Creatinine

What is creatinine?

Creatinine is the breakdown product of creatine, the molecule muscle uses to store quick-release energy. A small, roughly constant fraction of the body's creatine pool turns into creatinine every day, so production depends on how much muscle a person carries rather than on what they did that day. The kidneys filter it freely and reclaim almost none of it, which makes the blood level a convenient shorthand for filtering capacity: when filtration slows, creatinine accumulates. Serum creatinine is not a measure of kidney function in itself — it is the raw input from which estimated GFR (eGFR) is calculated.

What is creatinine made of, and where does it come from?

Creatinine is a small nitrogen-containing molecule (113 daltons) formed when creatine and creatine phosphate spontaneously lose water — no enzyme is involved, which is why the rate is so steady. Laboratories measure it by one of two methods: the older Jaffe reaction with alkaline picrate, which is cheap but reacts with glucose, ketones, bilirubin and some antibiotics, or an enzymatic method that is less prone to interference. Both are standardised against a reference method (IDMS), so results from different laboratories are comparable, though the units differ by region — mg/dL in some countries, µmol/L in others (1 mg/dL = 88.4 µmol/L).

What does creatinine do in the body?

Creatinine has no function in the body; it is a by-product on its way out. Its diagnostic value rests on two facts: production is steady from day to day, and clearance depends almost entirely on how much blood the kidneys filter. A small share (around 10-20%) is actively secreted by the kidney tubules rather than filtered, which becomes relevant when drugs block that secretion. The relationship between creatinine and filtration is inverse and strongly curved — filtration can fall by roughly half before creatinine leaves the laboratory's reference interval, and thereafter small further losses produce large rises. This 'creatinine-blind' range is the main reason eGFR, not raw creatinine, is used to grade kidney function.

What does a high creatinine mean?

A raised creatinine most often means the kidneys are filtering less than expected for a person of that size — either from a long-standing reduction in kidney function or from a short-term drop such as dehydration, a urinary blockage or a medication effect. Before that conclusion is drawn, the non-kidney explanations below have to be excluded, because heavy muscle, creatine supplements, a recent meat-rich meal and a handful of common drugs all raise creatinine without any change in filtration. KDIGO 2024 defines chronic kidney disease by an eGFR below 60 mL/min/1.73 m² or other markers of kidney damage persisting for more than three months, so a single high creatinine on its own establishes nothing.

Common causes:

• Reduced kidney filtration — chronic kidney disease from diabetes, high blood pressure or glomerular disease, or an acute kidney injury.
• Dehydration or reduced blood flow to the kidneys, which raises both creatinine and urea and usually reverses with fluids.
• High muscle mass — athletes, heavy resistance trainers and large-framed men run higher because they make more creatinine, not because they filter less.
• Creatine supplements and a large portion of cooked meat, both of which add creatinine directly.
• Medications that block tubular secretion without harming the kidney — trimethoprim, cimetidine, fenofibrate and some HIV drugs (dolutegravir, cobicistat) — plus non-steroidal anti-inflammatory drugs and some blood-pressure drugs that genuinely reduce filtration temporarily.

What does a low creatinine mean?

A low creatinine nearly always reflects low muscle mass rather than 'excellent' kidney function. Older adults, people who are underweight or bed-bound, and those with muscle-wasting conditions simply produce less of it. Pregnancy also lowers creatinine because blood volume expands and the kidneys filter more. The practical consequence of a low creatinine is that any eGFR calculated from it will overestimate kidney function, which is why cystatin C is recommended as a cross-check in people with unusually little muscle.

Common causes:

• Low muscle mass — advancing age, sarcopenia, malnutrition, amputation, muscular dystrophy or prolonged immobility.
• Pregnancy, in which filtration rises by up to 50% and plasma volume expands.
• Advanced liver disease, which reduces the liver's synthesis of creatine, the precursor.
• Very low meat intake or a long-standing vegan diet, which removes the dietary contribution.

What moves creatinine apart from disease?

Muscle mass — the dominant non-kidney factor either direction

Creatinine production is proportional to muscle, so a muscular 25-year-old man and a frail 80-year-old woman can have the same filtration rate and creatinine values that differ by half. Every eGFR equation tries to correct for this using age and sex, but it can only assume average muscle for that age and sex.

Creatine supplements raises it

Creatine monohydrate, widely used in strength sports, is converted to creatinine at a steady rate and raises the blood level for as long as it is taken, with no effect on true filtration. Studies in healthy adults have not shown kidney harm, but the raised creatinine will make a calculated eGFR look worse than the kidneys really are.

A cooked-meat meal in the preceding hours raises it

Cooking converts the creatine in meat into creatinine, which is absorbed directly. A large steak or a stewed-meat dish can raise serum creatinine by a noticeable margin for several hours — enough to move a borderline eGFR across a threshold.

Hydration and blood volume either direction

Dehydration reduces the blood flow reaching the kidneys and raises creatinine, typically together with a larger rise in urea. Over-hydration in the hours before a draw has a much smaller effect in the opposite direction. Neither reflects any change in the kidneys themselves.

Drugs that block tubular secretion raises it

Trimethoprim (in co-trimoxazole), cimetidine, fenofibrate, dolutegravir and cobicistat compete with creatinine for the same tubular transporter. Each can push creatinine up by 10-30% within days of starting, with filtration unchanged; the level returns to baseline when the drug stops. Cystatin C is unaffected and settles the question.

Assay interference either direction

The Jaffe method reads falsely high with ketones (fasting, ketogenic diets, uncontrolled diabetes) and some cephalosporins, and falsely low with high bilirubin. Enzymatic assays avoid most of this; a report from a different laboratory using a different method can differ for that reason alone.

