What is Creatinine?

Creatinine is a waste product that your body produces naturally as part of normal muscle activity. Every day, your muscles break down a compound called creatine (which helps supply energy to muscle cells), and creatinine is the byproduct of this process. Under normal circumstances, creatinine is produced at a fairly constant rate and is eliminated almost entirely by your kidneys—filtered from your blood and excreted in your urine.[1][2]

Think of creatinine like the exhaust from a car engine, as a car produces exhaust as a byproduct of burning fuel, your muscles produce creatinine as a byproduct of their normal energy metabolism. And just as a car's exhaust system removes those fumes, your kidneys remove creatinine from your blood. When your kidneys are working well, they efficiently clear creatinine from your bloodstream. When kidney function declines, creatinine builds up in your blood—which is why measuring creatinine levels provides valuable information about how well your kidneys are working.[3][4]

Creatinine has been used as a marker of kidney function for over a century, and it remains the most commonly used test for assessing kidney health worldwide. It's simple, fast, inexpensive, and widely available—making it an ideal screening tool for detecting kidney problems.[4][5]

How Creatinine Reflects Kidney Function

Your kidneys are remarkable organs that filter approximately 180 liters (about 47 gallons) of blood every day. Their primary job is to remove waste products and excess fluid from your blood while keeping the substances your body needs. The rate at which your kidneys filter blood is called the glomerular filtration rate, or GFR—essentially a measure of how efficiently your kidneys are cleaning your blood.

Under steady-state conditions, the amount of creatinine your body produces equals the amount your kidneys excrete. When kidney function declines, less creatinine is filtered out, and blood levels rise. This inverse relationship—as kidney function goes down, creatinine goes up—is the foundation for using creatinine to assess kidney health.[1][6]

However, it's important to understand that creatinine is an indirect marker of kidney function. It doesn't measure GFR directly; rather, doctors use mathematical equations that incorporate your creatinine level along with factors like age and sex to estimate your GFR (called eGFR, for "estimated GFR").[6][7]

Understanding Your Creatinine Numbers

Creatinine is measured in milligrams per deciliter (mg/dL) in the United States. Normal values vary considerably based on several factors:[8][9][10]

Sex differences: Men typically have higher creatinine levels than women because they generally have more muscle mass. Average values are approximately:

• Women: 0.7-1.0 mg/dL

• Men: 0.9-1.2 mg/dL

Age effects: Creatinine levels tend to increase slightly with age, though this relationship is complex. In older adults, muscle mass typically decreases, which would lower creatinine production—but kidney function also naturally declines with age, which raises creatinine levels. These opposing effects can sometimes mask kidney disease in elderly patients.[8][9]

Muscle mass: Because creatinine comes from muscle, people with more muscle mass (such as athletes or bodybuilders) naturally have higher creatinine levels, while those with less muscle mass (such as frail elderly individuals or those with muscle-wasting conditions) have lower levels.[2][4]

A key point to understand is that a "normal" creatinine level doesn't necessarily mean normal kidney function. Because creatinine production varies so much between individuals, a creatinine of 1.0 mg/dL might represent excellent kidney function in a muscular young man but significantly impaired function in a small elderly woman. This is why doctors now use eGFR equations rather than relying on creatinine alone.[5]

From Creatinine to eGFR: Estimating Your Kidney Function

Because raw creatinine values are difficult to interpret without context, clinical laboratories now routinely calculate and report estimated GFR (eGFR) whenever creatinine is measured. The eGFR uses your creatinine level along with your age and sex to estimate how well your kidneys are filtering blood.[6][7]

The currently recommended equation in the United States is the CKD-EPI 2021 creatinine equation, which was developed to provide accurate estimates across diverse populations without using race as a variable.[11][12] This equation replaced earlier versions that included a race coefficient, following recommendations from the National Kidney Foundation and American Society of Nephrology.[12]

eGFR is expressed in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m²). The categories are:[6][11]

