Creatinine is a small harmless protein that comes from the muscles. The only way it goes out of the body is via the kidneys into the urine. Normally it is present in a concentration of 0.5 to 1.2mg/dl in the blood (plasma). When the concentration of creatinine rises in the blood it means that the kidneys are not able to take it out effectively
.... i.e. the kidneys are working less. Creatinine as such is not a toxin / poison but a higher concentration indicates kidney disease and shows indirect evidence that other unmeasurable toxins of kidney failure are also in high levels.