Creatinine test: what it measures, normal levels, what a high creatinine means, and creatine vs creatinine

Every time a muscle contracts it burns creatine phosphate for energy, and a small, predictable fraction of that creatine breaks down into creatinine. The amount produced each day depends mainly on how much muscle you carry, which is why a large, muscular man runs higher than a small woman and why levels are so steady from week to week in the same person. Creatinine has no job in the body; it is simply a waste product that drifts into the blood and has to be removed.

The kidneys do the removing. Blood passes through roughly a million filtering units in each kidney, the glomeruli, which let creatinine through into the urine while holding back cells and proteins. A little more is actively pushed into the urine by the kidney tubules. Because production is constant and clearance is almost entirely by filtration, the concentration left in blood is a direct read on filtering capacity: when filtration halves, creatinine roughly doubles.

A serum creatinine test measures that concentration in milligrams per deciliter (mg/dL). It is included in the basic and comprehensive metabolic panels, the kidney or renal panel, and most routine blood work, so most people have had one without asking for it. On its own the number is only half the story; the lab uses it to calculate eGFR, described below, and clinicians read it alongside BUN and, increasingly, cystatin C.

Chronic kidney disease causes no symptoms until it is advanced, so the people who most need a creatinine test are the ones at risk rather than the ones who feel unwell. The National Kidney Foundation and the American Diabetes Association recommend at least yearly testing for anyone with diabetes or high blood pressure, the two conditions behind most kidney disease in the United States. Cardiovascular disease, a family history of kidney failure, obesity, and age over 60 are the other common reasons to check regularly.

Medications are a second large group. Non-steroidal anti-inflammatory drugs taken daily, lithium, certain antibiotics and antivirals, some chemotherapy agents and the contrast dye used in CT scans can all injure the kidneys, and a baseline creatinine before starting them, then periodic checks, is standard practice. Metformin, several blood-pressure medicines and many other drugs are dosed according to kidney function, so the result also changes how other prescriptions are written.

People on GLP-1 medicines, testosterone therapy or peptide protocols usually have creatinine on their monitoring panel, and anyone who takes creatine or eats a very high-protein diet should know their baseline, because both raise creatinine without harming the kidneys, as explained in the creatine vs creatinine section. Symptoms that do warrant a test rather than a routine check include swelling of the ankles or around the eyes, foamy urine, urinating far more or less than usual, persistent fatigue with nausea, and muscle cramps or itching that has no other explanation.

Creatinine is rarely ordered alone. The usual routes are a basic metabolic panel, which adds glucose, electrolytes and BUN; a comprehensive metabolic panel, which adds liver markers and proteins as well; or a dedicated kidney panel that pairs it with BUN, electrolytes and eGFR. You can have any of these ordered by your doctor, or order one online from TestWell and have blood drawn at a Quest or Labcorp patient service center, with a licensed physician reviewing the order and results in your portal within a few business days.

Preparation is simple but matters more than people expect. Arrive normally hydrated; dehydration concentrates the blood and can push creatinine above the reference range in a person with healthy kidneys. Skip strenuous exercise for 24 to 48 hours, because muscle breakdown after a hard workout releases extra creatinine. Avoid a large meal of cooked meat the evening before or the morning of the draw, since cooking converts the creatine in meat into creatinine that is absorbed directly into the blood. Fasting is not required for creatinine itself, though the metabolic panels it travels in are often drawn fasting for the glucose result.

Tell the ordering physician about creatine supplements, trimethoprim or cimetidine (both block the tubular secretion of creatinine and raise the level without changing filtration), and any recent contrast scan. If a result is being compared with an earlier one, having it drawn at the same lab, at a similar time of day and under similar conditions removes most of the noise.

Reference ranges differ by sex because they differ by muscle mass. Mayo Clinic Laboratories reports 0.74 to 1.35 mg/dL for adult men and 0.59 to 1.04 mg/dL for adult women; other laboratories publish ranges within a few hundredths of these, and the report you receive will show the range for the lab that ran the sample. Children run lower, older adults with less muscle run lower for the same kidney function, and a very muscular adult can sit slightly above the range with perfectly healthy kidneys.

