Porphyria does indeed have modern standard diagnostic numbers.
That does not mean, of course, that we can clearly determine whether George III had it. That must make one very critical of the sources. It was Ida Macalpine and Richard Hunter who first proposed the retrospective diagnosis of porphyria in King George, beginning with an article in the Brit. Journal in Jan. 1966 and followed up in other articles. BUT it should be noted that a number of physicians who dealt with patients who suffered from porphyria wrote critical replies in the medical journals to the Macalpine and Hunter theory, raising many doubts about the soundess of their view.