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Preface:
Some reports also show the urea-to-creatinine ratio (UCR) index. Some laboratories no longer report this index. Some instructors have asked about this index. So today, let's talk about this index and see if this is the answer you want.
PART01: What are urea and creatinine?
★ Urea (UREA/BUN): is the main final product of protein metabolism in the human body. Amino acids undergo deamination producing NH3 and CO2, which synthesize urea in the liver. Every gram of protein metabolized produces 0.3g of urea. Urea contains nitrogen accounting for 28/60, nearly half. Usually, the kidney is the main organ for urea excretion. Urea is filtered by the glomerulus and can be reabsorbed along all segments of the tubule. However, the faster the urine flow rate in the tubules, the less reabsorption occurs, reaching maximum clearance. Similar to serum creatinine, blood urea nitrogen can be within the normal range during early kidney impairment. Blood urea nitrogen concentration rises rapidly only when glomerular filtration rate declines below 50% of normal.
★ Creatinine (CREA): is a product of muscle metabolism in the human body; every 20g of muscle metabolism produces 1mg of creatinine. Creatinine is mainly excreted by glomerular filtration. Blood creatinine originates from both exogenous and endogenous sources. Exogenous creatinine is the metabolic product after meat intake; endogenous creatinine is produced by muscle tissue metabolism within the body. When meat intake is stable and muscle metabolism does not significantly change, creatinine production remains relatively constant.
★ From the textbook explanation above: both are metabolic products. Urea is a product of protein metabolism, while creatinine is a product of muscle metabolism. Since both are metabolized through the kidneys, when kidney function is abnormal, it leads to increased levels of these metabolic products.
PART02: What is the significance of the urea-creatinine ratio (UCR)?
★ Under normal kidney function (urea nitrogen and creatinine expressed in mg/dl; nowadays often expressed in mmol/l or umol/l, calculation of ratio requires ×250), the normal range of UCR is generally (10–20):1.
★ UCR can assist in diagnosing kidney injury, but elevated UREA and CREA levels are not always caused by kidney problems. For example, causes such as urea reabsorption or post-renal obstruction lead to elevated UREA without CREA increase. Simply put, UCR > 20 may indicate pre-renal or post-renal disease (understood as increased UREA while CREA remains unchanged or decreases). A decreased UCR may indicate intrinsic renal disease (understood as unchanged or decreased UREA and increased CREA).
★ Since both UREA and CREA are metabolic products — one from protein and one from muscle — abnormal protein and/or muscle metabolism can also cause abnormal UCR. Conditions such as fever, corticosteroid and tetracycline medications, stress states, high protein diets (especially with renal insufficiency), gastrointestinal bleeding, etc., can cause UCR elevation. Dehydration, heart failure, hypoproteinemia, reduced blood volume, hepatorenal syndrome can also increase UCR. Conversely, starvation, low protein diet, severe liver failure, diuretics, dialysis may cause UCR reduction. Haines et al. identified UCR as a metabolic marker of the persistent catabolic state in critically ill patients.
★ It is important to note that the factors influencing this ratio are complex. It is mainly related to liver and kidney function but many other factors (protein intake, muscle mass, water intake, cardiac function, medications, lifestyle, etc.) can affect this index. Therefore, careful attention should be paid during actual analysis. Don’t conclude organ problems simply based on this single ratio. Furthermore, for kidney diseases, changes in GFR and CCR are more critical and may be more suitable for clinical diagnosis than CREA/UREA and UCR.
Siman says: Regarding the UCR index, personally, I think its significance is limited. In the laboratory I belong to, this ratio is not reported because there are many influencing factors and incomplete clinical data. Using this index alone to analyze a patient’s current condition has some flaws. It is better to calculate CCR, GFR, etc. I have always advocated specific analysis for specific problems and not being rigid in thinking. Nothing more to say. The old saying continues: for testing, details determine success or failure, test for truth, verify with integrity!
Note: Due to my limited knowledge and experience, this article mainly organizes past literature and practical experience for communication, learning, and sharing only. Original copyright belongs to the original author. Content is not intended as clinical diagnosis or treatment reference. If errors exist, please private message me. Non-commercial use only, for professionals’ reference and communication. If there is infringement, please contact me for deletion. Thank you!
References:
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Jan Gunst, Kianoush B. Kashani, and Greet Hermans. The urea-creatinine ratio as a novel biomarker of critical illness-associated catabolism
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Shang Hong (Chief Editor). National Clinical Laboratory Operation Specifications (Fourth Edition)
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Roche Diagnostic Biochemical Reagent Instructions
Nan Huai-Chin:
A person can only rely on himself. No one is reliable. There is no reliable person in this world, not even your parents or your children. Only yourself. … If you do not stand up yourself and rely on others, you will never succeed.
