Serum creatinine: assessment of kidney function

Creatinine is a waste product formed during the normal breakdown of muscle tissue and is partly related to the digestion of dietary protein. The kidneys remove it from the blood and excrete it in the urine, so as the kidneys' filtration function declines, serum creatinine concentrations typically increase. This is why creatinine testing has remained a basic laboratory test for assessing kidney function for decades. [1]

Serum creatinine is convenient because it is readily available in almost any laboratory, is quick to perform, and is well suited for follow-up. It is widely used in preventive screenings, when acute kidney injury is suspected, when monitoring chronic kidney disease, and before prescribing medications whose doses depend on renal filtration. [2]

However, modern nephrology views this indicator differently than before. Creatinine is now viewed not as a standalone diagnosis or even as the best single indicator of kidney function, but as the initial laboratory basis for calculating the glomerular filtration rate. The estimated glomerular filtration rate better reflects the kidneys' filtration capacity than the creatinine number alone. [3]

It's also important to understand that early chronic kidney disease often has no symptoms. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) emphasizes that kidney disease typically doesn't cause any symptoms in its early stages, so laboratory testing allows for early detection. This is one reason why creatinine testing is still necessary, but no longer in isolation, but as part of a more comprehensive regimen. [4]

Today, the correct clinical question is not "what is the patient's creatinine level?" but "what is the estimated glomerular filtration rate, is there albuminuria, and are the changes stable over time." This shift distinguishes the current article from the outdated one. KDIGO 2024 recommends screening people at risk for chronic kidney disease using both glomerular filtration rate and urine albumin measurements, rather than creatinine alone. [5]

Table 1 shows what creatinine can actually tell a doctor and what it cannot tell on its own.[6]

What does creatinine show?	What it doesn't show on its own
Possible decrease in renal filtration function	The exact reason for the violation
The need to calculate the glomerular filtration rate	Full stage chronic kidney disease
Dynamics with repeated measurements	Presence or absence of albuminuria
A signal for a more detailed assessment	The exact degree of risk without taking into account urine and clinical context
Possible acute or chronic disorder	The onset of kidney damage in real time

What determines creatinine levels and why is the “normal” different for different people?

One of the main reasons for interpretation errors is that serum creatinine is not solely dependent on the kidneys. The National Kidney Foundation notes that creatinine production is influenced by age, body type, muscle mass, and diet. Therefore, the same creatinine level in two people may indicate different actual kidney function. [7]

In people with large muscle mass, creatinine may be higher without a true reduction in filtration. Conversely, in older individuals, in patients with sarcopenia, malnutrition, amputations, severe muscle wasting, or severe illness, creatinine may remain relatively low even with a significant decline in kidney function. This is why relying solely on the "reference range" is dangerous. [8]

Diet also plays a significant role. The National Kidney Foundation notes that large amounts of cooked meat can temporarily increase creatinine and affect the estimated glomerular filtration rate. Creatine supplements and recent intense physical activity can also alter levels. A vegetarian diet and low muscle mass can have the opposite effect. [9]

Some medications increase creatinine without actually decreasing filtration because they reduce tubular secretion of creatinine. The National Kidney Foundation lists such medications as trimethoprim, cimetidine, dronedarone, dolutegravir, cobicistat, rilpivirine, and some antineoplastic agents. This is a very important practical pitfall: the number on the form may increase, but the actual glomerular filtration rate will not change significantly. [10]

Finally, the laboratory method itself is important. The NIDDK points out that creatinine measurements can be distorted by endogenous and exogenous substances, while enzymatic methods are generally less susceptible to interference from drugs and some interfering substances than older methods. The National Kidney Foundation also emphasizes that accurate calculation of glomerular filtration rate requires standardized creatinine measurements traceable to isotope-based mass spectrometry. [11]

Table 2 helps to quickly understand which factors may bias the result upward or downward without direct relation to the true filtration of the kidneys.[12]

Factor	What happens most often to creatinine?	Why is this important?
Large muscle mass	Promotion	May give the appearance of kidney disease with normal filtration
Low muscle mass	Decrease	May mask actual decline in kidney function
Meat cooked the day before	Temporary increase	May temporarily impair estimated glomerular filtration rate
Creatine supplements	Promotion	May distort interpretation
Intense physical activity	Promotion	Sometimes it gives a short-term shift
Pregnancy	Decrease	May cause your creatinine levels to become unusually low
Cirrhosis	Decrease	Often masks a significant reduction in filtration
Trimethoprim, cimetidine, dolutegravir and some other drugs	Promotion	An increase is possible without a real decline in glomerular filtration

How to correctly assess kidney function using creatinine today

The current standard begins with converting serum creatinine to estimated glomerular filtration rate (EGFR). For adults, the 2021 race-neutral equations of the Chronic Kidney Disease Epidemiology Collaboration are widely used. In 2025, the College of American Pathologists recommended that laboratories use the 2021 race-neutral creatinine equation and automatically report the EGFR along with creatinine. [13]

