Is the blood marker lpa (lysophosphatidic acid) an effective screening tool for ovarian cancer?

No. Unfortunately, there are no good "screening" tests for ovarian cancer. When an ovarian cyst or mass is present and appears suspicious, your doctor may draw a ca-125 or ova-1 test. These tests can help predict the likelihood that it may or may not be concerning for cancer.
There are. No proven or reliable blood tests to screen for ovarian cancer. If you have a family history or want to check for brca 2 mutation, it is indeed reliably associated with ovca.