How to prepare for the «Creatinine» test: rules for taking it

Creatinine is the main laboratory indicator of kidney function. It is used to calculate the glomerular filtration rate (GFR), to determine the stage of chronic kidney disease and to adjust the doses of many medications. However, creatinine is a product of muscle metabolism, so its level depends on muscle mass, a meat-based diet, training and the creatine supplement popular among athletes. Our editorial team explains how to prepare for the test so that the conclusion about the kidneys does not turn out to be mistaken.

Where creatinine comes from and how kidneys are assessed by it

In muscles, creatine and creatine phosphate serve as a rapid source of energy. A small part of these compounds is converted every day, spontaneously and without the participation of enzymes, into creatinine. It enters the blood and is excreted predominantly by the kidneys through filtration in the glomeruli, and partly through secretion in the tubules.

If the kidneys filter worse, creatinine accumulates in the blood. GFR calculation formulas are based on this principle, in particular CKD-EPI (Levey et al., 2009) and its updated version without a race adjustment (Inker et al., 2021). The formulas take age and sex into account but do not account for individual muscle mass or diet.

Hence the main problem: with the same kidney function, a muscular athlete has a higher creatinine than a person with little muscle mass. The calculated GFR in an athlete may turn out underestimated, and kidney disease may be mistakenly suspected.

The KDIGO guidelines on chronic kidney disease (2012, update 2024) directly mention factors that change creatinine independently of GFR and recommend, in doubtful cases, determining an additional marker — cystatin C.

Food and water before the test

Boiled or fried meat contains creatinine, which forms from creatine during heat processing. A classic study by Mayersohn and colleagues (1983) showed that after a portion of cooked meat the concentration of creatinine in the plasma rose noticeably and remained elevated for several hours.

Therefore the evening before it is better to do without a large portion of meat, a steak or a barbecue, and in the morning to give blood on an empty stomach. For people on a high-protein diet it is useful simply not to make dinner a «meat record».

Hydration is also important. Dehydration reduces blood flow in the kidneys and can temporarily raise creatinine — especially after a sauna, a prolonged workout in the heat or «making weight». The day before, drink water in your usual amount; in the morning you may drink a glass of water.

Diuretics, which are sometimes used for «cutting» or rapid weight loss, are dangerous in themselves: they disrupt electrolyte balance and can cause acute kidney injury. They must not be used without a prescription.

Training, muscle mass and creatine

Intense and prolonged exertion raises creatinine because of increased breakdown of creatine phosphate, muscle damage and dehydration. The effect is usually temporary, but for a scheduled test it is better to avoid heavy workouts for at least 24–48 hours, and after competitions or unaccustomed exertion — longer.

The creatine monohydrate supplement is one of the most studied sports products. The position of the International Society of Sports Nutrition (Kreider et al., 2017) recognizes it as safe for healthy people at recommended doses. At the same time, part of the creatine is converted into creatinine, so its level in the blood may rise moderately without any kidney injury.

If you take creatine, be sure to tell the doctor. When an accurate assessment of kidney function is needed, the doctor may advise temporarily stopping the supplement before a repeat test or ordering cystatin C, which creatine does not affect.

Anabolic steroids increase muscle mass and thereby raise creatinine. In addition, their non-medical use is associated with hypertension and described cases of kidney injury, so for people who have used them a correct assessment of kidney function under a doctor's supervision is especially important.

Medications and laboratory methods

Some drugs block the tubular secretion of creatinine and raise its level in the blood without changing filtration. Classic examples are trimethoprim (including as part of co-trimoxazole) and cimetidine. Other drugs, for example non-steroidal anti-inflammatory agents in high doses, can actually reduce kidney function, especially against a background of dehydration.

There is no need to discontinue prescribed drugs on your own before the test, but the doctor should know the full list, including over-the-counter painkillers, which athletes often take after injuries.

The result also depends on the measurement method. The older Jaffe method can react to other substances in the blood — ketone bodies, some cephalosporins, high bilirubin — and produce false deviations. Enzymatic methods are more accurate. People on a ketogenic diet or prolonged fasting should check which method the laboratory uses.

• trimethoprim, cimetidine — a false increase due to blocked secretion;
• NSAIDs, ACE inhibitors, diuretics — can actually change GFR;
• ketosis (keto diet, fasting) — possible interference in the Jaffe method;
• vitamin C in high doses — an effect on some measurement methods has been described.

Checklist and interpretation

Let us summarize the rules. Blood is drawn in the morning on an empty stomach, after a normal dinner without a large amount of meat. The day before, do not train intensively, do not visit the sauna, do not consume alcohol and drink water as usual. Before the draw, sit for 10–15 minutes.

A single deviation in creatinine is not a diagnosis. The KDIGO guidelines define chronic kidney disease only when changes persist for more than three months, so the doctor usually repeats the test and also assesses the urine (albumin, urine creatinine).

In athletes with a large muscle mass, the GFR calculated from creatinine is often underestimated. A combined formula based on creatinine and cystatin C (Inker et al., 2012; 2021) gives a more accurate assessment in such people.

Compare results at one laboratory and under the same conditions. The dynamics of creatinine in a particular person are often more informative than a single figure compared with a population norm.

Editorial conclusions

Creatinine reflects not only the work of the kidneys but also muscle mass, diet and supplement intake. For people who train and consume a lot of protein, this is especially important.

The most important rules: no large portion of meat the day before, no intense workouts for 1–2 days, usual hydration and informing the doctor about creatine, medications and hormonal drugs.

With a doubtful result in a muscular person, it is advisable to discuss determining cystatin C with the doctor rather than drawing conclusions about kidney disease from a single figure.

We also recommend reading our materials on preparing for the cystatin C test, on the safety of creatine for the kidneys and on monitoring blood pressure and kidney function in athletes.

References

1. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013;3(1):1–150.
2. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int. 2024;105(4S):S117–S314.
3. Levey AS, Stevens LA, Schmid CH, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150(9):604–612.
4. Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737–1749.
5. Inker LA, Schmid CH, Tighiouart H, et al. Estimating glomerular filtration rate from serum creatinine and cystatin C. N Engl J Med. 2012;367(1):20–29.
6. Mayersohn M, Conrad KA, Achari R. The influence of a cooked meat meal on creatinine plasma concentration and creatinine clearance. Br J Clin Pharmacol. 1983;15(2):227–230.
7. Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18.