Details of Renal Function Tests

When patients are doubted to have renal injury or they have diagnosed as Kidney Disease, they are always required to receive renal function tests. So what does renal function tests test for indeed? Kidney is a very important organ with the function of excreting water, metabolic waste, toxin and medicine, reserving necessary substances for human body in order to maintain the balance of water, electrolyte as well as acid and base. Besides, kidney still has the function of internal secretion like synthesis and secreting renin and erythropoietin. Based on the above, renal function tests mainly test these functions of serum creatinine, urea nitrogen, internal creatinine clearance and serum uric acid, etc.

Firstly, Blood Uria Nitrogen (BUN) tests: there are more than 15 kinds of non-protein nitrogen in blood, and urea is the most, occupying about 50%. Then amino acid, uric acid, creatinine, creatine and nitrogen follow after. Urea is the final production of protein metabolism, synthesized in liver and transported to kidney through blood circulation. Then urea is excreted out of human body after glomerular filtration. When renal parenchyma is injured, glomerular filtrating rate also decreases at the same time, which can increase urea concentration of blood. For this reason, Blood Uria Nitrogen can be taken as a reflecting index of glomerular filtrating rate.

Secondly, serum creatinine tests: serum creatinine can be divided into endogenous creatinine and exogenous creatinine. Generally, per 20g muscle metabolizes 1mg creatinine per day. If people do not take strenuous exercise, endogenous creatinine secretion is steady every day. Serum creatinine is excreted out of human body after glomerular filtration, and renal tubule seldom reabsorb creatinine with less excretion. Under the condition of controlling exogenous creatinine intake, concentration of serum creatinine mainly depends on glomerular filtration. Therefore, concentration of serum creatinine is a reflection of glomerular filtrating function.

Thirdly, creatinine clearance rate test: plasma liquid amount filtrated by glomerulus per unit time is called glomerular filtrating rate (GFR). Main function of glomerulus is filtrating function, while the objective index of glomerular filtrating function is GFR. In per unit time, the amount of cleared endogenous creatinine of some certain volume plasma is called endogenous creatinine clearance rate. After filtrated by glomerulus, creatinine is difficult to be absorbed and excreted by renal tubule. So under the condition of strictly limiting diet and muscle activity, endogenous creatinine clearance rate should be some equal to GFR. For this reason, this test is a relative effective way to test glomerular filtrating function.

Fourthly, serum uric acid (UA) test: uric acid is the final production of purine metabolism in human body. Nucleic acid of food can be decomposed into purine, while nucleic acid in organic tissue can be decomposed into purine nucleoside. Both purine and purine nucleoside can produce uric acid after the process of hydrolysis, deamination and oxidization. Besides small part uric acid can be broke up in liver, most of uric acid is excreted out of body by kidney. All uric acid is filtrated by glomerulus, and 98% to 100% is reabsorbed by near-end glomerulus. So in normal condition, uric clearance rate is really low, which means that kidney can excrete creatinine easily but excrete uric acid difficultly. In early stage Kidney Disease, uric acid concentration increases firstly. This is beneficial to make an early diagnosis of renal injury.


Because kidney has so many functions and strong reservation ability, even the most sensitive test methods can not test out early and light renal injury. Besides, test result can be influenced by Heart Disease, Anemia and other non-Kidney Disease. Therefore, judgement of renal function can not be made only depending on some single test result. An exact diagnosis should also depend on comprehensive analysis of clinical symptoms.