Kidney markers in blood: what creatinine, GFR, and cystatin C show
The essentials at a glance
Kidney markers show how well the kidneys cleanse the blood. The best-known is creatinine, a breakdown product from the muscles. The glomerular filtration rate is calculated from it, abbreviated GFR: it indicates how much blood is filtered per minute and is between 85 and 135 milliliters in healthy people (IQWiG, 2024). A single abnormal result is not yet a diagnosis.
This article explains what the numbers mean and what they depend on. It provides context without diagnosing and identifies at each point where the significance of a value ends. Where an institution gives a number, its name and year are included here. Where no such figure exists, that is stated as well.
You will first read why the kidneys remain silent for so long. This is followed by creatinine, the GFR and cystatin C, three common misconceptions, warning signs, an overview of all the values, and preparation for a conversation with your doctor. The limitations come at the end.
What to expect in this article
1. What kidney markers actually show
2. Why the kidneys remain silent for so long
3. Creatinine: the standard value and its known weakness
4. The GFR: what the number means
5. Cystatin C: what this value adds
6. Three common statements about kidney markers
7. When kidney results should be discussed at a doctor's office
8. What puts strain on the kidneys and what does not affect them
9. How to prepare for a conversation about your results
10. The overview: what each value shows
11. What a blood test shows here—and what it does not
12. Who should have a measurement and who should not
13. Limitations: what kidney markers do not tell you
14. What you can do about an abnormal result
Frequently asked questions
Sources
What kidney markers actually show
The kidneys work as filters. They remove from the blood what the body no longer needs and excrete it in the urine. Kidney markers do not measure the kidneys themselves, but what they leave behind.
That is the key idea behind each of these values: a substance in the blood that is excreted through the kidneys is measured. If its level rises even though the body is not producing more of it, this suggests that less of it is being removed.
This leads to a feature that makes many test results difficult to interpret. A kidney marker is always an indirect measure. It depends on two things: how well the kidneys filter and how much of the substance is produced in the first place.
Key message
A kidney marker never measures the kidney directly, but rather a substance in the blood that the kidneys excrete. That is why each of these values depends on two factors: the filtering capacity and how much of the substance is produced in the body in the first place.
Who is affected
According to the Institute for Quality and Efficiency in Health Care (IQWiG), around nine million people in Germany have chronic kidney disease (2024). Most of those affected are over 60.
The course varies considerably. In around 100,000 people, the disease has progressed so far, according to experts, that dialysis is necessary; around 2,000 donor kidneys are transplanted each year (IQWiG, 2024).
These figures are not a reason for alarm, but provide context. Of the nine million people affected, only a very small proportion reach the stage at which dialysis is required. For everyone else, the aim is to know how the disease is progressing.
At the same time, another figure from the same source shows why this issue matters: In Germany, nearly 27,000 people die each year from the consequences of chronic kidney disease (IQWiG, 2024). This is not an argument for fear, but for paying attention to a value that is available early.
What the kidneys do besides filtering
Filtering is the best-known function, but not the only one. The kidneys regulate the body's fluid balance, help keep blood pressure in balance, control the acid–base balance, and are involved in the production of red blood cells.
This explains why declining kidney function later becomes apparent in ways that initially seem unrelated to the kidneys: through rising blood pressure, fluid retention, and blood values that were previously unremarkable.
When reading a test result, this means that kidney values rarely stand alone. Anyone who wants to interpret them looks at the blood pressure alongside them and at long-term blood sugar, because both contribute to answering the same question.
The reverse is also true. Anyone receiving treatment for high blood pressure has a reason to keep an eye on their kidney values—and to do so before anything changes, not afterward.
Why the kidneys remain silent for so long
The most important characteristic of the kidneys for this topic is their reserve capacity. They can lose a significant part of their function without anyone noticing.
