The CA 72.4 test (Cancer Antigen 72.4) is a blood test measuring a protein often elevated in gastrointestinal cancers, especially stomach (gastric) cancer, used to monitor treatment response or recurrence, not for initial diagnosis, as levels can rise in other conditions too. “OS” likely refers to On Site or simply denotes it’s a lab test. It helps track tumor activity alongside other markers like CEA, indicating if cancer is growing or shrinking