CA 19.9
CA 19.9 is a tumor-associated antigen, originally isolated from a human colon cancer cell line. It is present on gangliosides in tissues, but is carried by glycoproteins in serum. The oligosaccharide, bearing the CA 19.9 antigen, is related to sialylated Lewis A blood group antigen. Lewis A antigen must be present before CA 19.9 can be expressed. CA 19.9 is synthesized by normal cells in pancreatic and bile ducts, gastric and colonic mucosa, bronchial and salivary glands, endometrium, and prostate. Secretions from these organs are rich in CA 19.9, but very little antigen appears in the blood of healthy individuals.
Even though CA 19.9 was originally isolated from a colon cancer cell line, its greatest clinical utility is detecting pancreatic cancer. CA 19.9 is not a very useful screening test for pancreatic cancer in asymptomatic patients. The maximum achievable sensitivity is 95%, since 5% of the U. S. population are Lewis a & b nonsecretors and do not synthesize CA 19.9. In practice, the sensitivity for early cancer is much less. Serum levels are elevated in less than 30% of patients with stage 1 cancers.
Elevated CA 19.9 levels are not specific for pancreatic cancer, but are elevated in other benign and malignant disorders.Other GI malignancies are also associated with elevated CA 19.9 levels, but not usually to the same extent as pancreatic cancer. CA 19.9 is elevated in 67% of bile duct cancers, 41% of gastric cancers, 34% of colon cancers, 22% of esophageal cancers, 49% of hepatomas, and 14% of non-gastrointestinal tract cancers.Most benign disorders cause lower elevations of serum CA 19.9. However, two benign conditions, cirrhosis and acute cholangitis, can cause high CA 19.9 levels. In acute cholangitis, levels rapidly return to normal following decompression of the common duct.
CA 19.9 is the most useful circulating tumor marker for evaluating patients who present with signs and symptoms of a chronic pancreatic disorder. In this population the prevalence of cancer is about 17%. Using an upper limit of normal of 40 U/mL, serum CA 19.9 assays have a sensitivity of 81%, specificity of 90%, and a positive predictive value of 72%. The specificity and positive predictive value for cancer increase with higher CA 19.9 values. The higher the CA 19.9 level, the greater the likelihood of cancer. Levels >300 have a positive predictive value of 92%.
The location of the tumor within the pancreas does not appear to affect sensitivity. Both tumors of the head and the body-tail region cause elevations of similar magnitude. Tumor size is an important determinant of sensitivity. CA 19.9 is elevated in approximately 90% of patients with tumors greater than 3 cm in diameter, but in only 50% of patients with tumors between 2 and 3 cm. Less than 30% of patients with tumors smaller than 2 cm have elevated CA 19.9 levels. Histologic grade also affects CA 19.9 sensitivity. Poorly differentiated tumors do not express CA 19.9 as often as well differentiated tumors.
CA 19.9 can be used in conjunction with CT scan, arteriogram, and endoscopic retrograde cholangio-pancreatography to determine tumor resectability. High levels strongly suggest that the tumor is unresectable. Essentially all patients with CA 19.9 levels above 1000 U/mL have tumors greater than 5 cm in diameter and only 5% of this group have resectable tumors. Half of the patients with pre-treatment CA 19.9 levels below 1000 U/mL have a resectable tumor.
CA 19.9 levels are also useful in predicting survival and recurrence after surgery. Patients with elevated pre-surgical CA 19.9 levels, which normalize after surgery, have a better prognosis than patients whose CA 19.9 levels remain elevated. Progressively rising CA 19.9 levels after surgery predict cancer recurrence several months before CT scan.
Another use for CA 19.9 is to detect cholangiocarcinoma in patients with primary sclerosing cholangitis. CA 19.9 levels are increased in 15% of patients with primary sclerosing cholanitis without cancer and in 90% of patients with cancer. The sensitivity of a CA 19.9 level greater than 100 U/mL for cholangiocarcinoma in primary sclerosing cholangitis was 89% and the specificity was 86%.
Reference range is 0-55 U/mL.
Specimen requirement is one SST tube of blood.
