The kidneys are vital organs responsible for excreting waste products from the body, regulating fluid and electrolyte balance, and producing hormones. Thirty-seven million Americans have kidney disease, 90% of whom are unaware they have it. Kidney disease can lead to heart attack, stroke, kidney failure, and death. However, this is preventable, and the progression of kidney dysfunction can often be stopped and slowed. The key to kidney disease prevention is adopting a healthy lifestyle and frequently monitoring kidney function for early disease detection. The creatinine blood test is a key marker of kidney function. Understanding the creatinine blood test is imperative for the early detection and management of kidney dysfunction. (14)

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What Is Creatinine?

Creatinine is a metabolic waste product formed during the breakdown of creatine phosphate in muscle cells. Creatinine is produced at a constant rate, released into the bloodstream, and filtered from the blood by the kidneys. Creatinine levels, therefore, can serve as an indicator of kidney function. Healthy kidneys efficiently filter out waste products like creatinine, maintaining stable blood creatinine levels. Elevated creatinine levels may signify compromised kidney function or other underlying health issues. Clinical tests often measure creatinine levels in blood and urine to assess kidney function and overall health, with creatinine clearance tests providing more detailed information about kidney efficiency in handling and eliminating creatinine. Serum creatinine is also utilized in estimating glomerular filtration rate (eGFR). eGFR equations are superior to serum creatinine in assessing kidney function and classifying kidney disease because they include race, age, and gender variables. (10)

The Creatinine Blood Test: Procedure and Preparation

The creatinine blood test is typically conducted to assess kidney function and overall health. It can be ordered independently or as part of a more detailed panel called the comprehensive metabolic panel (CMP). These tests are available for order through Rupa Health as Creatinine or Comprehensive Metabolic Panel by Access Medical Laboratories. During the blood draw procedure, a healthcare professional will typically locate a suitable vein, often in the arm, by applying a tourniquet to make the veins more visible and accessible. The area will then be cleaned with an antiseptic. The actual blood draw involves inserting a needle into the vein to collect a sample of blood, which is then sent to a laboratory for analysis. Patients may feel a brief pinch or sting during the needle insertion, but the procedure is generally quick and well-tolerated. Following the blood draw, patients are advised to apply pressure to the site to minimize bleeding and may be given a bandage.

Alternatively, a Creatinine Clearance test, such as the one offered by Doctor's Data, measures the rate at which the kidneys filter and excrete creatinine from the blood. It is the best measure of actual GFR. This test involves a 24-hour at-home urine collection. At the beginning of the collection period, the patient should be instructed to void and discard the specimen. The patient then continues to collect all urine, including the final specimen, voided during the 24-hour collection period. (16)

To prepare for a creatinine test, your doctor may recommend that you fast overnight before a creatinine blood test, but this is generally not required. The following medications and supplements should be avoided for 48 hours before and during the test collection period (for the serum and urine tests):

- Cephalosporin antibiotics

- Non-essential medications*

- Dietary supplements, especially those containing creatine

*Do not discontinue prescription medications without consulting your healthcare provider first

Normal and Abnormal Creatinine Levels

Serum creatinine is reported as milligrams per deciliter of blood (mg/dL). Normal creatinine levels may vary slightly based on individual labs, but the typical normal range for serum creatinine is (5, 7, 11):
- Adult men: 0.74 to 1.35 mg/dL (65.4 to 119.3 micromoles/L)

- Adult women, 0.59 to 1.1 mg/dL (52.2 to 97.2 micromoles/L)

- Adolescents: 0.5 to 1.0 mg/dL

- Children: 0.3 to 0.7 mg/dL

- Infants: 0.2 to 0.4 mg/dL

- Newborns: 0.3 to 1-2 mg/dL

Creatinine clearance is reported as milliliters of creatinine per minute. The typical range for creatinine clearance is (6, 17):

- Adult men: 97 to 139 mL/min

- Adult women: 87 to 128 mL/min

- Newborns: 40 to 65 mL/min

Factors Influencing Creatinine Levels

High and low serum creatinine levels and creatinine clearance can provide important information about kidney function and overall health.

Elevated serum creatinine levels and reductions in creatinine clearance often indicate impaired kidney function. The kidneys are crucial in filtering and excreting creatinine from the blood. When kidney function is compromised, creatinine clearance decreases, leading to an accumulation of creatinine in the bloodstream. High serum creatinine levels may suggest conditions such as acute or chronic kidney disease, glomerulonephritis, urinary tract obstruction, dehydration, or rhabdomyolysis (muscle breakdown). (5, 17)

Abnormally low levels of serum creatinine are less common but may be associated with conditions such as muscle atrophy or severe malnutrition. Increased creatinine clearance (kidney hyperfiltration) may occur during pregnancy. (11, 17)

Several factors can influence creatinine levels, including age, ethnicity, gender, muscle mass, diet, and certain medications. It's crucial for healthcare professionals to be aware of these factors and to consider them in the context of each individual's health status. They should review patients' medical history, medications, and dietary habits to identify potential contributors to creatinine fluctuations. Serial monitoring of creatinine levels over time can also provide valuable information about trends and changes in kidney function. (4, 11)

Factors that can lead to higher creatinine levels include (4):

- Eating large amounts of cooked meats

- Taking creatinine supplements

- Taking certain medications: sulfamethoxazole/trimethoprim, cimetidine, cephalosporin antibiotics, cobicistat, dolutegravir, fenofibrate, ritonavir

- Recent high-intensity exercise

- High muscle mass

Factors that can lead to lower creatinine levels (4):

