The best method to evaluate the success of chelation is to measure 24 hr urine Gd. I think the minimum is a 24 hr urine for Gd within 1 week prior to chelation (maybe the easiest and most accurate is the day before, bringing the urine in the day of chelation). The most dramatic urine Gd content should be the day after the first chelation, which is after Ca-DTPA (if using the standard Ca-/Zn-DTPA protocol) or after the sole injection of Zn-DTPA (if using the Zn-DTPA only infusion), so collection starting immediately after chelation finished. I think the next day collection, after Zn (if using the Ca-/Zn-DTPA protocol) is good as well, but not as essential. I would do the same at the 5th chelation, so pre-chelation gives a baseline of how much Gd has been generally eliminated (reflected by the amount eliminated without provocation), and after the first day following chelation informs if Ca-DTPA is still promoting elimination of Gd (as a reflection of the amount of mobilizable Gd is still in the system).
The end-point of treatment is when the individual feels normal/near normal because some residual Gd in bone may be there forever. Note also that the patient may still be sick from GDD even if no detectable Gd is present any more in urine, as the disease can take on a pure auto-immune type disease status. This is why mitigating the auto-immune type process is also essential for most individuals.