IQWiG explicitly describes this connection in its guide to chronic kidney disease:
Documented source
“At the beginning of the disease, the kidneys can still adequately filter the blood despite impaired function. That is why the disease is usually symptom-free in its early stages.”
Institute for Quality and Efficiency in Health Care (IQWiG)
Chronic kidney disease (renal insufficiency), as of 2024
This sentence explains why kidney values exist in the first place. An organ that signals early does not need laboratory monitoring. An organ that only becomes noticeable late does.
In everyday terms, this means that the absence of symptoms is not proof that everything is fine. Conversely, a single abnormal value does not mean that something is wrong. Both conclusions miss the point.
How chronic kidney disease is diagnosed
A blood test alone is not sufficient. The IQWiG names five components: a detailed discussion, a physical examination including blood pressure measurement, blood tests, urine tests, and an ultrasound examination of the abdominal area and kidneys (2024).
A self-test cannot provide three of these five components. This is not a weakness of blood values, but the reason they are a starting point rather than a result.
Creatinine: the standard value and its familiar weakness
Creatinine is the value that appears on almost every laboratory report. It is readily available, inexpensive, and has been established for decades. Its weakness appears in the same line as its origin.
The IQWiG describes creatinine as a breakdown product produced in the muscles (2025). That is precisely the limitation: In addition to filtration performance, the amount of creatinine entering the blood depends on how much muscle mass a person has.
Who this affects in both directions
A very muscular person produces more creatinine and therefore has a higher value, without the kidneys working less well. A person with little muscle mass produces less and may have an unremarkable value even though filtration performance has already declined.
This affects two very different groups: strength athletes on the one hand, and older people with reduced muscle mass and people recovering from a prolonged illness on the other. In both cases, the value tells a story that is not only about the kidneys.
Diet also plays a role. A very meat-heavy meal can temporarily increase the value. That is why a single measurement says less than two measurements taken weeks apart.
What creatinine clearance measures additionally
In addition to the blood value, there is creatinine clearance. According to the IQWiG, it indicates how well the kidneys remove creatinine from the blood (2025), and it is determined using both blood and urine values.
The institute gives reference values of 95 to 160 milliliters per minute per 1.73 square meters of body surface area for women over 18 and 98 to 156 for men over 18 (2025). These values depend on the laboratory and method; the information on your own test report is always decisive.
The figure of 1.73 square meters each is due to standardization: filtration performance is converted to an average body surface area so that people of different sizes can be compared. It is a calculated value, not a measured one.
Why two measurements tell you more than one
A creatinine value fluctuates from day to day. Fluid intake, meals, strenuous exercise the day before, and the time of the blood draw all affect the same number.
That is why the most informative measure is not the value itself but its direction. Two measurements taken weeks to months apart show whether an abnormality is stable or changing—and that is the information that matters.
There is a time-based criterion for the term chronic: chronic kidney disease refers to impairment that persists for months, not to a single abnormal value.
GFR: what the number means
The glomerular filtration rate is the number people search for most often and the one explained least often. It directly describes filtration performance.
IQWiG puts it this way: The glomerular filtration rate indicates how much blood is filtered per minute; in healthy people, this is between 85 and 135 milliliters per minute (2024). The lower the number, the less blood is filtered in the same amount of time.
Why GFR is usually estimated rather than measured
Measuring filtration performance directly is time-consuming. In everyday practice, it is therefore calculated from a blood value, usually creatinine, together with age and sex. Test results then list eGFR, where the e stands for estimated.
The estimated GFR therefore inherits all the weaknesses of the value from which it is calculated. A GFR calculated from creatinine depends on muscle mass, even if the number itself no longer appears to.
The five stages and what they mean
Based on filtration performance, chronic kidney disease is divided into five stages. In the first stage, urine tests show damage while overall function remains normal. After that, kidney function is mildly, moderately, and severely impaired, until kidney failure occurs in the fifth stage, when the kidneys can no longer adequately filter the blood (IQWiG, 2024).
