Not disputing the complexity/variability but isn't PGD2 a 24hr urine test?
Yep, and with a half life of seconds to minutes I think that is highly unreliable. An immediate blood test is needed. You might be able to get away with freezing the blood sample in liquid nitrogen which will buy you more time.
Now its not clear what secondary metabolites are stable that might appear in urine. However most of these have alternative causes, I don't know you can be sure you are measuring those accurately in urine.
If you don't think this is right, look up the PGD2 half life online. It varies by study, but 30 minutes is the longest I found, and one study was a couple of minutes.
If they have a stable reliable half-life for a reliable metabolite I would like to see their validation studies.
I think the all-time highest PGD2 would be found in African Sleeping Sickness.
It also looks like just sleeping badly will raise PGD2, which will induce fatigue related to needing to sleep. Almost everyone with ME, and maybe everyone, sleeps poorly. I expect most of us will have elevated PGD2 much of the time, quite separately from mast cell disorders.
I cannot say much about the validity of other tests, but I am wary of 24-hour urine testing for PGD2 unless they have very good reasons to do so, that are published, and preferably validated multiple ways ... even better if they are independently validated.