Sometimes when the kidney is diseased they can't do their job of excreting creatinine. Rule out dehydration, too much exercise
, too much protein in the diet
, bleeding within the intestine, hypothyroidism, adrenal cortical insufficiency, glomerulonephritis, amyloidosis, pyelonephritis, nephrosis, calcium nephropathy (also called lymphosarcoma), kidney cancer, and hydronephrosis.
As far as I know, there is not a problem w/ the egfr as long as it is below 60.