Rising CA 19.9 over 6 months

Hi,
My father had Whipples surgery done in May 2005 (at the age of 56) followed by a liver surgery to remove solitary metastasis in Oct 2006. Since then, he has been constantly monitoring his blood sugar levels, body weight, RBC and WBC counts, CA 19.9, Liver Function Test (LFT) and ESR.
But the last 3 CA 19.9 blood tests have showed an increase:
CA 19.9 in May-2011: 45.86
CA 19.9 in June-2011: 51.23
CA 19.9 in Oct-2011: 61.72
But Liver Function Test in Oct-2011 still showed normal values. CT scan and ultrasound did not reveal any abnormality except liver pneumobilia and dilated pancreatic duct (these two conditions have been there since 2006).

Could anyone know any reason behind this increase in CA 19.9 and please let me know if there is any combination of blood tests that would help to detect any metastasis.
Thanks a lot.