Quoted straight from Erowid:-
I believe the half life of MDMA is 9 hours (+/- 2.25 hours)[1]. If you redo your otherwise correct calculation, you get less than 1% remaining at 72 hours (rather than 48).
However, this is only one step of the calculation as far as how long MDMA is detectable in urine. MDMA is metabolized into a variety of other molecules including HMMA, MDA, and HMA. Respectively these have half-lives of approximately 11, 25, and 37 hours. MDA triggers the standard Amphetamine urine test, while MDMA itself will trigger a Methamphetamine urine test. We don't have any information about whether HMMA or HMA metabolites will trigger the amphetamine/methamphetamine test in a urinalysis and whether the quantities present from MDMA metabolism are sufficient for such a trigger.
As you also state, there are many factors which go into determining how long a substances is detectable in urine. The specific metabolism of the person being tested, the amount and frequency used, the method of administration, fluid intake, amount of exercise, etc.
Traditional urine tests are somewhat unreliable at detecting MDMA use with an Amphetamine test, but there are several new test designs out which are supposed to be quite a bit more reliable[2]. At this point we believe that MDMA is detectable for approximately the same duration as Amphetamine/Methamphetamine (3 days average, up to 5 days with heavy frequent use), but we haven't seen conclusive data on this.
References
1. Pharmacology of MDMA in humans,
de la Torre R, Farre M, Roset PN, Hernandez Lopez C, Mas M, Ortuno J, Menoyo E, Pizarro N, Segura J, Cami J
Ann N Y Acad Sci, 2000; 914:225-37
2) Profiles of urine samples taken from Ecstasy users at Rave parties: analysis by immunoassays, HPLC, and GC-MS.
Zhao H, Brenneisen R, Scholer A, McNally AJ, ElSohly MA, Murphy TP, Salamone SJ
J Anal Toxicol 2001 May-Jun;25(4):258-69
