Could ovarian cyst cause a false pregnancy test?

Yes, but rare: Only one type of cyst can cause a false pregnancy test. This is a tumor that can release hCG. It is very rare and is very unlikely causing your pregnancy test to be positive. If the false positive is blood related then this could be due to antibodies in your blood interfering with the test. If this is true then the urine test will be negative since the antibodies can't be urinated.

Unlikely: An ovarian cyst is unlikely to produce sufficient HCG to cause a false positive on a pregnancy test. However, there are cases where a benign corpus luteum cyst has interfered with pregnancy testing.