Creatinine, to be distinguished from creatine, is an essential marker for assessing the proper functioning of the kidneys. When faced with symptoms such as intense fatigue or muscle pain, some people wonder about their state of health, especially if their last blood test showed a high level of creatinine.
What are the normal levels and how can they be interpreted?
Between the different levels in men and women, but also in adults practicing high-intensity sports such as crossfit, sprinting, rugby or street workout, it's easy to get confused. High creatinine levels are not always a bad sign.
Here is a clear table showing blood creatinine levels considered normal:
| Gender | Creatinine level (µmol/l) | Creatinine level (mg/l) |
|---|---|---|
| Men | 65 to 120 µmol/l | 7 to 13 mg/l |
| Women | 50 to 100 µmol/l | 6 to 11 mg/l |
We're shedding some light on the subject here, but it's not the purpose of this article to make a diagnosis in place of the doctor!
Creatine and creatinine in the blood: close links
The first thing to know is to distinguish creatine from creatinine before looking at blood levels.
What is creatine?
It's an amino acid derivative manufactured in part by the body. The other part comes from a diet rich in animal proteins, such as meat and fish, and/or from supplementation.
Eggs contain very little creatine (0.1g/kg), while meat can contain up to 5g/kg and fish up to 10g/kg.
Creatine content of feed (g/kg)
The body regulates its endogenous production according to exogenous food intake, to maintain a constant reserve, 95% of which is found in the muscles.
The reason the body needs it is to provide energy. At the start of any physical effort, the body releases adenosine triphosphate (known as ATP), and to renew it rapidly, the muscles use creatine.
Phosphorylated, it is transformed into creatine phosphate (or phosphocreatine) at 70%, and it is this breakdown that rapidly releases ATP. The remainder circulates in the form of "free" creatine, which is eliminated in the urine after passing through the bloodstream.
Creatinine in the body
Creatinine is a chemical waste product of creatine¹ present in skeletal muscle.
The greater an individual's muscle mass, the higher his or her blood creatinine level.
After passing through the bloodstream, it is filtered and eliminated via the urine, thanks to the action of the kidneys. This is called glomerular filtration, and its rate can be measured: it's the GFR (for Glomerular Filtration Rate).
Why measure creatinine?
The level of creatinine in the blood, also known as serum creatinine or creatinemia, is therefore a marker of kidney function. It reflects the state of the kidneys, to show whether they're doing well or not.
For example, if the kidneys fail to filter creatinine properly, the level is likely to rise. People with chronic renal failure generally have a GFR of less than 60 mL/min/1.73 m², which corresponds to high creatinine levels.
Creatinine levels are a simple and inexpensive way to get an idea of GFR, but they are not very accurate, and the result may be distorted by particular pathophysiological conditions (high muscle mass, obesity, etc.).
To calculate GFR with the utmost precision, it is possible to use direct calculation methods, via so-called exogenous radiopharmaceuticals, filtered by the kidneys, whose blood concentration is monitored. This method is, of course, more restrictive, complicated and costly.
In short, let's remember that creatinine has no direct function in the body. It is a waste product of creatine and, in human medicine, is an indirect and potentially imprecise (but simple and rapid) indicator of renal health.
So it's too simplistic to systematically associate elevated blood creatinine with renal pathology, even though, in the majority of cases, abnormally high blood creatinine = renal abnormality.
The influence of creatine on creatinine levels
Yes, creatine mechanically increases blood creatinine concentration, but that doesn't mean it's responsible for kidney pathology.
For years, we've heard a lot of questions about the possible side effects of creatine supplementation on the kidneys, particularly in the loading phase when dosage is high². However, no research has shown that supplementation causes kidney dysfunction.
It's important to remember that creatinine levels are related to creatine levels, which in turn are related to muscle mass. The greater the muscle mass, the higher the creatinine levels are likely to be.
Creatinine, whether in the blood or urine, increases following ingestion of creatine monohydrate or creatine-rich foods such as meats². It is excreted in the urine, where it can reach relatively high levels during supplementation (especially in the loading phase).
Also, more creatine in the muscles potentially means more waste from creatine utilization, and therefore higher creatinine levels. This gave rise to the idea that, if creatinine levels and creatinuria increase, the kidneys could become overloaded, causing kidney damage.
In reality, these elevated levels are temporary and related to the treatment, but in no way reflect a reduction in renal function.
In addition, a review of studies on creatine supplementation showed no increase in serum creatinine in 12 studies. A further 8 studies showed an increase within the normal range, with only 2 studies showing an above-average increase.
The precautionary principle remains when it comes to ingesting dietary supplements. However, over thousands of cases and several decades, research proves that creatine supplementation is risk-free.
Two conditions must be met:
- ingested at the indicated doses and in healthy individuals, and
- that it is indeed pure and not contaminated by undesirable products (traceability and production control), which is why the Creapure® label distinguishes the best creatines.
Creatine Creapure®
The best for maximizing muscle power in a healthy way and optimizing intensive performance
- Standardised without doping substances
- No additives, no sweeteners
- Made in Germany, packaged by Protéalpes in Albertville
Creatine levels in the blood: what are the normal levels?
Reference values by gender and age
As with all blood tests, there are reference values, also known as standards, which vary according to gender.
Thus, creatinine levels in men's blood range from 65 to 120 µmol/l, or 7 to 13 mg/l. In women, averages range from 50 to 100 µmol/l, corresponding to a level of between 6 and 11mg/l.