Two figures mark practical thresholds. Below a GFR of 30, preparations for dialysis should begin; below 10, most people develop kidney failure (IQWiG, 2024).
For the vast majority of findings, these figures are far off. They are included here to make clear where the scale ends, with most values lying at its upper end.
Cystatin C: what this value adds
Cystatin C is a protein that all nucleated cells in the body continuously produce. It is excreted through the kidneys, and its production rate does not depend on muscle mass.
The S2k guideline on rational laboratory diagnostics for kidney damage, issued by the German Society of Nephrology and the German Society for Clinical Chemistry and Laboratory Medicine, describes the synthesis rate of cystatin C as stable and independent of acute-phase reactions, liver disease, muscle mass, and dietary habits (AWMF 115/001, 2021). As an advantage over creatinine, it explicitly cites no dependence on muscle mass or sex.
Creatinine also reflects the muscles. Cystatin C reflects only filtration capacity.
Creatinine also reflects the muscles. Cystatin C reflects only filtration capacity. This precise difference makes the second value useful where the first becomes unreliable.
When the guideline recommends cystatin C
The guideline is specific on this point. If reduced filtration capacity is suspected, cystatin C should be measured when there is doubt about the clinical assessment of creatinine (AWMF 115/001, 2021).
For three groups, estimation of filtration capacity based on cystatin C is expressly required: people with very high muscle mass, people with sarcopenia, meaning pathological muscle loss, and children (AWMF 115/001, 2021).
Thus, cystatin C is not a substitute for creatinine but a complement with a clearly defined area of application. People whose body composition differs from average receive a more accurate assessment through this value.
Where cystatin C also has limitations
A more accurate value is not a complete value. Cystatin C describes filtration capacity, and only that. Damage that first appears in the urine remains hidden from it.
That is precisely what the first of the five stages concerns: overall function is still normal there, and the damage can be detected only through urine tests (IQWiG, 2024). A blood value—whatever it may be—comes too late at this point.
This leads to the most honest statement in this chapter: Cystatin C answers the question of filtration capacity more accurately than creatinine. It does not answer the question of whether the kidneys are damaged.
Three common statements about kidney values
The following three statements appear in many forums and advice articles. All three sound plausible, and none of them withstands scrutiny.
Checked against the evidence
Widespread
“If something is wrong with the kidneys, you notice it.”
Supported by evidence
In the early stages, chronic kidney disease is usually asymptomatic because the kidneys can still filter the blood sufficiently despite impaired function (IQWiG, 2024).
Widespread
“A normal creatinine level means healthy kidneys.”
Supported by evidence
Creatinine is produced in the muscles (IQWiG, 2025). With low muscle mass, the level may remain unremarkable; therefore, the clinical guideline requires estimation using cystatin C in sarcopenia (AWMF 115/001, 2021).
Widespread
“An abnormal result means kidney disease.”
Supported by evidence
A single laboratory result usually cannot establish a disease (IQWiG, 2025). Determining the cause involves a discussion, physical examination, blood and urine tests, and an ultrasound (IQWiG, 2024).
Anyone searching for a kidney function result usually has a genuine concern: A report contains a number that is flagged. The concern is justified, but jumping to conclusions is not. The reliable approach is a second measurement and a discussion with a doctor.
When kidney function results should be taken to a medical practice
This chapter identifies the observations for which reading further would be the wrong answer to the right question.
These observations require medical evaluation
An abnormal kidney function result on a report
regardless of how far it is from the reference range
Significantly less or significantly more urine than usual
especially if the amount you drink has not changed
Foamy or noticeably discolored urine
both should be clarified through a urine test, not a search query
Fluid retention in the legs, ankles, or face
especially in the morning around the eyes or in the evening on the lower legs
A history of high blood pressure, diabetes, or both
then kidney function should be checked regularly, even without symptoms
Continuous use of painkillers
over weeks and without medical supervision—this should be discussed
This list does not replace an examination, nor is it a diagnostic checklist. It describes reasons to have a conversation, not to interpret things on your own.
