MaggieMag,
It is not uncommon to see immature white blood cells in the blood when you have infections or other illnesses that cause the marrow to work overtime. Viral infections can frequently allow immature white blood cells (not necessarily blasts) to go into the blood stream, although these immature granulocytes (IGs like metamyelocytes, myelocytes, and promyelocytes) don't really belong and could indicate marrow failure as well.
Rarity - as for the LDH, there are several different types (i believe 5), and one of them is actually used to determine if a patient has had a heart attack recently, or has liver issues. I am linking an article that goes into some detail.
https://www.healthline.com/health/la...drogenase-test