However, an estimated glomerular filtration rate based on creatinine alone is not always sufficient. The NIDDK emphasizes that a combination of creatinine and cystatin C is preferable and more accurate than using creatinine alone, especially when the estimated glomerular filtration rate is near a clinically important threshold, such as when deciding drug dosing or pre-transplant evaluation. [14]

KDIGO 2024 takes another important step and recommends, whenever possible, adding cystatin C to creatinine for the diagnosis and staging of chronic kidney disease. This is particularly useful in situations where creatinine may be inaccurate due to atypical muscle mass, severe comorbidity, cirrhosis, muscle wasting, severe obesity, and a number of other conditions. [15]

The second essential component of modern assessment is urine albumin. KDIGO 2024 specifically recommends screening individuals at risk for chronic kidney disease using two markers: an estimate of glomerular filtration rate (EGFR) and a urine albumin measurement. The NIDDK and the National Kidney Foundation emphasize that albuminuria can be present even when the estimated glomerular filtration rate (EGFR) is above 60 mL per minute per 1.73 m² and appears "normal." [16]

Therefore, modern interpretation of creatinine is not a search for a single "normal number," but a comparison of four elements: serum creatinine, estimated glomerular filtration rate, urine albumin-to-creatinine ratio, and the dynamics of these parameters over time. This approach allows us to distinguish temporary laboratory abnormalities from actual chronic kidney disease and prevent early kidney damage from being missed. [17]

Table 3 shows the components that should comprise a basic kidney assessment today.[18]

Evaluation component	Why is it needed?
Serum creatinine	Initial laboratory value for calculating filtration
Estimated glomerular filtration rate	The main indicator of the filtration function
Urine albumin to creatinine ratio	Detection of glomerular filter damage and risk stratification
Cystatin C when indicated	Improving accuracy when creatinine may be unreliable
Repeated measurement in time	Confirmation of sustainability of changes and assessment of dynamics

KDIGO 2024 defines chronic kidney disease as abnormalities in kidney structure or function that have been present for at least 3 months and are significant for health. Classification is based on the cause, glomerular filtration rate, and albuminuria category. This means that a single creatinine reading alone does not permit a diagnosis of chronic kidney disease without confirmation of the chronic nature and urine testing. [19]

Table 4 reflects the glomerular filtration categories used in the current KDIGO classification.[20]

Category	Estimated glomerular filtration rate
G1	90 ml or more per minute per 1.73 m²
G2	60-89 ml per minute per 1.73 m²
G3a	45-59 ml per minute per 1.73 m²
G3b	30-44 ml per minute per 1.73 m²
G4	15-29 ml per minute per 1.73 m²
G5	less than 15 ml per minute per 1.73 m², renal failure

Albuminuria is as important as filtration. KDIGO continues to use three categories of albuminuria: A1, A2, and A3. The higher the urine albumin-to-creatinine ratio, the higher the risk of kidney disease progression and cardiovascular complications, even if the reduction in filtration is still moderate. [21]

Table 5 shows the categories of albuminuria that should be assessed along with creatinine and estimated glomerular filtration rate.[22]

Category	Urine albumin to creatinine ratio
A1	less than 30 mg per g
A2	30-300 mg per g
A3	more than 300 mg per g

When creatinine rises or falls and why it's not always kidney disease

Elevated serum creatinine levels often indicate decreased renal filtration. This occurs in chronic kidney disease, acute kidney injury, urinary tract obstruction, severe dehydration, decompensated heart failure, and a number of other clinical situations. However, modern practice requires that one always remember: a high level does not automatically indicate chronic kidney disease. [23]

On the other hand, low creatinine doesn't always mean "perfect kidneys." The National Kidney Foundation lists vegetarian diets, low muscle mass, pregnancy, amputations, muscle wasting, and severe cirrhosis among the causes of low creatinine. Under these conditions, the estimated glomerular filtration rate based on creatinine alone may appear better than it actually is. [24]

Of particular importance is the group of patients in whom "normal" creatinine may conceal significant filtration impairment. These include the elderly, patients with sarcopenia, cancer, severe chronic illness, cirrhosis, and significant muscle wasting. This is where a combined assessment with cystatin C can be particularly useful. [25]

The reverse error is also common. A young, muscular individual, an athlete, a patient following intense exercise, or a large meal the day before may have higher-than-average creatinine levels without a significant reduction in filtration. In such a situation, without calculating the glomerular filtration rate, without evaluating the urine, and without understanding the context, a false impression of "renal failure" can arise. [26]

Therefore, modern interpretation relies on comparison with baseline, repeat measurement, and assessment of whether creatinine was measured at a stable state. The NIDDK specifically emphasizes that for screening for chronic kidney disease, the estimated glomerular filtration rate should be calculated from a stable serum creatinine, and KDIGO recommends repeating abnormal values to confirm the presence of chronic kidney disease. [27]

Table 6 summarizes the most common clinical interpretations of the result. [28]

Scenario	The most likely interpretation	What to do next
Creatinine is elevated, estimated glomerular filtration rate is decreased, albuminuria is present	There is a high risk of kidney disease	Confirm the chronic nature, look for the cause, stage
Creatinine is elevated, but the patient is very muscular and there is no albuminuria.	The severity of the problem may be overestimated	Recalculate filtration, estimate cystatin C if necessary
Creatinine is "normal" but the patient is severely undernourished	Kidney function may be worse than it appears	Consider cystatin C and a complete nephrology evaluation
Creatinine increased after trimethoprim or cimetidine	Possible drug effect on creatinine secretion	Compare with clinical and other data
Creatinine increased randomly	A diagnosis cannot be made based on one test.	Repeat the analysis and evaluate the urine

Creatinine in acute and chronic kidney disease: what does the modern algorithm look like?

