How to Prepare for a Creatinine Blood Test: Rules for Accurate Results
Creatinine is one of the most commonly ordered blood tests in the NHS, used to estimate how well the kidneys are working. For people who train hard, eat plenty of protein or take creatine, it is also one of the tests most likely to produce a confusing result. Our editorial team explains how to prepare so that the number reflects your kidneys rather than yesterday's workout or steak.
What creatinine and eGFR measure
Creatinine is a waste product formed from the breakdown of creatine and creatine phosphate in muscle. It is produced at a fairly constant rate, released into the blood and filtered out by the kidneys. If kidney filtration falls, creatinine accumulates and its blood level rises.
In the UK, serum creatinine is reported in micromoles per litre (µmol/L). Laboratories then use it to calculate the estimated glomerular filtration rate (eGFR), expressed in mL/min/1.73 m², which is what doctors mostly use to stage kidney function.
The calculation relies on equations such as CKD-EPI (Levey et al., 2009), which take into account age and sex. These equations assume average muscle mass for a person of that age and sex — an assumption that does not hold well for very muscular individuals.
Because creatinine depends on both muscle and kidney function, anything that changes muscle breakdown or creatine intake can shift the result without any change in the kidneys themselves. A review by Kashani and colleagues (2020) describes these non-renal influences in detail.
Factors that distort the result
Several everyday factors can temporarily raise or lower creatinine. Knowing them helps you avoid an unnecessary scare — or a repeat appointment.
| Factor | Effect on creatinine | Why |
|---|---|---|
| Cooked meat in the previous 12 hours | Raises | Cooking converts creatine in meat to creatinine |
| Intense or prolonged exercise in the past 1–3 days | Raises | Increased muscle breakdown |
| Creatine supplements | Raises | More creatine is converted to creatinine |
| Dehydration | Raises | Reduced kidney blood flow and concentration |
| High muscle mass | Raises | Larger creatine pool |
| Certain medicines (e.g. trimethoprim, some others) | Raises | Reduce tubular secretion of creatinine |
| Low muscle mass, amputation, frailty | Lowers | Smaller creatine pool |
The meat effect is well documented. Preiss and colleagues (2007) showed that a cooked-meat meal significantly increased serum creatinine and lowered calculated eGFR for several hours. NICE guidance on chronic kidney disease accordingly advises people not to eat meat in the 12 hours before an eGFR blood test.
Hydration status matters too. A blood test taken after a long run in hot weather, without adequate fluid replacement, may show a temporarily higher value.
Practical preparation checklist
A creatinine test does not usually require fasting unless other tests, such as glucose or lipids, are taken at the same time. However, a little preparation significantly improves reliability.
- Avoid meat — especially cooked red meat — for 12 hours before the test.
- Avoid strenuous or unusual exercise for 48–72 hours beforehand; light walking is fine.
- Drink normally and avoid both dehydration and excessive water loading.
- Tell the doctor or nurse about any supplements, especially creatine and protein powders.
- List your medicines, including over-the-counter painkillers such as ibuprofen.
- Try to be tested at a similar time of day for repeat measurements.
Do not stop prescribed medication without speaking to your GP or pharmacist, even if it is known to influence creatinine. The doctor can interpret results in the context of your treatment.
If you are having a creatinine test as part of a sports medical, it is worth scheduling it during a lighter training week, not straight after a competition or a heavy block.
Athletes and creatine users
Muscular people often have creatinine values at or above the upper end of the laboratory reference range. Banfi and Del Fabbro (2006) found that elite athletes had higher serum creatinine than sedentary controls, with differences depending on sport.
Creatine monohydrate supplementation also raises serum creatinine modestly without harming the kidneys, as noted in the International Society of Sports Nutrition position stand (Kreider et al., 2017). If you want a "clean" baseline, discuss with your doctor whether to pause creatine for a few weeks before testing.
When creatinine-based eGFR is doubtful, clinicians can measure cystatin C, a protein produced by all cells and much less influenced by muscle. The newer CKD-EPI equations combining creatinine and cystatin C (Inker et al., 2021) give a more accurate estimate in many people.
A urine albumin-to-creatinine ratio (ACR) is another useful test, since it detects kidney damage through protein leakage even when filtration is normal.
Interpreting results and next steps
A single slightly raised creatinine result in a fit, muscular adult is common and often harmless. What matters is context: symptoms, blood pressure, urine tests, trends over time and other risk factors such as diabetes.
In the NHS, chronic kidney disease is not diagnosed from one test. Abnormal results are usually repeated, and CKD requires evidence of reduced function or kidney damage persisting for more than three months.
A sudden, substantial rise in creatinine, however, is a different matter, particularly after extreme exercise with muscle pain and dark urine, which can indicate rhabdomyolysis. This requires urgent medical attention.
People using anabolic steroids or other performance drugs should know that these can cause genuine kidney injury, not just a measurement artefact. Being open with the doctor allows proper assessment.
Conclusions of the editorial team
Creatinine is a valuable test, but it reflects muscle and diet as well as kidney function. For athletes, this can produce falsely worrying results.
Avoiding meat for 12 hours, resting from hard training for a couple of days, staying normally hydrated and disclosing supplements and medicines are the key steps to an accurate result.
When doubt remains, cystatin C and a urine ACR test help distinguish genuine kidney problems from harmless variation.
You may also find our articles on whether whey protein affects the kidneys, on creatine safety and on liver enzymes after training useful.
References
- Levey AS, Stevens LA, Schmid CH, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150(9):604–612.
- 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.
- Preiss DJ, Godber IM, Lamb EJ, Dalton RN, Gunn IR. The influence of a cooked-meat meal on estimated glomerular filtration rate. Ann Clin Biochem. 2007;44(1):35–42.
- Banfi G, Del Fabbro M. Serum creatinine values in elite athletes competing in 8 different sports: comparison with sedentary people. Clin Chem. 2006;52(2):330–331.
- Kashani K, Rosner MH, Ostermann M. Creatinine: from physiology to clinical application. Eur J Intern Med. 2020;72:9–14.
- 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.
- National Institute for Health and Care Excellence. Chronic kidney disease: assessment and management (NG203). London: NICE; 2021.
