Cancer Antigen 125 (CA 125) is a protein measured in the blood that serves as a biomarker. This substance is found on the surface of many cells and is often elevated in the presence of certain medical conditions. The measurement of CA 125 is most commonly associated with the investigation of ovarian cancer, though the test is not a definitive diagnostic tool for the disease. It is used when a doctor suspects a condition that could cause its level to rise.
Defining Normal and Elevated Levels
The concentration of CA 125 in the blood is measured in units per milliliter (U/mL). A level of 35 U/mL is the widely accepted threshold separating normal from elevated results. Generally, a CA 125 level below 35 U/mL is considered within the normal range. This cutoff includes approximately 99% of healthy individuals in the general population.
Any reading above 35 U/mL is classified as elevated. However, a single high result does not automatically equate to a cancer diagnosis. Even values slightly above the threshold (35 to 65 U/mL) are often caused by non-cancerous conditions. Significantly higher values prompt further investigation.
Conditions That Raise CA 125 Levels
A high CA 125 reading can be caused by a wide variety of conditions, broadly categorized as either malignant (cancerous) or non-malignant (benign). This lack of specificity is why the test is not considered a reliable screening tool for the general population.
Ovarian cancer is the most recognized malignant condition associated with a rise in CA 125, particularly the epithelial type, where elevated levels are seen in up to 85% of advanced cases. Other cancers of the reproductive system, such as fallopian tube cancer and primary peritoneal cancer, can also cause the level to rise. Elevations are also observed in cancers outside the gynecological system, including endometrial, pancreatic, and lung cancers.
Many non-malignant conditions also cause an elevation in CA 125, sometimes to very high levels. Endometriosis, where tissue similar to the uterine lining grows outside the uterus, is a common cause of a false positive result. Other benign gynecological conditions that can lead to an elevated reading include pelvic inflammatory disease (PID), uterine fibroids, and ruptured ovarian cysts.
Physiological states and inflammatory diseases can also cause the protein level to increase temporarily. These include a woman’s menstrual period, the first trimester of pregnancy, and liver diseases such as cirrhosis. Recent surgical procedures or inflammatory conditions in the abdominal cavity, like peritonitis, may also result in a temporary rise. The test is generally more reliable in postmenopausal women because many of these benign conditions are no longer present.
How Doctors Utilize the Test Results
Doctors utilize the CA 125 test primarily as a monitoring tool for patients diagnosed with ovarian cancer. Serial testing tracks the effectiveness of treatment, such as chemotherapy, by observing whether the protein level decreases over time. A consistent drop in the CA 125 level suggests that the cancer is responding favorably to therapy.
The test is also used for surveillance after treatment to detect potential recurrence. A rising CA 125 level in a previously treated patient is a reliable indicator that the disease may have returned, sometimes before any symptoms appear. However, the result is always considered alongside imaging scans and the patient’s clinical symptoms.
The CA 125 test is not recommended for routine screening of ovarian cancer in women at average risk. This limitation is due to the test’s relatively low sensitivity for early-stage cancer and the high rate of false positives caused by benign conditions. In high-risk individuals, such as those with a strong family history or a known gene mutation like BRCA, the test may be used as part of a specialized surveillance program, often combined with other diagnostic tools. A high result always necessitates a thorough clinical evaluation, as an isolated number is insufficient for a diagnosis.