In a lecture, delivered before the Harvey Society two years ago, Folin1 said:
I venture to predict that we shall learn more concerning the abnormal or subnormal metabolism of the sick on the basis of creatinin and creatin determinations alone than could be learned in another thirty years by means of the nitrogen determinations of the past.
Creatin is methyl guanidin acetic acid and may be converted into creatinin by the withdrawal of one molecule of water. It would seem remarkable that creatin, which is the most important of the extractives of muscle and the chief constituent of broths and beef extracts, and creatinin, a constant constituent of the urine, should have received so little attention in clinical medicine. This seems to be even more striking when we consider that uric acid, which appears usually in smaller quantity, both absolutely and in nitrogen content, in human urine, has been
SEDGWICK JP. CREATININ AND CREATIN METABOLISM IN CHILDREN. JAMA. 1910;55(14):1178–1180. doi:10.1001/jama.1910.04330140022007