Kidney function is a topic that causes many people searching for information to feel anxious. That anxiety is understandable, and it is rarely justified: By far the majority of abnormal individual results are clarified on a second measurement or by an explainable cause.
What harms the kidneys and what does not
Few organs are surrounded by as many everyday rules as the kidneys. Some of them are supported by evidence; others are not.
The two main causes are known
High blood pressure and diabetes are the leading causes of chronic kidney disease. Over the years, both damage the tiny blood vessels through which filtration takes place, and both can remain symptom-free for a long time.
This leads to the most practically important recommendation in this article: Anyone with either of these two diagnoses should have their kidney function checked regularly, regardless of how they feel. This is not a precautionary measure but the standard practice.
Painkillers over a longer period
Certain over-the-counter painkillers can strain the kidneys when taken continuously. The problem rarely arises from a single tablet, but rather from weeks and months without medical supervision.
Anyone who regularly uses such products should mention them at their next appointment. Not to stop taking them, but so that someone can take them into account.
What cannot be substantiated
There is no reliable information from a German institution that could be cited here on how much drinking improves kidney values. Therefore, this article makes no recommendation on fluid intake.
The same applies to detox cures, kidney teas, and products intended to improve kidney function. Where there is no evidence, none is claimed here either.
Chapter at a glance
The two most important causes of chronic kidney disease are high blood pressure and diabetes; both can remain symptom-free for a long time. Anyone with either diagnosis should have their kidney values checked regularly. Long-term use of painkillers should be discussed. There is no reliable information from a German institution on fluid intake, kidney teas, or detox cures—so this article makes no recommendation on them either.
How to prepare for a conversation about your values
The following four steps prepare you for a conversation at your doctor's office; they do not replace it. They are intended for situations in which a kidney value is flagged on a test result and the next question is what to do now.
Look for old test results
A single value says little. Two values measured months apart say much more.
List your medications
Everything taken regularly, including over-the-counter medicines, with an approximate start date.
Record your blood pressure
For one week, in the morning and evening. This value is part of the kidney assessment.
Take two questions with you
What was the value before? And would a urine test be useful?
The benefit of this preparation lies in the second question. According to IQWiG, a urine test is one of the components used to diagnose chronic kidney disease (2024), and it is missing from a purely blood-based test result.
The first question is just as important. A trend over several years distinguishes a stable deviation from a change, and no single measurement can make that distinction.
The overview: what each value shows
The following table compares four tests according to the same three criteria. It provides context but does not make a diagnosis: in each row, it first states what is measured, then what else the value depends on besides kidney function, and finally what it does not show.
| Value | What is measured | What else it depends on | What it does not show |
|---|---|---|---|
| creatinine in the blood | a breakdown product produced in the muscles and excreted through the kidneys | muscle mass, age, and temporarily after a very meat-rich meal | early limitations in people with little muscle mass |
| GFR or eGFR | the filtration rate: how much blood is filtered per minute, 85 to 135 ml/min in healthy people | from the value used to calculate it—calculated from creatinine, it inherits its dependence on muscle mass | the cause of an impairment; it only quantifies its extent |
| Cystatin C in the blood | a protein continuously produced by all nucleated cells and excreted through the kidneys | according to the clinical guideline, not on muscle mass, sex, liver diseases, or diet | damage that appears only in the urine |
| Urine test | substances in urine that indicate damage to the filters | depending on the type of sample and the time of collection | the filtration rate as a number—for that, blood values are still needed |
There is no reliable figure from a German institution on how often an abnormal individual result returns to normal on the second measurement. Therefore, this table contains no percentage.
What a blood test shows here and what it does not
An honest statement is needed here: mybody®x (MYBODY Lab GmbH) measures neither creatinine nor the GFR calculated from it. Both are explained in this article because people search for them, not because a test provides them.
