I've only just started thinking about LDH because, although I've had MGUS for 2.5 years, I only had it measured when I changed hospitals in August this year. Since then I've had 2 LDH tests. The 1st one was 392, the 2nd (last week) was apparently 140.
I am not sure what has caused the drop but I understand that the test result can be affected by how the blood was taken - ie the cells can be damaged during the blood test.
I understand the normal range for LDH is 150-500 so if your other bloods are normal I wouldn't worry too much.
Paul