Introduction
- Creatinine is an important nitrogenous waste product formed from creatine phosphate metabolism in skeletal muscle.
- It is produced continuously in the body at a fairly constant rate depending on muscle mass.
- Creatine phosphate acts as an energy reserve in muscles, and its non-enzymatic breakdown produces creatinine.
- After formation, creatinine enters blood circulation and is excreted mainly by kidneys through glomerular filtration.
- Very little creatinine is reabsorbed by renal tubules, so serum creatinine is a reliable indicator of kidney filtration function.
- Serum creatinine estimation is one of the most commonly performed renal function tests.
- It is usually interpreted together with serum urea and estimated glomerular filtration rate (eGFR).
- Increased serum creatinine usually indicates reduced kidney filtration capacity.
Principle
- Serum creatinine estimation is commonly based on Jaffe’s kinetic method.
- In alkaline medium, creatinine reacts with picrate reagent.
- An orange-red colored complex is formed.
Reaction
Creatinine + Alkaline Picrate → Orange-red Creatinine-Picrate Complex
Principle of Measurement
- The intensity of color formed is directly proportional to creatinine concentration.
- Absorbance is measured at 505 nm.
- Kinetic reading reduces interference from non-creatinine chromogens.
Specimen
Sample Type
- Serum is preferred specimen
- Plasma may also be used
- Heparinized plasma acceptable
Precautions
- Use non-hemolyzed sample
- Fresh specimen preferred
- Avoid contaminated sample
Storage
- Sample should be analyzed as early as possible
- Refrigeration recommended if delayed
Reagents
Working Reagent
- Picric acid
- Sodium hydroxide
Standard
- Creatinine standard concentration = 2 mg/dL
Role of Reagents
- Picric Acid – Reacts with creatinine to form colored complex
- Sodium Hydroxide – Provides alkaline medium for reaction
Materials Required
- Test tubes
- Micropipette
- Pipette tips
- Colorimeter / semi-auto analyzer
- Cuvette
- Timer
- Creatinine reagent kit
Procedure
| Components | Standard | Test |
|---|---|---|
| Working reagent | 1000 µL | 1000 µL |
| Standard | 50 µL | — |
| Sample | — | 50 µL |
Reading
- Mix properly
- Measure initial absorbance after 30 seconds (A1)
- Read again exactly after 120 seconds (A2)
- Measure against reagent blank
Calculation of Change
- ΔA = (A2 − A1)
Calculation
Formula
Creatinine (mg/dL) = Absorbance of Sample / Absorbance of Standard × 2
Example
- Sample absorbance = 0.45
- Standard absorbance = 0.50
Calculation
- 0.45 / 0.50 × 2 = 1.8 mg/dL
Normal Reference Values
| Group | Normal Value |
|---|---|
| Adults | 0.9 – 1.4 mg/dL |
Important Note
- Slightly lower in children
- Slightly higher in muscular individuals
Clinical Significance
- Serum creatinine is one of the most important biochemical markers used to assess kidney function.
- Creatinine is produced continuously from creatine phosphate metabolism in skeletal muscles.
- Because its production is relatively constant and mainly dependent on muscle mass, serum creatinine reflects renal filtration efficiency very accurately.
- Creatinine is filtered almost completely by the glomeruli and is minimally reabsorbed by renal tubules.
- Therefore even a small rise in serum creatinine may indicate reduced glomerular filtration rate (GFR).
- Serum creatinine is routinely used together with urea, electrolytes, and eGFR for renal assessment.
Increased Creatinine (Hypercreatininemia)
- Increased serum creatinine usually indicates reduced kidney filtration capacity.
- It occurs when kidneys fail to remove creatinine effectively from blood.
- Persistent elevation strongly suggests renal impairment.
Renal Failure
- Renal failure is the most important cause of elevated serum creatinine.
- In kidney failure, glomerular filtration decreases significantly.
- Creatinine accumulates in blood because excretion becomes inadequate.
Seen In
- acute kidney injury
- chronic kidney disease
- end-stage renal disease
Clinical Importance
- Rising creatinine indicates worsening renal function
- Serial monitoring helps assess disease progression
Glomerulonephritis
- Inflammation of glomeruli reduces filtration efficiency.
- Creatinine rises due to impaired glomerular function.
Clinical Importance
- Elevated creatinine indicates glomerular damage severity
Renal Obstruction
- Obstruction to urine flow causes creatinine retention.
Seen In
- kidney stones
- enlarged prostate
- urinary tract obstruction
Clinical Importance
- Sudden rise suggests obstructive renal pathology
Dehydration
- Severe dehydration reduces renal blood flow.
- Reduced perfusion lowers glomerular filtration.
- Serum creatinine rises mildly to moderately.
Shock
- Shock reduces renal perfusion significantly.
- Acute renal hypoperfusion causes creatinine elevation.
Seen In
- septic shock
- hemorrhagic shock
- cardiogenic shock
Cardiac Failure
- Congestive heart failure reduces renal circulation.
- Reduced blood flow decreases creatinine clearance.
Severe Muscle Injury
- Creatinine may rise when muscle breakdown increases.
Seen In
- trauma
- crush injury
- rhabdomyolysis
- muscular disorders
Clinical Importance
- Muscle damage increases creatinine production independent of kidney disease
High Protein Intake and Meat Intake
- Temporary mild increase may occur after heavy meat consumption.
- Creatinine production rises transiently.
Drug-Induced Increase
Drugs Causing Increased Creatinine
- aminoglycosides
- NSAIDs
- contrast agents
- nephrotoxic drugs
Clinical Importance
- Monitoring required during nephrotoxic therapy
Decreased Creatinine
- Decreased serum creatinine is less clinically significant but may indicate reduced muscle mass or increased renal clearance.
Reduced Muscle Mass
- Low muscle mass reduces creatinine production.
Seen In
- malnutrition
- muscle wasting
- elderly patients
Pregnancy
- Pregnancy increases glomerular filtration rate.
- Creatinine becomes slightly lower than normal.
Severe Liver Disease
- Reduced creatine synthesis may lower creatinine production.
Diagnostic Importance
- Serum creatinine is one of the best routine indicators of glomerular filtration rate.
- It is more specific than urea for renal function assessment.
Clinical Uses
- assesses renal filtration
- detects renal impairment
- monitors kidney disease
- evaluates acute kidney injury
- monitors dialysis patients
Creatinine Clearance
- Serum creatinine is used for creatinine clearance calculation.
- Creatinine clearance estimates glomerular filtration rate.
Clinical Importance
- detects early renal dysfunction
- assesses nephron loss
Estimated GFR (eGFR)
- Serum creatinine is used in eGFR formulas.
- eGFR helps classify kidney disease stages.
Clinical Importance
- early detection of chronic kidney disease
- monitoring progression
Urea and Creatinine Together
- Creatinine is interpreted along with urea.
- Urea/creatinine relationship helps differentiate cause of renal dysfunction.
Monitoring Importance
- Follow-up of chronic kidney disease
- Monitoring renal therapy
- Assessing dialysis adequacy
- Detecting nephrotoxic drug effect
