subgroup analysis regarding OS based on detection methods was performed to explore the source of heterogeneity and the results revealed that elevated YKL-40 expression had a significantly poor OS effect on breast cancer patients both by IHC(HR=1.39, 95%CI=1.12-1.71) but not by ELISA/RIA(HR=1.60, 95%CI= 0.84-3.07), with a reduced heterogeneity. The gene discussed is CHI3L1; the disease is breast cancer.