What is the average index of the lactate dehydrogenase (LDH) for all patients with lactate dehydrogenase (LDH) within the normal range.
average index of the lactate dehydrogenase (LDH) refers to AVG(LDH); (LDH) within the normal range refers to LDH < 500
SELECT AVG(LDH) FROM Laboratory WHERE LDH < 500