Use

Carbohydrate Antigen 19‑9 (CA 19‑9) measured in pleural fluid may aid in evaluating suspected malignancy, but it is not specific and should not be used as the sole diagnostic basis; results must be interpreted alongside cytology, imaging, and clinical findings. Low or negative results (e.g. <20 U/mL) may be misleading in Lewis nonsecretor individuals or non‑CA 19‑9- secreting tumors, and CA 19‑9 may also be elevated in benign conditions such as liver cirrhosis, pancreatitis, gallstones, and cholecystitis. Interference from heterophile antibodies (HAMA) may occur and must be considered. (Source: Mayo Clinic Laboratories)