GFR, the Glomerular Filtration Rate
This is how much blood is filtered each minute. It is the
best single measure of kidney function. It
falls as kidneys fail (end-stage CKD → very low GFR).
Creatinine is a waste product from muscle breakdown that the kidney clears. As GFR drops, creatinine rises → so we use it to stage kidney disease.
BUN (Blood Urea Nitrogen) is urea from protein breakdown. It also rises when the kidneys cannot clear it.