In children, the values decrease, as they do in the elderly.
Variations in blood levels
Creatinine levels are variable and differ from one individual to another, even in the absence of renal dysfunction¹.
The criteria influencing blood creatinine levels are:
- gender;
- age;
- ethnicity;
- weight, including muscle mass;
- diet;
- fasting habits.
In short, many parameters are responsible for these variations in blood creatinine levels.
Why do blood creatine levels vary?
The impact of physiological factors
Scientific literature proves that exercise acutely increases creatinine concentrations in the blood³. As the muscle mass of athletes is more developed, their blood creatine and creatinine levels tend to be higher.
This elevation (physiological and healthy) also disturbs the assessment of filtration rate (GFR).
Beyond muscles, diet comes into play. Vegetarians and vegans will have different rates from someone on a meat-rich diet, for example.
The same applies to people who take creatine monohydrate as a course of treatment, compared with those who do not supplement.
Hydration also impacts these levels, since dehydration increases blood concentration and alters the kidneys' ability to filter⁴.
Filtration is the final point explaining the variation in levels. Creatine is eliminated by glomerular filtration, but also by so-called tubular secretion. The latter varies greatly from one person to another, or under the influence of medication.
The specific impact of aging
The elderly are susceptible to a drop in blood creatinine levels. The reason for this lies in their muscles: muscle degeneration or deficiency leads to a drop in creatinine.
Conversely, a pathology involving significant lysis of muscle mass (extreme and abrupt melting = rhabdomyolysis) leads to a short-term increase in creatinine linked to muscle catabolism.
For example, sarcopenia is a disease linked to old age and characterized by progressive muscle wasting. This is why older people are likely to have lower creatinine levels.
Pathological causes
A high level of creatinine in the blood can be explained by various pathologies:
- kidney problems: renal failure, infection or pyelonephritis, diabetic nephropathy, etc. ;
- problems transporting blood to the kidneys, such as congestive heart failure;
- urinary problems, such as obstruction of the urinary tract ;
- muscular pathologies, such as myopathies;
- pregnancy-related risks such as preeclampsia.
Conversely, a low level of creatinine in the blood can be explained by :
- muscle wasting due to sarcopenia or significant weight loss;
- a nutritional problem;
- liver pathologies.
If you have any doubts or unusual symptoms, the first thing to do is consult your GP, who will prescribe the necessary tests to check these famous levels.
How do you measure creatinine levels?
Two different examinations
Two tests are prescribed by the doctor if there is any doubt about creatinine levels in the blood:
- measurement of serum creatinine by laboratory blood sampling;
- measurement of creatininuria by collecting urine over a 24-hour period, according to a precise protocol.
These blood and urine tests are performed to evaluate what is known as direct creatinine clearance. This measures the rate of filtration by the kidneys, as the ratio between the rate of elimination by the kidneys and the concentration of creatinine in the blood.
As seen previously, it is also possible to measure GFR via exogenous radiopharmaceuticals, in order to calculate directly the capacity of the kidneys to filter this substance and thus estimate GFR.
Results to be interpreted
It's the doctor who interprets the results of these tests to decide whether or not to treat the condition. Sometimes, dietary recommendations combined with physical activity and treatment may suffice. On the other hand, for more serious pathologies, such as severe renal failure, dialysis and even transplants may have to be considered.
In all cases, this medical interpretation depends on the precise context of the patient, his or her symptoms, antecedents, treatments, diet and so on. It is unique to each subject, and only a doctor can make it.
Conclusion
Muscle creatine is metabolized to creatinine, a waste product transported in the bloodstream for elimination by the kidneys.
Its potential accumulation in the blood, and therefore its blood dosage, provides an insight into renal health, and can also help to identify the subject's muscular metabolism. Slightly different levels from normal can sometimes be observed, but it should be borne in mind that these variations depend on many factors, such as age, sex, muscle mass and diet.
High readings are not necessarily indicative of kidney disease, just as low levels can be physiologically explained without associated disorders. In case of doubt, only a healthcare professional can interpret the results, which take into account the individual context.
Further information
- Find out if creatine makes your belly bigger and if this weight gain is reversible.
- Optimize your supplementation: find out if it's really worth taking creatine on rest days during your cure.
- Simplify your sports routine: check whether you can mix creatine and protein to maximize your results.
- If your goal is muscle hypertrophy, find out why taking creatine for mass gain is an often-recommended option.
- Bloating, gas or diarrhoea? Here's what you need to know if you experience stomach pains on creatine.
- Is supplementation suitable for all ages? Find out at what age creatine can be taken without risk.
- It is entirely possible to produce more creatine naturally by optimizing your daily diet.
- Do you practice running, cycling or other endurance sports? Learn how to optimize your performance with creatine in endurance sports.
- Specific needs, hormonal variations... here's why creatine for menopausal women can be a real plus.
- Convenient format or maximum efficiency? Compare the options to find out whether creatine is better in capsule or powder form.
- Are you hesitating between the two? Here's what you need to know to choose between BCAA or creatine depending on your goal.
- If you combine several supplements, learn how to manage dosages with our complete guide to the whey + BCAA + creatine mix.
- Finally, are you wondering whether creatine can help you dry out? We take a look at creatine's effectiveness for fat loss.
