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Renal Function Test. Detailed class notes

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Biochemical Tests of Renal Function • Measurement of GFR ?Clearance tests ?Plasma creatinine ?Urea, uric acid and β2-microglobulin Biochemical Tests of renal function In acute and chronic renal failure, there is effectively a loss of function of whole nephrons  Filtration is essential to the formation of urine  tests of glomerular function are almost always required in the investigation and management of any patient with renal disease.  The most frequently used tests are those that assess either the GFR or the integrity of the glomerular filtration barrier. Measurement of glomerular filtration rate GFR can be estimated by measuring the urinary excretion of a substance that is completely filtered from the blood by the glomeruli and it is not secreted, reabsorbed or metabolized by the renal tubules.  Clearance is defined as the (hypothetical) quantity of blood or plasma completely cleared of a substance per unit of time. GFR = (Uinulin  V) Pinulin V is not urine volume, it is urine flow rate  Clearance of substances that are filtered exclusively or predominantly by the glomeruli but neither reabsorbed nor secreted by other regions of the nephron can be used to measure GFR.  Inulin 1. The Volume of blood from which inulin is cleared or completely removed in one minute is known as the inulin clearance and is equal to the GFR. 2. Measurement of inulin clearance requires the infusion of inulin into the blood and is not suitable for routine clinical use Biochemical Tests of Renal Function • Measurement of GFR ?Clearance tests ?Plasma creatinine ?Urea, uric acid and β2-microglobulin Creatinine  1 to 2% of muscle creatine spontaneously converts to creatinine daily and released into body fluids at a constant rate.  Endogenous creatinine produced is proportional to muscle mass, it is a function of total muscle mass the production varies with age and sex  Dietary fluctuations of creatinine intake cause only minor variation in daily creatinine excretion of the same person.  Creatinine released into body fluids at a constant rate and its plasma levels maintained within narrow limits  Creatinine clearance may be measured as an indicator of GFR.

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