肌肉代谢的终产物，经肾脏滤过排出体外。血液中肌酐水平升高通常提示肾功能受损，因肾脏无法有效排泄；是评估肾小球滤过功能的重要指标。
Creatinine is a waste product generated by the normal breakdown of muscle tissue in the body. It is filtered out of the blood by the kidneys and excreted in urine. Because its production is relatively constant (depending on muscle mass) and it is primarily removed by the kidneys, creatinine levels in blood and urine are key indicators of kidney function. Below is a detailed breakdown of creatinine, including its role, reference values for blood and urine, factors affecting levels, and clinical significance.
What is Creatinine?
Creatinine is a byproduct of the metabolism of creatine phosphate, a molecule that provides energy to muscles. Unlike many other waste products, creatinine production is relatively steady in individuals with stable muscle mass—this makes it a reliable marker for assessing how well the kidneys are filtering waste from the blood (a process called glomerular filtration).
1. Blood Creatinine (Serum or Plasma Creatinine)
Blood creatinine is the most commonly measured form, as it directly reflects how efficiently the kidneys are clearing waste. Elevated blood creatinine often indicates reduced kidney function, as the kidneys are unable to filter it out effectively.
Reference Values for Blood Creatinine
Normal ranges vary slightly by lab, method, and population, but typical values are:
| Group | Reference Range (μmol/L) | Reference Range (mg/dL) |
|---|---|---|
| Adult males | 60–110 μmol/L | 0.7–1.2 mg/dL |
| Adult females | 45–90 μmol/L | 0.5–1.0 mg/dL |
| Children (2–12 years) | 27–62 μmol/L | 0.3–0.7 mg/dL |
| Newborns | 18–35 μmol/L | 0.2–0.4 mg/dL |
2. Urine Creatinine
Urine creatinine measures the amount of creatinine excreted in urine over a specific period (usually 24 hours) or in a single “spot” sample. It helps assess kidney function when paired with blood creatinine (e.g., to calculate the creatinine clearance rate) or to normalize other urine markers (e.g., urine protein/creatinine ratio).
Reference Values for Urine Creatinine
a. 24-Hour Urine Creatinine (most standardized)
This reflects total daily creatinine excretion, tied to muscle mass:
| Group | Reference Range (mmol/day) | Reference Range (mg/day) |
|---|---|---|
| Adult males | 7–18 mmol/day | 800–2000 mg/day |
| Adult females | 5.3–16 mmol/day | 600–1800 mg/day |
| Children (2–12 years) | 1.8–6.3 mmol/day | 200–700 mg/day |
b. Random (Spot) Urine Creatinine
A single urine sample (not 24-hour) is less standardized but typically ranges from:
- 3.6–20.5 mmol/L (40–230 mg/dL) for adults.
Spot urine creatinine is rarely interpreted alone; it is often used in ratios (e.g., urine protein/creatinine ratio) to account for variations in urine concentration (e.g., dehydration can make urine more concentrated, increasing creatinine levels).
3. Creatinine Clearance Rate (CCR)
This test estimates how well the kidneys filter creatinine, calculated using both blood and 24-hour urine creatinine levels. It is a more direct measure of glomerular filtration rate (GFR), the gold standard for kidney function.
Reference Values for CCR
- Adults: 80–120 mL/min
- Declines naturally with age (e.g., ~70 mL/min in adults over 70).
Factors Affecting Creatinine Levels
- Muscle mass: Athletes or individuals with more muscle have higher creatinine production (and thus higher blood/urine levels). Conversely, low muscle mass (e.g., in older adults, malnutrition) lowers levels.
- Hydration: Dehydration reduces blood volume, increasing blood creatinine temporarily. It also concentrates urine, raising urine creatinine.
- Diet: Eating large amounts of meat (which contains creatine) can slightly increase blood creatinine for a few hours.
- Kidney function: Impaired kidneys (e.g., chronic kidney disease) reduce creatinine clearance, leading to high blood creatinine and low urine creatinine.
- Medications: Some drugs (e.g., ACE inhibitors, diuretics) or supplements (e.g., creatine) can affect levels.
Clinical Significance
- High blood creatinine: May indicate kidney damage, reduced GFR, or acute/chronic kidney disease.
- Low blood creatinine: Rare but can occur with low muscle mass, liver disease (reduced creatine production), or severe malnutrition.
- High urine creatinine: Often due to dehydration (concentrated urine) or high muscle mass.
- Low urine creatinine: Suggests reduced kidney filtration (e.g., kidney disease) or low muscle mass.
- Urine protein/creatinine ratio: Elevated values (as in your earlier results) indicate proteinuria (excess protein in urine), a sign of kidney damage.
Key Takeaway
Creatinine is a critical marker for kidney health, but interpretation depends on context (e.g., age, muscle mass, hydration). Always discuss results with a healthcare provider, who will consider other tests (e.g., GFR, urine protein) and your overall health to make a diagnosis.