Creatinine is a waste product derived from the digestion of protein and the normal breakdown of muscle tissue. The kidneys filter creatinine from the blood, and it is excreted in urine. A serum creatinine test, a common blood test, assesses how well your kidneys are filtering your blood.
What is Creatinine?
Creatinine is a waste product resulting from protein digestion and muscle tissue breakdown. Everyone has some creatinine in their blood, but elevated levels can indicate a potential kidney problem.
Purpose of the Serum Creatinine Test
The serum creatinine test is a blood test used to evaluate kidney function. It is often part of a routine health check, included in a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP). This test is frequently ordered to:
- Check kidney health in individuals at high risk for chronic kidney disease (CKD) or those with symptoms of acute kidney injury (AKI).
- Monitor changes in kidney function over time in people with CKD.
- Help healthcare professionals determine if medication adjustments are needed.
The Test: Risks and Preparation
The serum creatinine test is a low-risk procedure. Some people might experience slight pain or bruising where the needle is inserted, but these symptoms usually resolve quickly.
Most people don't need special preparation for the test. However, your doctor might give you specific instructions, such as fasting (avoiding food and drink except water) for several hours beforehand. In some cases, you might be asked to avoid eating cooked meat the night before the test, as it can temporarily increase creatinine levels and affect the estimated glomerular filtration rate (eGFR) results.
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During the test, a healthcare professional will draw a small amount of blood from a vein in your arm using a small needle. This usually takes less than five minutes. There are typically no restrictions after the test.
Interpreting Creatinine Test Results
While the serum creatinine test is common, it's not the sole indicator of kidney health. A "normal" creatinine level varies based on age, sex, body size, and other factors. Some individuals with a "normal" creatinine level may still have kidney disease, while others with a "high" level may not have kidney disease or may have a less severe condition than it appears.
The best way to assess kidney function is to look at your eGFR, which is calculated using your serum creatinine level, age, and sex. Cystatin C levels can also be used instead of or in addition to serum creatinine for eGFR calculation. An eGFR calculated using both creatinine and cystatin C is more accurate.
Factors Affecting Creatinine Levels
Certain factors can cause higher or lower creatinine levels, independent of kidney health. These factors can affect the interpretation of eGFR results.
Factors that can lead to higher creatinine levels:
- Eating large amounts of cooked meats
- Taking creatine supplements
- Recent high-intensity exercise
- High muscle mass
- Certain medications, such as cimetidine, cobicistat, dolutegravir, fenofibrate, ritonavir, or trimethoprim
Factors that can lead to lower creatinine levels:
- Following a vegan or vegetarian diet
- Low muscle mass
- Pregnancy
- History of amputation or muscle-wasting disease
- Severe liver disease (cirrhosis)
The actual effect of these factors on creatinine levels varies significantly from person to person. If you have concerns about interpreting your creatinine or eGFR level, consult your healthcare professional.
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Questions to Ask Your Healthcare Team
- When was my last serum creatinine test?
- What is my eGFR level based on my most recent creatinine level?
- When should I have my creatinine checked again?
- Are there any factors that might make my creatinine levels higher or lower than expected?
Rapid Weight Loss and Creatinine: What the Studies Say
Rapid weight loss (RWL) can affect kidney function and creatinine levels. Studies on athletes, particularly those in combat sports, have shown that RWL can lead to increased creatinine levels and other markers of acute kidney injury (AKI).
Rapid Weight Loss in Athletes
Athletes in weight-class sports often employ RWL methods before competitions. RWL is defined as a rapid reduction in body mass over a short period, typically 2%-10% in the week leading up to a competition. Methods used include increased physical activity, plastic suit training, caloric deficit, reduced fluid intake, and sauna use.
A study on male wrestlers found that high-intensity specific training combined with RWL significantly increased kidney function markers, including blood urea nitrogen, uric acid, and serum creatinine. This suggests that RWL, combined with intense training, can negatively affect kidney function.
Dehydration and Kidney Injury
Dehydration is a significant concern with RWL, and even mild dehydration can be a risk factor for kidney disease progression. Since athletes in combat sports may reduce their body mass multiple times yearly, frequent dehydration can impact kidney function.
Early detection of AKI in athletes who regularly use RWL methods is essential to prevent more severe kidney disease. The combination of RWL and high training workloads can disrupt homeostasis, leading to pathological biochemical kidney marker values.
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The Impact of Bariatric Surgery
Studies on patients undergoing bariatric surgery (Roux-en-Y gastric bypass) have also provided insights into the relationship between weight loss and kidney function. A prospective intervention study found that a large weight loss following RYGB was associated with a reduction in absolute measured glomerular filtration rate (mGFR), while mGFR adjusted for body surface area (BSA) was unchanged.
Plasma creatinine was reduced, leading to increases in creatinine-based eGFR (MDRD and CKD-EPI), while cystatin C-based eGFR remained unchanged. The reduction in lean limb mass (a surrogate for muscle mass) correlated with the reduction in plasma creatinine, suggesting that changes in muscle mass influence creatinine levels.
Weight Loss and Sympathetic Nervous Activity
Obesity is associated with elevated sympathetic nervous activity, which can contribute to renal complications. A study on overweight or obese Japanese men found that weight loss improved renal function, as evidenced by measures of creatinine and creatinine clearance (CCr). Basal plasma norepinephrine levels and total body fat mass were predictors of improvement in renal function associated with weight loss.
Weight Loss: Benefits and Risks
While weight loss is generally recommended for obese individuals to prevent diabetes and cardiovascular diseases, it's not always beneficial for kidney function. Some studies have reported U-shaped associations between body mass index (BMI) and incident proteinuria and microalbuminuria, as well as between weight change and incident CKD.
A study on healthy people in Japan found that decreasing BMI change was associated with decreasing eGFR change in males with high normal BMIs, while decreasing BMI change was associated with increasing eGFR change in overweight males. This suggests that the effects of weight loss on kidney function can vary depending on an individual's initial BMI.
Other Kidney Function Markers
Besides serum creatinine and eGFR, other markers can help assess kidney health:
- Cystatin C: A protein produced by body cells. Like creatinine, it's removed by the kidneys. High levels can indicate kidney problems.
- Measured Glomerular Filtration Rate (mGFR): A direct measure of how well the kidneys are removing waste products.
- Blood Urea Nitrogen (BUN): A waste product from protein breakdown. BUN levels are usually compared to creatinine and eGFR levels to evaluate kidney health.
- Urine Albumin-Creatinine Ratio (uACR): Measures albumin and creatinine in urine. High levels can be a sign of albuminuria.
- Urine Protein-Creatinine Ratio (uPCR): Measures all proteins in urine.
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