Introduction
- Serum creatinine is a key biochemical parameter used to evaluate renal function.
- Creatinine is a waste product formed by the breakdown of creatine phosphate in muscles, released into the blood at a relatively constant rate.
- The kidneys filter creatinine from the bloodstream, and its serum level reflects the glomerular filtration rate (GFR).
- Elevated or reduced serum creatinine levels can indicate various renal or systemic conditions.
Principal
The serum creatinine is determined based on its reaction with picric acid in an alkaline medium (Jaffe’s method) to form a colored complex. This reaction is measured spectrophotometrically. Alternatively, enzymatic methods utilize creatininase, creatinase, or sarcosine oxidase to provide more specific and accurate readings.
Method
Two common methods for serum creatinine determination are:
- Jaffe’s Reaction:
- The reaction of creatinine with alkaline picrate produces a reddish-orange color.
- Interference from proteins, glucose, and other substances is a limitation.
- Enzymatic Method:
- Enzymes (e.g., creatininase) catalyze the conversion of creatinine to intermediates, producing measurable products like ammonia or hydrogen peroxide.
- These methods are highly specific and less prone to interference.
Requirements
Materials
- Patient serum sample
- Reagents:
- Picric acid solution (for Jaffe’s method) or enzyme reagents (for enzymatic methods)
- Alkaline buffer (e.g., sodium hydroxide)
- Distilled water
Instruments
- Spectrophotometer or automated biochemistry analyzer
- Pipettes and cuvettes
Procedure
Jaffe’s Method
| T | S | B | |
| PPF | 3 ml | – | – |
| Standard creatinine solution | – | 3 ml | – |
| Distilled water | – | – | 3 ml |
| Picric acid | 1 ml | 1 ml | 1 ml |
| NaOH | 1 ml | 1 ml | 1 ml |
- Incubate at room temperature for a specific time (e.g., 15 – 20 minutes).
- Measure the absorbance at 520 nm using a spectrophotometer.
- Perform a blank and standard control reading to calibrate results.
Enzymatic Method
- Add serum to a reagent containing enzymes that catalyze creatinine breakdown.
- Incubate the reaction mixture at a defined temperature and duration.
- Measure the absorbance or another quantifiable change (e.g., color development or fluorescence).
Calculation
The serum creatinine concentration is calculated using the formula:
Creatinine concentration = (Sample Absorbance – Blank Absorbance)/(Standard Absorbance – Blank Absorbance)×Standard Concentration (mg/dL)
Normal Values
- Adults (Male): 0.7–1.3 mg/dL (62–115 µmol/L)
- Adults (Female): 0.5–1.1 mg/dL (44–97 µmol/L)
- Children: 0.3–0.7 mg/dL (26–62 µmol/L)
Clinical Significance
- Increased Serum Creatinine:
- Acute or chronic kidney disease
- Dehydration
- Urinary tract obstruction
- Rhabdomyolysis or muscular dystrophy
- Decreased Serum Creatinine:
- Reduced muscle mass (e.g., in elderly or malnourished individuals)
- Pregnancy (due to increased GFR)