You have probably already looked up the test, but here's a link that explains what an elevated LDH could possibly indicate. As you can see, there are many possible causes and even some false elevations, so perhaps it's just a fluke?
http://www.labtestsonline.org/understanding/analytes/ldh/test.html

I know that I am being very closely followed at MD Anderson and have labwork every 3 months, and they do not do an LDH test...

Ginny M. SCC of Left lateral tongue Dx 04/06,Surgery MDACC 05/11/06: Partial glossectomy with selective neck dissection. T1N0M0 - no radiation. Phase III clinical trial ("EPOC" trial)04/07 thru 04/08 because tests showed a 65% chance of recurrence. 10 Year Survivor!