Estimation of Urinary Creatinine

Introduction

• Urinary creatinine is one of the most commonly measured biochemical parameters in clinical laboratories.
• It serves as an important indicator of kidney function, muscle metabolism, and the completeness of 24-hour urine collection.
• Since creatinine is produced at a relatively constant rate and is freely filtered by the kidneys, its measurement plays a vital role in diagnosing renal disorders, monitoring kidney function, and calculating creatinine clearance.

• Creatinine is a nitrogen-containing waste product formed from the normal metabolism of creatine phosphate in skeletal muscles.

• Creatine phosphate serves as a rapid energy reserve during muscle contraction.

• During muscle contraction:

  ATP → ADP + Pi + Energy

  To replenish ATP rapidly, the body utilizes the Lohmann Reaction:

  ADP + Phosphocreatine ⇌ ATP + Creatine
  (Catalyzed by Creatine Kinase)

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Principle

• Creatinine reacts with picric acid in an alkaline medium to form an orange-yellow colored complex called creatinine picrate.

Creatinine + Picric Acid + NaOH → Orange Creatinine-Picrate Complex

• The intensity of the colored complex is directly proportional to the concentration of creatinine present in the urine sample.
• The absorbance is measured at:
  • 520 nm
  • Blue-green filter in a colorimeter

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Reagents Required

• Picric acid (0.04 M)
• Sodium hydroxide (0.75 N)
• Stock creatinine standard (1 mg/mL)
• Working creatinine standard (0.01 mg/mL)
• Distilled water

Sample Preparation

Dilute:

5 mL urine → 500 mL

This produces a 1:100 dilution.

Procedure

Tube	Blank	Standard	Test
Diluted Urine	—	—	3.0 mL
Creatinine Standard	—	3.0 mL	—
Distilled Water	3.0 mL	—	—
Picric Acid	1.0 mL	1.0 mL	1.0 mL
Sodium Hydroxide	1.0 mL	1.0 mL	1.0 mL

Procedure Steps

1. Prepare blank, standard, and test tubes.
2. Add reagents as shown above.
3. Mix thoroughly.
4. Allow the reaction to stand for 15 minutes.
5. Measure absorbance within the next 30 minutes at 520 nm.

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Calculation

Urinary Creatinine (g/L)

= (OD Test − OD Blank) ÷ (OD Standard − OD Blank)

(The dilution factor has already been incorporated in the calculation.)

Normal Values

Daily Creatinine Excretion

Population	Normal Range
Males	14–26 mg/kg/day
Females	11–20 mg/kg/day

The value gradually declines with age because of reduced muscle mass.

Total Urinary Creatinine

0.7–1.2 g/day

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Interpretation

Urinary Creatinine Increases In

Urinary creatinine excretion may increase in the following conditions:

• Strenuous exercise – Increased muscle metabolism leads to higher creatinine production.
• Acromegaly – Increased muscle mass results in greater creatinine formation.
• Gigantism – Larger skeletal muscle mass increases creatinine excretion.
• Diabetes mellitus – Hyperfiltration in early diabetes may increase urinary creatinine excretion.
• Hypothyroidism – Reduced metabolism with increased muscle breakdown may elevate urinary creatinine.
• Acute infections and fever – Enhanced tissue metabolism can increase creatinine production.
• High-protein or meat-rich diet – Cooked meat contains creatinine, temporarily increasing urinary excretion.
• Athletes and bodybuilders – Greater muscle mass produces more creatinine.
• Creatine supplementation – May slightly increase urinary creatinine levels.

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Urinary Creatinine Decreases In

Urinary creatinine excretion may decrease in the following conditions:

• Chronic kidney disease (CKD) – Reduced glomerular filtration decreases creatinine excretion.
• Acute kidney injury (AKI) – Sudden decline in kidney function reduces urinary creatinine.
• Advanced renal failure – Severe loss of renal function markedly decreases creatinine excretion.
• Hyperthyroidism – Muscle wasting reduces creatinine production.
• Anemia – Reduced muscle metabolism may lower creatinine excretion.
• Muscular dystrophy – Progressive muscle loss decreases creatinine production.
• Paralysis – Muscle atrophy reduces creatinine formation.
• Neurogenic muscle atrophy – Loss of muscle mass lowers urinary creatinine.
• Polymyositis and inflammatory muscle diseases – Muscle damage decreases creatinine production.
• Severe malnutrition or cachexia – Reduced muscle mass leads to lower creatinine excretion.
• Leukemia – Associated muscle wasting may decrease urinary creatinine.
• Vegetarian diet – Lower intake of dietary creatinine results in slightly reduced urinary levels.
• Elderly individuals – Age-related decline in muscle mass reduces creatinine excretion.