Table schema:
CREATE TABLE table_name_28 (prothrombin_time VARCHAR, partial_thromboplastin_time VARCHAR, platelet_count VARCHAR, bleeding_time VARCHAR)

Question: Which Prothrombin time has a Platelet count of unaffected, and a Bleeding time of unaffected, and a Partial thromboplastin time of normal or mildly prolonged?
SQL: SELECT prothrombin_time FROM table_name_28 WHERE platelet_count = "unaffected" AND bleeding_time = "unaffected" AND partial_thromboplastin_time = "normal or mildly prolonged"