There are no blood tests that will detect any cancer.
There are some proteins the can be measured in the blood once cancer has become established. An example is the sugary protein called CA125.
This is often (but not always) elevated when there is any disease affecting the peritoneal cavity. The peritoneum is the inner lining of the abdomen and it is wrapped around the ovaries. This chemical is often detected in the body fluids and blood when an ovary or peritoneal cancer has become established. However, it is worth noting that this is not usually a sign of early cancer.
Testing blood for these chemicals can sometimes help make a diagnosis or monitor the level of disease. A CA125 test is sometimes used for women who suffer from progressive bloating to see if they would benefit from an ultrasound scan because a normal result usually reassures the GP that there is nothing irritating the peritoneum. It is also helpful for surgeons planning an operation for an ovarian cyst because a normal level usually means that the growth is benign and the surgery can be limited to removal of the ovary.
The blood tests cannot be used to prevent cancer. Tests looking at blood chemicals, as a way of screening the cancer, has been shown to be unhelpful, presumably because they are usually elevated once the disease is established and consequently at a stage after early detection is crucial.