What the Men’s Wellness Check includes is Cystatin C. This is the value required by the clinical guideline when there are doubts about the assessment of creatinine (AWMF 115/001, 2021). It does not replace a medical examination or a urine test.
Blood test using capillary blood
Men’s Wellness Check | Men’s health test
Includes Cystatin C as a kidney marker, as well as gamma-GT and GPT for the liver, HbA1c, the cholesterol profile, plus testosterone, cortisol, PSA, and homocysteine—17 values from a single blood sample. What the test does not provide: It measures neither creatinine nor the GFR calculated from it, and it does not replace a urine test. An abnormal result is a reason to discuss it, not a diagnosis.
Lab results 3–5 business days after the sample is received
Blood test using capillary blood
Women’s Wellness Check | Women’s health test
Includes liver, kidney, blood sugar, and cholesterol levels in the organ profile, as well as the thyroid profile consisting of TSH, fT3, and fT4, plus ferritin, B12, D3, cortisol, prolactin, and SHBG—18 values from a single blood sample. What remains unclear: The product page mentions the kidneys in the organ profile without naming the marker. Anyone specifically looking for Cystatin C will find it explicitly included in the Men’s Wellness Check. The test does not measure cycle hormones.
Lab results 3–5 business days after the sample is received
Who may benefit from a measurement and who may not
A home measurement is not an answer to every kidney question. It answers one specific question, and the two columns below distinguish between the cases.
Makes sense for you if …
you want to keep an eye on your values without there currently being any specific suspicion.
you are very muscular or have little muscle mass and know that creatinine is difficult to interpret in your case.
you want to have several values checked anyway and the kidney question is one of them.
Probably not if …
an abnormal kidney value is already listed on a report. In that case, scheduling an appointment is the right order of events, not taking a second self-test.
you have high blood pressure or diabetes. In that case, monitoring belongs under medical supervision, not in your own hands.
you expect a diagnosis. That requires urine, an ultrasound, and a conversation with a doctor—none of which a blood test can replace.
Limitations: what kidney values do not tell you
A laboratory value is an indication, not a finding about a person. A disease usually cannot be inferred from a laboratory value alone (IQWiG, 2025). This applies especially to the kidneys, because each of these values depends on more than filtration performance.
There is also a limitation inherent in the method. Filtration performance is estimated in everyday practice rather than measured, and an estimate carries uncertainty that is not printed on the report.
The data also has limitations. IQWiG provides specific figures on the frequency of chronic kidney diseases, the filtration performance of healthy people, and the reference values for creatinine clearance. There is no comparable figure for how often an abnormal individual value falls back within the normal range on the second measurement. That is why this article does not provide one.
The tests also have the limitation stated in the product cards: They provide blood values. The urine test and ultrasound that are part of diagnosing chronic kidney disease are not included and cannot be replaced by them.
What you can do about an abnormal value
If you take away one action from this article, let it be this: Find your most recent report that contains the same value and place the two side by side. Two numbers with a date in front of them are the starting point for any meaningful interpretation.
The reason is unremarkable. A single value leaves open whether it has always been that way or has changed—and that is precisely the question asked first in medical practice.
It began with the observation that the kidneys remain silent for a long time. That is the uncomfortable part. The other part is that they can be measured long before they make themselves known. Thinking about both together gives you the advantage.
Frequently asked questions
What does a GFR value that is too low mean?
The glomerular filtration rate indicates how much blood is filtered per minute; in healthy people, this is between 85 and 135 milliliters per minute (IQWiG, 2024). A lower value initially means that less blood is being filtered in the same amount of time. It does not identify a cause and is not a diagnosis in itself. Because GFR is usually calculated from creatinine in everyday practice, it also depends on muscle mass.
Why is cystatin C more accurate than creatinine?