In acute kidney injury, creatinine is particularly important because it is included in the international definition of this condition. KDIGO defines acute kidney injury as an increase in serum creatinine of 0.3 mg/dL or more over 48 hours, or an increase of 1.5 times or more from baseline within 7 days, or a decrease in urine output of less than 0.5 mL/kg/hour over 6 hours. This is one of the most practically meaningful applications of the test. [29]

But even in acute situations, creatinine has limitations. It is not an ideal marker of the "point of injury" because it changes over time and is assessed dynamically, not as an instantaneous reflection of what is happening in the nephron. The NIDDK emphasizes that any calculation of glomerular filtration rate is an estimate, and a trend in the values is often more informative than a single measurement point. [30]

For chronic kidney disease, the logic is different. KDIGO 2024 defines the disease as impairment of kidney structure or function lasting at least 3 months. This means that a persistent decrease in the estimated glomerular filtration rate (EGFR) of less than 60 ml/min per 1.73 m², persistent albuminuria of 30 mg/g or greater, or other evidence of kidney damage must persist or be confirmed over time, unless the process is clearly chronic based on history or imaging. [31]

From a practical standpoint, the current algorithm is as follows: If creatinine is elevated or the estimated glomerular filtration rate is decreased, it is necessary to assess whether the situation is acute, then check albuminuria, compare the data with previous tests, and repeat the tests for confirmation. If the estimated glomerular filtration rate is less than 15 ml per minute per 1.73 m², this is consistent with renal failure and requires consideration of renal replacement therapy. [32]

Finally, the question of who should be screened more actively is important. The NIDDK recommends testing kidney function in people with diabetes, hypertension, cardiovascular disease, and a family history of kidney failure. It is in these groups that creatinine remains a useful marker, but only as part of a modern assessment system, not as a single number from a biochemical analysis. [33]

FAQ

1. What does serum creatinine indicate?
Creatinine reflects how well the kidneys remove muscle waste products from the blood. Its increase is often associated with a decrease in filtration function, but it does not equate to accurate kidney function and does not replace the estimated glomerular filtration rate. [34]

2. Can creatinine alone tell you if your kidneys are okay
? No. The National Kidney Foundation clearly states that some people with “normal” creatinine may already have kidney disease, while some people with elevated creatinine may not have kidney disease. [35]

3. What is more important today: creatinine or estimated glomerular filtration rate?
For clinical interpretation, the estimated glomerular filtration rate, calculated based on creatinine, and in some cases based on creatinine and cystatin C, is more important. It better reflects the filtration function of the kidneys. [36]

4. Why can an elderly person have a "normal" creatinine level even though their kidneys are failing?
Because low muscle mass produces less creatinine. Because of this, the number may appear normal even though filtration is actually reduced. [37]

5. Can exercise increase creatinine without kidney disease?
Yes. High muscle mass, intense exercise, cooked meat, and creatine supplements can raise the level without causing actual kidney damage. [38]

6. What medications can increase creatinine without actually causing a decline in kidney function?
Common examples include trimethoprim, cimetidine, dronedarone, dolutegravir, cobicistat, rilpivirine, and some other drugs that reduce tubular secretion of creatinine. [39]

7. Why is it important to test urine albumin along with creatinine?
Because albuminuria reveals kidney damage and the risk of progression. It can occur even with an estimated glomerular filtration rate (EGFR) above 60 ml per minute per 1.73 m². [40]

8. When is cystatin C needed
It is particularly useful when creatinine may be inaccurate due to unusual muscle mass, cirrhosis, significant comorbidity, severe obesity, muscle wasting, or when the result is near a clinically important threshold. [41]

9. How is acute kidney injury recognized by creatinine?
KDIGO uses the change in creatinine over time: an increase of 0.3 mg/dL or more over 48 hours, or an increase of 1.5 times or more from baseline over 7 days, or a decrease in urine output. [42]

10. What is the main modern finding about creatinine testing
? Creatinine remains a very important test, but today it should only be read in conjunction with estimated glomerular filtration rate, urine albumin, clinical context, and time course. [43]

Key takeaway

Modern serum creatinine assessment is based on three principles. First, creatinine alone does not equate to kidney function. Second, the main markers for assessing chronic kidney disease today are the estimated glomerular filtration rate and albuminuria. Third, in complex cases, the addition of cystatin C improves accuracy. This approach is consistent with KDIGO 2024, the NIDDK, the US National Kidney Foundation, and current laboratory guidelines. [44]