What is the Urea-to-Creatinine Ratio (UCR)
Urea comes from protein metabolism, and creatinine (CREA) comes from muscle metabolism; both are excreted through the kidneys.
The difference between the two is that after urea is filtered by the glomerulus, it will still be reabsorbed in the renal tubules. One important purpose of this reabsorption is to help with water reabsorption (the exact details are not yet fully understood and will not be expanded here). In contrast, creatinine, after being filtered by the glomerulus, basically is not reabsorbed in the renal tubules (whether or not the two substances are reabsorbed is the root of this issue).
By comparing the urea-to-creatinine ratio (UCR), it can help determine where the problem is occurring in the kidney.
Stable UCR
When kidney function is impaired, both substances cannot be normally excreted through the glomerulus, so blood CREA and blood UREA both rise proportionally, and the urea-to-creatinine ratio (UCR) remains basically stable.
Elevated UCR
Prerenal causes leading to elevated UCR: When blood volume deficiency, renal artery stenosis, or other factors cause insufficient renal perfusion leading to decreased glomerular filtration rate (GFR), both substances increase, but blood UREA rises faster because increased urea reabsorption is needed to promote water reabsorption. In comparison, blood CREA rises more slowly, so UCR elevates.
Postrenal causes leading to elevated UCR: Such as urinary tract stones, prostate enlargement, and other causes of postrenal obstruction, urine cannot be excreted at a normal speed, so the renal tubules have more time to reabsorb UREA leading to elevated blood UREA. Meanwhile, CREA is almost not reabsorbed, so blood CREA remains unchanged, resulting in…
In addition, since both UREA and CREA are metabolic products of the body—one from protein and one from muscle—situations involving abnormalities in protein and/or muscle metabolism may also cause UCR abnormalities. Conditions such as fever, use of steroids and tetracycline, and stress states—e.g., high-protein diet (especially in renal insufficiency), gastrointestinal bleeding—can all cause UCR elevation.
Decreased UCR
Diuretics increase the flow rate of tubular fluid, shortening the contact time between urea and renal tubular epithelial cells, reducing CREA reabsorption, thereby leading to decreased blood CREA.
Additionally, starvation, low-protein diet, severe liver failure, use of diuretics, and dialysis can all cause a decrease in UCR.