The Signatera test (by the company Natera) is intended to detect cancer recurrences by looking for low levels of DNA derived from your cancer in your blood. So it serves the same purpose as testing for CA125. How well it works is a subject of debate. There are definitely false negatives. If it's positive, that means there are cells derived from your cancer somewhere in your body. But apparently sometimes the source is hard to find, and a tumor may not grow. So these are considered false positives.