CA-125 is commonly used as a tumor marker to monitor response to chemotherapy of recurrent disease. It is the best marker available but is not perfect. For example it can go up or down by 25% from test to test just due to normal variations of biological function cycles in your body. Many non-cancerous conditions can cause fluctuations during therapy that limit pinpoint accuracy in assessing response. However, the overall trend is important and often used to determine whether or not to change chemotherapy drugs.
Certainly a single measurement cannot be definitive. If an elevation is noted on chemotherapy, ask if the same lab is running the tests or if there was a recent change. In addition, even when the same lab is used, a variation of up to 14% can occur in the same patient. So it is reasonable to ask that the CA-125 be repeated before any consideration of change in therapy. Also, does it correlate with scan or physical examination evidence of progression? A slight increase in CA-125 alone does not necessarily mean that your cancer is progressing.
For incompletely clear reasons accurate trends of CA-125 may variably develop depending upon which chemotherapy drug is used. In particular, patients treated with Doxil,a drug commonly used for recurrence, may initially see an unchanged or even increased CA-125. Studies show that this can be misleading with a good clinical response documented later.So for this particular drug, as long as there is no obvious progression on scans or examination, it is recommended that a minimum of four cycles be given before a judgment is made regarding response. Of course, the dose may need to be modified if there are signs of toxicity such as severe mouth sores or skin rashes.