• Normal or high: ≥90 mL/min/1.73 m²

• Mildly decreased: 60-89 mL/min/1.73 m²

• Moderately to severely decreased: 30-44 mL/min/1.73 m²

• Kidney failure: 15 mL/min/1.73 m²

Chronic kidney disease (CKD) is defined as an eGFR less than 60 mL/min/1.73 m² or markers of kidney damage (such as protein in the urine) persisting for more than three months.[6][11]

The Limitations of Creatinine: What It Can and Cannot Tell You

While creatinine is invaluable as a screening tool, it has important limitations that both patients and healthcare providers should understand.[1][7][13][14] Creatinine is a "lagging" indicator. One of the most significant limitations is that creatinine levels may not rise until kidney function has already declined substantially. Studies suggest that normal serum creatinine can be maintained until GFR has been reduced by nearly 50%.[5][13] This means that by the time creatinine becomes abnormal, significant kidney damage may have already occurred.

Why Kidney Function Matters: The Link to Cardiovascular Disease and Mortality

Understanding your kidney function isn't just about your kidneys—it's about your overall health. Reduced kidney function is strongly and independently associated with increased risks of death, cardiovascular disease, and hospitalization.[15][16][17]

A landmark study of over 1.1 million adults found a graded relationship between declining eGFR and adverse outcomes. Compared to people with normal kidney function (eGFR ≥60 mL/min/1.73 m²), those with progressively lower eGFR had dramatically higher rates of death, cardiovascular events, and hospitalization:[18]. This figure demonstrates that even moderate reductions in kidney function—before patients reach the point of needing dialysis—are associated with substantially increased health risks. The relationship is continuous: the lower the eGFR, the higher the risk.

A massive meta-analysis of over 27 million individuals from 114 global cohorts confirmed these findings, showing that lower eGFR and higher levels of albumin in the urine were each independently associated with increased risks of 10 adverse outcomes, including kidney failure, all-cause mortality, cardiovascular mortality, heart attack, stroke, heart failure, and hospitalization.[15]

Importantly, Mendelian randomization studies (which use genetic variants to establish causal relationships) have provided evidence that mild-to-moderate kidney dysfunction is causally related to coronary heart disease risk—not just a marker of other underlying problems.[19] This highlights the potential value of preventive approaches that preserve kidney function.

Elevated creatinine and reduced eGFR can result from a range of conditions, most commonly diabetes and hypertension, which damage the kidney’s filtering units over time, as well as glomerulonephritis, polycystic kidney disease, urinary tract obstruction, nephrotoxic medications (such as NSAIDs, certain antibiotics, or contrast dye), and episodes of acute kidney injury from dehydration, infection, or drug reactions. Because many of these drivers are modifiable, kidney function can often be protected by maintaining blood pressure around 130/80 mm Hg (preferably with ACE inhibitors or ARBs when indicated), optimizing glycemic control in diabetes (target A1c around 7% for most people), and using kidney-protective therapies such as SGLT2 inhibitors when appropriate. [2][14][6][11][16]

References

1. Acute Kidney Injury. Ostermann M, Lumlertgul N, Jeong R, et al. Lancet (London, England). 2025;405(10474):241-256. doi:10.1016/S0140-6736(24)02385-7.

2. Chronic Kidney Disease. Webster AC, Nagler EV, Morton RL, Masson P. Lancet (London, England). 2017;389(10075):1238-1252. doi:10.1016/S0140-6736(16)32064-5.

3. Creatinine Test. National Library of Medicine (MedlinePlus).

4. The Metabolism of Creatinine and Its Usefulness to Evaluate Kidney Function and Body Composition in Clinical Practice. Ávila M, Mora Sánchez MG, Bernal Amador AS, Paniagua R. Biomolecules. 2025;15(1):41. doi:10.3390/biom15010041.

