This is most likely a benign cyst such as an endometrioma, but any cyst in a postmenopausal patient needs to be followed to ensure resolution. Size and complexity of the cyst on imaging are taken into account in determining management. A blood test can be done in conjunction with ultrasound, but it is more to guide the Obgyn in need for referral to a GynOncologist. There is not a reliable screening test for ovarian cancer yet so age of the patient, menopausal status, characteristics of the cyst, size of the cyst, change or growth noted on ultrasound or other imaging are criteria used to decide management. Family history such as breast or ovarian cancer, or known pathogenic mutations such as BRCA1/2 are also a factors to consider. A family history of ovarian cancer or multiple breast cancers, especially early/premenopausal cancers can be a reason to screen for hereditary cancer syndromes such as BRCA.