Estimation of creatinine in serum – Kidney function tests

Kidney function tests (KFT)
Estimation of creatinine in serum
  • It is a collective term for a variety of tests that can be done to evaluate how well kidneys are functioning
  • Many conditions can affect the ability of kidneys to carry out their vital functions , some lead to rapid decline(acute) in kidney functions and others lead to gradual decline(chronic)in function
  • Both chronic and acute result in a buildup of toxic waste substances in the blood

Kidney function tests as:
  • Creatinine
  • Urea
  • Blood urea nitrogen (BUN)
  • Uric acid
  • Levels of other elements regulated by kidney as : sodium,  potassium ,chloride ,calcium ,magnesium ,phosphorus


Estimation of creatinine in serum

Introduction
  • Creatinine is a chemical waste product that is generated from muscle metabolism
  • Creatinine is produced from creatine ,a molecule of major importance for energy production in muscles
  • Creatinine is transported through blood stream to the kidneys , the kidney filter out most of creatinine blood and dispose it in the urine
  • A high blood level of creatinine indicates that the kidneys may not be working properly
  • Creatinine is usually more accurate marker of kidney function than urea

Principle
  • Creatinine in alkaline medium reacts with picric acid to form a yellowish red complex of creatinine picrate
  • The intensity of color is measured at 520 nm
  • The rate of formation of color is proportional to the creatinine in the sample


Normal range
  • In adult male : 0.6 – 1.2 mg %
  • In adult female : 0.5 – 1.1 mg %
  • Note : females have a lower creatinine than males because they have less muscle mass

Reagents
  • R1 : Alkaline buffer
  • R2 : Picric acid

Procedure
  • Depend on a kinetic technique
  • Add 500µ R1 + 500 µ R2 incubate for 5 min at 37 , then add 100 µ serum that incubated also for 5 min at 37 then mix well and read


Serum creatinine levels are increased in:
  • The most common causes of chronic kidney disease in adults are high blood pressure and diabetes mellitus
  • Damage or swelling of blood vessels in kidney caused by infection or autoimmune diseases and prostate diseases
  • Bacterial infections of the kidney
  • Kidney stones or other causes of urinary tract obstruction
  • Acute tubular necrosis (death of cells in the kidneys’ tubes) caused by drugs or toxins

Serum creatinine levels are decreased in:
  • Mean lower muscle mass or muscle disease
  • Pregnancy can also cause low blood creatinine levels
  • Liver disease which affect creatine production which can cause low creatinine

 


Creatinine clearance
  • Creatinine clearance test measures creatinine levels in both a sample of blood and a sample of urine from a 24 hour urine collection
  • The results are used to calculate the amount of creatinine that has been cleared from the blood and passed into urine
  • The amount of creatinine produced in the body depend on muscle mass and is relatively constant for an individual
  • The amount of creatinine removed from the blood depend on both the filtering ability of the kidneys and the rate at which blood is carried to the kidneys
  • There are several versions of the creatinine clearance calculation , all of them include the measurement of the amount of creatinine in a blood sample collected just before or after the urine collection , the amount of creatinine in urine sample and the 24 hour urine volume
  • Creatinine clearance gives better information than the blood creatinine test on how well the kidneys are working
  • It can be calculated if values of creatinine urine concentration , urine flow rate and creatinine serum concentration are knownCreatinine clearance



Normal range
  • In males : 90 – 140 ml/min
  • In females : 87 – 170 ml/min

Creatinine clearance levels increased in:
  • Excessive exercise
  • Muscle injury
  • Carbon monoxide poisoning
  • Commonly seen during pregnancy
  • In patients with diabetes
  • Occur with large dietary protein intake

Creatinine clearance levels decreased in:
  • Serious kidney damage that result from:
  • Urinary tract blockage or low blood flow to the kidneys
  • Infection
  • Cancer


Glomerular filtration rate (GFR)
  • GFR is a test used to check how well the kidneys are working specially , it estimates how much blood passes through the glomeruli each minute (glomeruli are the tiny filters in the kidney that filter waste from the blood)
  • GFR can be calculated by combining the creatinine level in blood and some factors as : age , blood creatinine , height , weight , gender , ethnicity
  • GFR decrease with age
GFR


Normal range
  • GFR values can be classified as follows:
  • Normal : 100 – 140 ml/min
  • Mild kidney failure : < 90 ml/min
  • Moderate kidney failure : <60 ml/min
  • Severe kidney failure : < 30 ml/min
  • End- stage  kidney failure : < 15 ml/min which is incompatible with life , without dialysis or transplantation

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