Creatinine Clearance (CrC), Glomerular Filtration Rate (GFR)
- 24 hours of urine is collected along with serum (clotted blood 3-5ml).
- When start urine collection notes the time and discards the first sample of urine. When 24 hours completed, then empty the bladder, last urine sample in the same urine container.
- Refrigerate the urine during collection or keep on ice.
- Collect the blood at the mid of urine collection.
- The patient should be hydrated so that there is >2 mL urine output per minute.
- Avoid coffee, tea, drugs during the test.
- Stop the medication (Drugs) like cortisone, or ACTH treatment.
- Exercise increases the creatinine level.
- An incomplete collection of the urine gives a false value.
- Meat rich diet increases the creatinine.
- Drugs. like aminoglycosides e.g. gentamicin, heavy metals, and nephrotoxic drugs increase the creatinine level.
- When GFR falls below 10 mL/min, the test becomes less accurate.
- In the case of high plasma protein, creatinine secretion is increased, leading to marked overestimation of GFR.
- Creatinine clearance reflects the Kidneys ability to excrete creatinine.
- Creatinine clearance is used to measure the glomerular filtration rate (GFR ) of the kidney.
- This test will give information on the renal functions like:
- Obstruction of the kidney.
- Acute or chronic renal failure.
- Dysfunction due to other causes like heart failure.
- Creatinine is a breakdown of creatine phosphate which has an important role in the contraction of muscles.
- The daily production of creatine and creatinine depends upon the muscle mass, which fluctuates very little.
- Creatinine is freely and mainly excreted by the kidney so this will reflect the filtration power of the kidney.
- Creatinine excretion by the kidney will depend upon the number of a millimeter of filtrate (urine) per minute is called glomerular filtration rate (GFR ).
- The filtrate will depend upon the amount of blood to be filtered.
- Further filtration will depend upon the ability of the glomeruli to act as a filter.
- The creatinine clearance is a measure of GFR.
- Creatinine clearance is roughly equal to the glomerular filtration rate (GFR).
- Plasma creatinine 15 to 20% is filtered and the rest goes back to the systemic circulation, renal vein.
- Amount filtered = Amount excreted
- GFR x Plasma creatinine = Urine creatinine x Urine volume
- The amount of blood to be filtered is decreased in renal artery atherosclerosis, dehydration, and shock.
- The ability of filtration by glomeruli will be decreased in glomerulonephritis, acute tubular necrosis, and other primary renal diseases.
- If one kidney is knocked out then another kidney is normal, can compensate for the filtration, and GFR will be in the normal limit.
- Creatinine clearance depends upon:
- With each decade of life, CrC decreases 6.5 mL/min because of a decrease in GFR.
- Urine collection is for 24 hours, so any error in the collection will give false results.
- Muscle mass varies among the people will also affect the CrC.
- decrease muscle mass will give decreased values.
- Ingestion of a large amount of meat for the time will increase the CrC.
Glomerular filtration rate (GFR)
Definition: GFR is defined as the number of millimeters of urine Filtrate made by the kidneys per minute.
- It is defined as the quantity of blood cleared of a substance per unit time and depends on the plasma concentration of the substance and excretion rate of the kidney which reflects GFR and renal plasma flow.
- Creatinine clearance is a measure of the glomerular filtration rate.
- Urine and serum creatinine levels are measured and the creatinine clearance rate is calculated.
Creatinine clearance is calculated as:
- Formula to calculate the Creatinine clearance: Urine volume X urine creatinine/plasma creatinine.
Corrected Creatinine clearance
Example if U = Urine creatinine in mg/dL
V = urine output in 24 hours (1440 minutes)
P = Plasma or serum creatinine in mg/dL
A = Body surface area in squamous meter
- U = number of mg /dL of creatinine excreted in the urine in 24 hours.
- V = volume of urine in mL per minute.
- P = serum creatinine mg/dL.
- 1.73/A is a body surface area.
- Can collect 2 hours sample of the urine. Because 24 hours collection is not as accurate as 2 hours.
- For good urine outflow, gives 500 ml of water 10 to 15 minutes before the start of the collection.
- Completely empty the bladder and discard this urine.
- When the patient feels full bladder, then start a collection of urine for exactly 2 hours and empty the last sample into the container at the end of 2 hours.
- With increasing age, the creatinine clearance also decreases because of a decrease in GFR.
- An incomplete collection of urine will give false values.
- Decreased muscle mass give decrease values.
- Increased ingestion of meat will increase the Creatinine clearance.
- Exercise may increase creatinine.
- drugs like gentamicin, cimetidine, and cephalosporin may give rise to increase the level.
|Age||mL /min / 1.73 m2|
|0 to 1 year||72|
|13 to 14 year||86|
|20 to 29 year||94 to 140||72 to 110|
|30 to 39 year||59 to 137||71 to 121|
|After this age with each decade, Value Decreases ∼6.5 mL/min|
- To convert to SI units x 0.00963 = Mean creatinine Clearance mL/s/m2.
- Male = 97 to 137 ml/min.
- Female = 88 to 128 ml/min.
- Children =70 to 140 ml/ min.
- Newborn = 40 to 65 ml / min.
Adult <40 years of age
- Male = 107 to 139 mL/min (1.78 to 2.32 ml/sec)
- Female = 87 to 107 mL/min (1.45 to 1.78 mL/sec)
- value decreases 6.5 mL/min/decade of life after the age of 20 years with decline in GFR.
Newborn = 40 to 65 mL/min
- Male = 85 to 125 mL/min
- Females = 75 to 115 mL/min
- Effect of the age:
- 50 to 75 years = subtract 5 mL for each 5 years interval.
- Over 75 years = subtract 8 mL for each 5 years interval.
- Artifacts that lower the value is:
- Incomplete urine collection.
- Presence of Ketones, Barbiturates.
- BSP, PSP, when the level is higher in the urine than the plasma.
- This has no clinical significance, suspect some error in the collection procedure.
- High cardiac output syndrome.
- When done with all precautions then it is a very sensitive indicator of decreased glomerular filtration rate.
- Diseases of the kidney with impaired renal function.
- Congestive heart failure.
- Cirrhosis with ascites.
- Dehydration (loss of body fluids).
- Bladder outlet obstruction.