Creatinine Test: Measuring Kidney Function

Understanding Serum Creatinine

Serum creatinine is a blood test measuring a waste product created during muscle metabolism. The kidneys filter creatinine into urine for excretion. Elevated creatinine indicates reduced kidney function. This simple test is fundamental in assessing kidney health and guides treatment decisions.

What is Creatinine?

Creatinine is produced continuously from creatine in muscles at a relatively constant rate. Production correlates with muscle mass. The kidneys excrete nearly all filtered creatinine. Serum levels reflect glomerular filtration rate (GFR), the best measure of kidney function.

Creatinine Levels

Normal Ranges

• • Males: 0.6-1.2 mg/dL (or 53-106 μmol/L)
• • Females: 0.5-1.1 mg/dL (or 44-97 μmol/L)
• • Elderly: May be slightly higher
• • Children: Lower due to less muscle mass

Limitations of Serum Creatinine

Serum creatinine alone is not sensitive for early kidney disease detection. Up to 50% kidney function can be lost before creatinine rises noticeably. Variables like age, sex, race, and muscle mass affect interpretation. GFR calculation provides better assessment.

Glomerular Filtration Rate (GFR)

GFR is calculated from serum creatinine, age, sex, and race. It more accurately reflects kidney function than creatinine alone. Normal GFR is 90 mL/min/1.73m² or higher. GFR stages kidney disease:

• • Stage 1: GFR ≥ 90 (Normal kidney function)
• • Stage 2: GFR 60-89 (Mild kidney disease)
• • Stage 3: GFR 30-59 (Moderate kidney disease)
• • Stage 4: GFR 15-29 (Severe kidney disease)
• • Stage 5: GFR < 15 (Kidney failure)

Causes of Elevated Creatinine

• ✓ Chronic kidney disease (diabetes, hypertension)
• ✓ Acute kidney injury
• ✓ Urinary tract obstruction
• ✓ Heart failure and dehydration
• ✓ Certain medications (NSAIDs, ACE inhibitors)
• ✓ Muscle injury and rhabdomyolysis

When is Creatinine Testing Recommended?

Creatinine testing is part of routine checkups, essential for diabetic and hypertensive patients, recommended before starting medications affecting kidneys, and needed when evaluating urinary symptoms or renal failure risk.

Other Kidney Function Tests

• ✓ Blood Urea Nitrogen (BUN) - Assess protein metabolism
• ✓ BUN/Creatinine Ratio - Distinguish prerenal vs renal azotemia
• ✓ Cystatin C - Alternative to creatinine for GFR calculation
• ✓ Urine Albumin-to-Creatinine Ratio (UACR) - Detect kidney disease

Protecting Kidney Function

Maintain healthy blood pressure and blood glucose, avoid excessive NSAIDs, stay well-hydrated, limit protein and sodium intake, avoid smoking, and get regular checkups. Early detection and management can slow kidney disease progression.