5. ACR Manual on Contrast Media 2025. ACR Committee on Drugs and Contrast Media. American College of Radiology.

6. Chronic Kidney Disease Diagnosis and Management: A Review. Chen TK, Knicely DH, Grams ME. JAMA. 2019;322(13):1294-1304. doi:10.1001/jama.2019.14745.

7. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International. 2024;105(4S):S117-S314. doi:10.1016/j.kint.2023.10.018.

8. Serum Creatinine Levels in the US Population: Third National Health and Nutrition Examination Survey. Jones CA, McQuillan GM, Kusek JW, et al. American Journal of Kidney Diseases : The Official Journal of the National Kidney Foundation. 1998;32(6):992-9. doi:10.1016/s0272-6386(98)70074-5.

9. Age- And Sex-Specific Reference Limits for Creatinine, Cystatin C and the Estimated Glomerular Filtration Rate. Hannemann A, Friedrich N, Dittmann K, et al. Clinical Chemistry and Laboratory Medicine. 2011;50(5):919-26. doi:10.1515/CCLM.2011.788.

10. Establishing Age/Sex Related Serum Creatinine Reference Intervals From Hospital Laboratory Data Based on Different Statistical Methods. Pottel H, Vrydags N, Mahieu B, et al. Clinica Chimica Acta; International Journal of Clinical Chemistry. 2008;396(1-2):49-55. doi:10.1016/j.cca.2008.06.017.

11. Chronic Kidney Disease: Prevention, Diagnosis, and Treatment. Goodbred AJ, Langan RC. American Family Physician. 2023;108(6):554-561.

12. The Primary Care Management of Chronic Kidney Disease (CKD) (2025). Jonathan Casey Brown DO MPH, Wendy Caesar-Gibbs RD, Cynthia Delgado MD FASN FNKF, et al. Department of Veterans Affairs.

13. Roadmap for Accelerating the Development of Biomarkers for Acute Kidney Injury. Raymond C. Harris MD FASN, Joseph V. Bonventre MD PhD FASN, Jacqueline Bowen, et al. Kidney Health Initiative (2022).

14. Performance and Pitfalls of the Tools for Measuring Glomerular Filtration Rate to Guide Chronic Kidney Disease Diagnosis and Assessment. Gama RM, Griffiths K, Vincent RP, Peters AM, Bramham K. Journal of Clinical Pathology. 2023;76(7):442-449. doi:10.1136/jcp-2023-208887.

15. Estimated Glomerular Filtration Rate, Albuminuria, and Adverse Outcomes: An Individual-Participant Data Meta-Analysis. Grams ME, Coresh J, Matsushita K, et al. JAMA. 2023;330(13):1266-1277. doi:10.1001/jama.2023.17002.

16. Chronic Kidney Disease. Herrington WG, Judge PK, Grams ME, Wanner C. Lancet (London, England). 2026;407(10523):90-104. doi:10.1016/S0140-6736(25)01942-7.

17. Association of Estimated Glomerular Filtration Rate and Albuminuria With All-Cause and Cardiovascular Mortality in General Population Cohorts: A Collaborative Meta-Analysis. Chronic Kidney Disease Prognosis Consortium, Matsushita K, van der Velde M, et al. Lancet (London, England). 2010;375(9731):2073-81. doi:10.1016/S0140-6736(10)60674-5.

18. Chronic Kidney Disease and the Risks of Death, Cardiovascular Events, and Hospitalization. Go AS, Chertow GM, Fan D, McCulloch CE, Hsu CY. The New England Journal of Medicine. 2004;351(13):1296-305. doi:10.1056/NEJMoa041031.

19. Mild-to-Moderate Kidney Dysfunction and Cardiovascular Disease: Observational and Mendelian Randomization Analyses. Gaziano L, Sun L, Arnold M, et al. Circulation. 2022;146(20):1507-1517. doi:10.1161/CIRCULATIONAHA.122.060700.