Which other tests are read together with creatinine?

Estimated GFR downstream

Estimated GFR is calculated from creatinine, age and sex. It is the number kidney function is actually graded on; creatinine is only its input.

Cystatin C same panel

Cystatin C is the other filtration marker, produced by every nucleated cell rather than by muscle. When creatinine and cystatin C disagree, muscle mass or a secretion-blocking drug is the usual reason, and the combined equation is the more accurate estimate.

Blood Urea Nitrogen read as a ratio

The urea-to-creatinine ratio separates causes. Urea rises out of proportion in dehydration, gastrointestinal bleeding, high protein intake and steroid use; both rise together when the kidney itself is the problem.

Uric Acid same panel

Uric acid is also cleared by the kidney, so it climbs as filtration falls, and chronically raised uric acid is associated with faster loss of kidney function.

Systolic Blood Pressure upstream

High blood pressure is, with diabetes, one of the two leading causes of chronic kidney disease, and kidney disease in turn raises blood pressure — the two are read together.

Glycated Hemoglobin (HbA1c) upstream

Diabetes is the single most common cause of kidney failure worldwide. Long-term glucose control is one of the strongest determinants of whether creatinine drifts up over the years.

Which foods affect creatinine?

• A large portion of cooked meat, especially stewed or fried — raises strong evidence
• Salt reduction to under 5 g a day (about 2 g sodium), the KDIGO 2024 target for people with kidney disease — supports strong evidence
• Plant-forward patterns such as DASH or Mediterranean, which are associated with slower loss of kidney function — supports moderate evidence
• Very high protein intake above about 1.3 g/kg a day, which KDIGO 2024 advises against in people at risk of kidney disease progression — raises moderate evidence

Which supplements are studied for creatinine?

Creatine monohydrate strong evidence

Creatine is not taken to change creatinine — it is taken for strength and muscle — but it raises creatinine as a predictable side effect, because the extra creatine converts to creatinine at the same steady rate as the body's own. Long-term studies in healthy adults at these doses have not shown harm to kidney function.
Typical dose: 3-5 g/day, the usual sports-nutrition maintenance dose
Cautions: Tell the clinician ordering the test that creatine is being taken; otherwise the raised creatinine may be misread as reduced kidney function. Safety has not been established in people who already have kidney disease, and it is usually advised against in that group. Cystatin C, which creatine does not affect, resolves any doubt about true filtration.

How does exercise affect creatinine?

Regular moderate aerobic activity strong evidence

How much: At least 150 minutes a week of moderate intensity, the KDIGO 2024 recommendation for people with kidney disease as well as the general population.
What to expect: Lowers blood pressure and improves glucose handling — the two main drivers of long-term kidney decline. It does not lower creatinine directly; the benefit is in slowing the rise over years.

Resistance training moderate evidence

How much: Two or three sessions a week covering the major muscle groups.
What to expect: Building muscle raises creatinine slightly and permanently — an expected, healthy change, not a kidney signal. A hard, unaccustomed session also raises creatinine for a day or two through muscle breakdown, so it is best kept away from a blood draw.

Which lifestyle factors affect creatinine?

Blood pressure control strong evidence

Sustained high blood pressure damages the kidney's filtering units, and kidney damage in turn raises blood pressure. Keeping pressure within the range a clinician has set is the intervention with the largest effect on how creatinine behaves over a decade.

Caution with over-the-counter painkillers strong evidence

Non-steroidal anti-inflammatory drugs (ibuprofen, naproxen, diclofenac) reduce kidney blood flow, and regular use is associated with faster kidney decline, particularly alongside dehydration or blood-pressure medication. Paracetamol does not carry this effect at normal doses.

Smoking cessation strong evidence

Smoking is an independent risk factor for developing and worsening chronic kidney disease. KDIGO 2024 recommends stopping as part of routine kidney care.

Herbal and traditional remedies strong evidence

Some herbal products, including any containing aristolochic acid (found in some Aristolochia and formerly in some 'Mu Tong' or 'Fang Ji' preparations), cause irreversible kidney damage. A full list of herbal and supplement products belongs in every consultation where creatinine is discussed.

How should you prepare for a creatinine test?

• Avoid a large meat meal for about 12 hours before the draw; overnight fasting takes care of this and is often requested anyway for the tests ordered alongside.
• Skip strenuous or unaccustomed exercise for 24-48 hours beforehand, and drink normally — neither dehydrated nor deliberately over-hydrated.
• Mention creatine supplements and every medication, especially trimethoprim, cimetidine, fenofibrate, NSAIDs and HIV drugs, since several raise creatinine without affecting the kidneys.
• Look at the eGFR the laboratory reports alongside creatinine, not creatinine alone, and check the units (mg/dL versus µmol/L) before comparing results from different laboratories.
• Where muscle mass is unusual — very athletic, very frail, an amputation, a muscle disease — a cystatin C test gives a filtration estimate that does not depend on muscle.

References

1. 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 — https://doi.org/10.1016/j.kint.2023.10.018
2. Inker LA et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. N Engl J Med 2021 — https://doi.org/10.1056/NEJMoa2102953
3. Delanaye P, Cavalier E, Pottel H. Serum Creatinine: Not So Simple! Nephron 2017 — https://doi.org/10.1159/000469669
4. Levey AS, Inker LA. Assessment of Glomerular Filtration Rate in Health and Disease: A State of the Art Review. Clin Pharmacol Ther 2017 — https://doi.org/10.1002/cpt.729