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In a woman with known ovarian cancer, a rise in CA-125 usually means a progression or recurrence of the disease. A decrease in CA-125 usually means the disease is responding to treatment.
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In a woman who has NOT already been diagnosed with ovarian cancer, an elevated CA-125 can mean a number of things. While it can indicate that she has ovarian cancer, it can also indicate other types of cancer, as well as several benign diseases such as endometriosis.
To understand why the CA-125 is not a good general screening test, it is important to understand some basic facts about diagnostic tests. If the result of a diagnostic test is abnormal, the test result is usually considered to be "positive" (meaning the person seems to have the disease). If the result is normal, the test is considered "negative" (the person doesn't seem to have the disease). However, tests are somewhat imperfect, and the results might be incorrect.
Therefore, we are left with four possibilities every time a test is done:
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True positive -- means that the test result is abnormal and the patient does, in fact, have the disease.
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False positive -- means that even though the test result was abnormal, the patient in fact does NOT have the disease.
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True negative -- means the test is normal and the patient does not have the disease.
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False negative -- means that even though the test is normal, the patient does in fact have the disease, and that the test simply missed it.
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