<P> In Season 12, Carter appears in a four episode arc, working with a fellow doctor in Darfur, Sudan, where he is joined by Dr. Pratt and Debbie (Mary McCormack). Pratt informs him that Luka and Abby have reunited and are expecting a baby . </P> <P> In the Season 15 episode, "The Book of Abby", long - serving nurse Haleh Adams shows the departing Abby Lockhart a closet wall where all the past doctors and employees have put their locker name tags . However, Carter's is missing; according to Haleh, he did not want to do it because it was "defacing government property ." </P> <P> Carter later returns in the Season 15 episode, "The Beginning of the End", in which he rejoins the ER at County General . He explains to Cate Banfield that he is relocating to Chicago, and is looking to pick up some shifts . She agrees, after finding out that one of his teachers at the hospital was Mark Greene . He visits the Joshua Makalo Carter Center . At the end of the episode, it is shown that Carter is on dialysis because of amyloidosis developing from schistosomiasis which irreparably damaged Carter's remaining kidney . He is back in Chicago to be placed on the US transplant list . Working in the ER once again, he is shown to still be a good doctor with good judgment, but is still catching up with the latest medicines and techniques being used in the US . Eventually, his condition worsens, causing him to collapse while attending to a patient . While being treated by Dr Gates and Dr Morris, he goes into V - tach, but is brought back thanks to Morris's quick - thinking . He is transferred to Northwestern Medical Center . </P> <P> In the episode "Old Times", while a patient at Northwestern, he is visited by his mentor and good friend Dr Peter Benton, to whom he reveals that his marriage with Kem is going through a rough patch . In the same episode, Dr. Benton acts a back - seat driver and supervises the operation to make sure Carter is well taken care of . Later, Carter gets a new kidney and the transplant is a success . </P>

Why did dr carter need a kidney transplant