Why does kidney disease result in glucose and protein being excreted in urine, but creatinine staying in the blood?
"If the filtration in the kidney is deficient, blood creatinine concentrations rise." https://en.wikipedia.org/wiki/Creatinine
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Healthy kidneys practically completely excrete creatinine, which means it is not reabsorbed in the kidneys. In kidney damage, less blood is filtered by the kidneys (decreased glomerular filtration), which means less creatinine is excreted, so it accumulates in the blood. So, creatinine excretion is not "impaired" but slowed down because of decreased blood flow through the kidneys.
In a healthy person, glucose glucose and proteins are not excreted into the urine in significant amounts. In kidney damage, there is increased permeability of glomerular capillary wall and decreased reabsorption of the proteins (proteinuria), so they can be excreted into the urine. Kidney disease only rarely results in the excretion of glucose into urine (glycosuria).
When blood glucose levels exceed "plasma glucose threshold," which is 180 mg/L (10 mmol/L), which is usually due to diabetes mellitus, some glucose will be excreted into the urine, even when the kidneys are not damaged (Diabetes, 2011).