CA-125 is the most frequently used biomarker for ovarian cancer detection.^[16] Medical societies including American Congress of Obstetricians and Gynecologistsrecommend against women with average risk of ovarian cancer having routine CA-125 screening or other screening for this cancer.^[17] Reasons for this include evidence that ambiguous test results are more likely to lead to further invasive, harmful, and unnecessary health care than they are likely to detect ovarian cancer in women who are at average risk of developing it.^[17]

Around 90% of women with advanced ovarian cancer have elevated levels of CA-125 in their blood serum, making CA-125 a useful tool for detecting ovarian cancer after the onset of symptoms.^[18] Monitoring CA-125 blood serum levels is also useful for determining how ovarian cancer is responding to treatment (with the duration of disease-free survival correlating with the rate of fall of CA-125)^[19] and for predicting a patient’s prognosis after treatment.^[20] This is because the persistence of high levels of CA-125 during therapy is associated with poor survival rates in patients.^[20] Also, an increase in CA-125 levels within individuals in a remission is a strong predictor of the recurrence of ovarian cancer.^[21] Indeed, a rising CA-125 level may precede clinical evidence of disease relapse by an interval of 3 to 6 months.

Prognosis relates to both the initial and post-treatment CA-125 values. A preoperative value >65 U/mL suggests a poor prognosis. Persistent elevations following chemotherapy indicate a poor prognosis. The half-life of CA-125 after chemotherapy correlates with prognosis (patients with CA-125 half-life <20 days show improved survival). Time-to-normalization (rate of fall of CA-125) affects prognosis with more rapid normalization within 3 cycles of chemotherapy correlating with improved survival.^[22]

In April 2011 the UK’s National Institute for Health and Clinical Excellence (NICE) recommended that women with symptoms that could be caused by ovarian cancer should be offered a CA-125 blood test.^[23] The aim of this guideline is to help diagnose the disease at an earlier stage, when treatment is more likely to be successful. Women with higher levels of the marker in their blood would then be offered an ultrasound scan to determine whether they need further tests.