MUC1 and hypersensitivity pneumonitis: Eleven studies (9, 22–27) reported the correlation between HP patients and KL-6 concentrations in a Caucasian population, suggesting that the KL-6 concentrations were higher in HP patients as compared to normal individuals (SMD = 1.75, 95% CI = 1.21-2.29, I2 = 84%, P < 0.0001).