Creatinine is produced in the muscles (IQWiG, 2025), so its level depends on muscle mass. By contrast, the S2k guideline of the German Society of Nephrology and the German Society for Clinical Chemistry and Laboratory Medicine describes the production rate of cystatin C as stable and independent of muscle mass, dietary habits, liver disease, and acute-phase reactions (AWMF 115/001, 2021). Estimation using cystatin C is expressly required for very muscular people, people with sarcopenia, and children.
Can you tell if your kidney values are poor?
Usually not. At the beginning of chronic kidney disease, the kidneys can still adequately filter the blood despite impaired function, which is why the early stages are generally symptom-free (IQWiG, 2024). That is precisely why kidney values are measured instead of waiting for symptoms. Symptoms such as fluid retention or a change in urine output only occur later and should be medically evaluated.
Is a blood test enough to assess the kidneys?
No. According to IQWiG, diagnosing chronic kidney disease involves a detailed consultation, a physical examination including blood pressure measurement, blood tests, urine tests, and an ultrasound examination of the abdomen and kidneys (2024). A blood test covers one part of this process. It is a starting point for a discussion, not a substitute for an examination.
How many people in Germany have chronic kidney disease?
Around nine million, and most affected people are over 60 years old (IQWiG, 2024). In approximately 100,000 people, the disease has progressed so far, according to experts, that dialysis is necessary; around 2,000 donor kidneys are transplanted each year. The vast majority of affected people do not reach this stage.
Next step
The trend first, then the measurement
If you are less interested in a single finding than in where your levels currently stand, a blood test can provide a starting point. It does not replace a medical evaluation or a urine test.
Men’s Wellness Check Women’s Wellness CheckRead more
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Sources
- Institute for Quality and Efficiency in Health Care (IQWiG): Chronic Kidney Disease (Kidney Failure) (as of 2024) – gesundheitsinformation.de
- Institute for Quality and Efficiency in Health Care (IQWiG): How Does Chronic Kidney Disease (Kidney Failure) Progress? (as of 2024) – gesundheitsinformation.de
- Institute for Quality and Efficiency in Health Care (IQWiG): Creatinine Clearance (as of 2025) – gesundheitsinformation.de
- German Society of Nephrology (DGfN) and German Society for Clinical Chemistry and Laboratory Medicine (DGKL): Interdisciplinary S2k Guideline “Rational Laboratory Diagnostics for the Evaluation of Acute Kidney Injuries and Progression of Chronic Kidney Diseases,” AWMF Registry No. 115/001 (2021) – register.awmf.org
The verbatim quotation regarding the absence of symptoms in the early stages comes from source [1], as do the figures on affected individuals, dialysis, and transplants, as well as the list of examinations. The glomerular filtration rate of 85 to 135 millilitres per minute, the five stages, and the thresholds of 30 and 10 come from [2]. The description of creatinine as a breakdown product from muscles, the reference values for creatinine clearance, and the statement that a disease usually cannot be inferred from a single laboratory value alone come from [3]. All statements about cystatin C—including its independence from muscle mass, sex, diet, and liver disease, as well as the recommendations for its use—come from [4]. Information on the price, sample type, and scope of analysis of the tests mentioned comes from the mybody®x product pages, accessed on 28/08/2026; processing times follow the central specification for blood tests. All sources were accessed and reviewed on 28/08/2026.
mybody®x Editorial & Expert Team
Blood analysis interpretation Laboratory diagnostics Nutritional science Nutrigenetics
This article was created by the mybody®x editorial and expert team. The team combines expertise in blood analysis interpretation, laboratory diagnostics, and nutritional science. Everyone who contributed to it is listed on the authors page.
Published on 28/08/2026 · Last updated on 28/08/2026
The content is intended for general information and does not replace medical advice, diagnosis, or treatment. Reference ranges depend on the laboratory, method, and age—always refer to the information on your test report.
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