ADA2 and Pleural effusion: However, due to non-standardized and costly ADA2 assays, total ADA levels in pleural effusion remain the primary marker in the clinical setting.The currently accepted ADA cut-off value is 40 U/L [9], but the threshold value ranges from 17.5 to 77 U/L among different studies and decrease with age [8, 9, 23–27].