- Low muscle mass

- Age: Reduced muscle mass often seen in elderly patients may lead to lower values

- Sex: Women often have a lower blood creatinine level than men due to less muscle massEating a vegan or vegetarian diet

- Pregnancy

- History of amputation or muscle-wasting disease

- Liver disease

Creatinine Test in the Context of Kidney Disease

Given the variability of creatinine levels influenced by the abovementioned factors, there are better ways to understand kidney health than using creatinine as a standalone test. The best way to understand kidney function is to order creatinine in conjunction with other kidney function tests, such as eGFR, blood urea nitrogen (BUN), albumin/creatinine ratio, and cystatin C. (4)

Creatinine levels are integral for estimating the GFR, a key indicator of kidney function and a parameter for classifying kidney disease. The Modification of Diet in Renal Disease (MDRD) Study equation and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation are the most widely used for estimating GFR in patients aged 18 and over. Once GFR is calculated, it can be used to classify kidney disease. (9)

A persistent elevation of creatinine over time, especially in conjunction with other abnormal kidney markers, may reflect the loss of kidney function and the presence of chronic kidney disease. Additionally, creatinine testing plays a role in identifying acute changes in kidney function, as elevated levels can indicate acute kidney injury, often seen in conditions like infections or drug toxicity. In the monitoring of kidney diseases, creatinine levels are tracked over time to assess changes in eGFR and disease progression. Regular monitoring also enables healthcare providers to adjust medication dosages, particularly for drugs excreted by the kidneys, optimizing treatment plans. Moreover, creatinine testing aids in evaluating the efficacy of interventions and allows for the early detection of complications associated with kidney dysfunction, facilitating timely and informed clinical decisions. (2, 12)

MDRD Equation

MDRD Equation: GFR (mL/min/1.73 m2) = 175 × (Scr)-1.154 × (Age)-0.203 × (0.742 if female) × (1.212 if African American)

Notes:

- Scr is serum creatinine in mg/dL

- The equation has not been validated in patients older than 70

CKD-EPI Equation

CKD-EPI Equation: GFR = 141 × min (Scr /κ, 1)α × max(Scr /κ, 1)-1.209 × 0.993Age × 1.018 [if female] × 1.159 [if black]

Notes:

- Scr is serum creatinine in mg/dL

- κ is 0.7 for females and 0.9 for males

- α is -0.329 for females and -0.411 for males

- min indicates the minimum of Scr /κ or 1

- max indicates the maximum of Scr /κ or 1

Kidney Disease Stages Based on GFR

The 5 Stages:

Stage 1: GFR > 90 ml/min/1.73 m²

Stage 2: GFR 60-89 ml/min/1.73 m²

Stage 3a: GFR 45-59 ml/min/1.73 m²

Stage 3b: GFR 30-44 ml/min/1.73 m²

Stage 4: 15-29 ml/min/1.73 m²

Stage 5: GFR < 15 ml/min/1.73 m²

Functional Medicine Perspective on Kidney Health

A functional medicine approach to maintaining kidney health emphasizes a holistic perspective that addresses the underlying factors contributing to kidney dysfunction and promotes overall well-being. Lifestyle and dietary changes play a pivotal role in this approach. Maintaining adequate hydration is fundamental for kidney health, ensuring optimal filtration and waste removal. A diet rich in fruits, vegetables, whole grains, and lean proteins supports a nutrient-dense, kidney-friendly nutrition profile. Reducing salt and sugar intake helps manage blood pressure and blood sugars, key determinants of kidney function. Regular physical activity promotes cardiovascular health, indirectly benefiting kidney function. Managing stress through practices like mindfulness and adequate sleep is integral, as chronic stress can impact kidney health. (12, 19)

Next Steps After Receiving Creatinine Test Results

Upon receiving creatinine test results, it is crucial to schedule a follow-up appointment with a healthcare provider to discuss and interpret the findings. The healthcare provider will assess kidney function by considering the implications of the results within the broader context of the individual's health history. If creatinine levels are elevated, the provider may explore potential underlying causes, such as kidney disease, dehydration, or certain medications. Lifestyle modifications and natural supplements may be discussed to optimize kidney health. Depending on the findings, the healthcare provider may recommend further testing or monitoring to assess kidney function over time. It is important to be aware of when to seek further evaluation, particularly if there are significant or concerning changes in creatinine levels. Following the healthcare provider's recommendations for interventions or lifestyle adjustments is crucial for ongoing kidney health monitoring and management. Regular communication with the healthcare team ensures a personalized approach to maintaining kidney health.

The following signs and symptoms may be indicative of abnormal kidney function, warranting evaluation by and follow-up with a healthcare professional:

- Fatigue and weakness

- Increased or decreased urine output

- Foamy or bubbly urineBlood in the urine

- Frequent nighttime urinationSwelling in the extremities

- Shortness of breath

- Nausea and vomiting

- Metallic taste in the mouth

- Skin itching and rashes

- Anemia

- Flank pain

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How to Interpret Your Creatinine Blood Test: Final Thoughts

Understanding and correctly interpreting creatinine test results hold paramount importance for kidney health. Monitoring creatinine levels provides valuable insights into kidney function, aiding in the early detection of potential issues and allowing for timely intervention. Regular health check-ups, including creatinine testing, are essential for comprehensive well-being. Consulting with healthcare professionals for personalized advice ensures that individuals receive tailored guidance, fostering a proactive and informed journey toward optimal kidney function and overall health.