When is the latest patient's medical data recorded? This patient should have an abnormal level of lactate dehydrogenase.
latest patient refers to ID with MAX('First Date'); abnormal level of lactate dehydrogenase refers to LDH > = 500;
SELECT T1.`First Date` FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.LDH >= 500 ORDER BY T1.`First Date` DESC LIMIT 1