Because the same creatinine value means different things in a 25-year-old bodybuilder and an 80-year-old woman, laboratories convert it into an estimated glomerular filtration rate. The equation in use across the United States since 2021, CKD-EPI 2021, combines creatinine with age and sex and no longer includes a race term. The result is expressed in mL/min/1.73 m², where anything at or above 90 is normal filtration, 60 to 89 is mildly reduced (which is common and usually insignificant in older adults without other signs of kidney damage), 45 to 59 is mildly to moderately reduced, 30 to 44 moderately to severely reduced, 15 to 29 severely reduced, and below 15 is kidney failure.

One limitation is important to understand: creatinine is a late marker. Filtration can fall by close to half before creatinine leaves the reference range, which is why an eGFR of 65 with a creatinine that still reads normal is worth attention, and why clinicians pair the blood test with a urine albumin-to-creatinine ratio. Albumin leaking into urine (30 mg/g or more) often shows kidney damage years before eGFR falls.

The common causes come first. In someone with no kidney history, a mildly high creatinine is most often explained by dehydration, a heavy workout in the previous day or two, a large meat meal, creatine supplementation or simply a lot of muscle. Repeating the test under standard conditions, well hydrated and rested, resolves many of these on its own. A creatinine that stays high on a repeat draw is where the kidney work-up begins.

Reduced kidney filtration is the cause that matters. Long-standing diabetes and high blood pressure account for most cases and damage the kidneys slowly, so the creatinine creeps up over years. Faster rises come from acute kidney injury: severe dehydration or blood loss, sepsis, a blockage in urine flow from an enlarged prostate or kidney stones, and drugs including NSAIDs, contrast dye and some antibiotics. Glomerular diseases such as lupus nephritis and IgA nephropathy, polycystic kidney disease and rhabdomyolysis (massive muscle breakdown, which floods the blood with creatinine and can itself injure the kidney) are less common but important causes.

Two things separate a kidney cause from a muscle or medication cause. The first is eGFR read together with cystatin C: cystatin C is produced by all cells rather than by muscle, so an eGFR calculated from cystatin C that agrees with the creatinine-based one confirms a filtration problem, while a normal cystatin C eGFR points back to muscle mass, diet or a secretion-blocking drug. The second is the urine: albumin, blood or protein in the urine indicates kidney damage, whereas a clean urinalysis with a high creatinine in a muscular person on creatine is reassuring.

A low creatinine almost never signals a kidney problem, because kidneys that filter well simply keep the level at the low end of normal. Instead it reflects low production: small body size, low muscle mass from age, inactivity or prolonged illness, an amputation, malnutrition or a very low-protein or vegan diet, and advanced liver disease, where less creatine is made in the first place. Pregnancy lowers creatinine too, because filtration rises by about half in the second trimester and dilutes the blood.

The practical consequence of a low creatinine is not the value itself but what it does to eGFR. Because the equation assumes average muscle mass, a frail or sarcopenic person can have a reassuring eGFR while true filtration is considerably lower. KDIGO recommends adding cystatin C in exactly this situation, and dosing decisions for kidney-cleared drugs in older or very thin patients often rely on it.

Creatine is a nutrient; creatinine is its waste product. Creatine is made by the liver and kidneys from amino acids, obtained from meat and fish, and stored in muscle as creatine phosphate, where it regenerates ATP during short bursts of effort. That is why creatine monohydrate is the most studied sports supplement. Creatinine is what creatine becomes when it is used and broken down, and it is measured only because it happens to be a convenient marker of kidney filtration.

Taking creatine raises creatinine. Loading doses can lift serum creatinine by roughly 0.1 to 0.3 mg/dL, sometimes enough to move a result just above the reference range and lower the calculated eGFR by several points, even though the kidneys are filtering normally. Multiple controlled trials in healthy adults have found no evidence that creatine at standard doses damages the kidneys; the elevated creatinine is a production artifact, not a filtration failure. The same is true of a diet very high in cooked meat.

If you supplement, tell the ordering clinician and either pause creatine for about a week before a baseline test or ask for cystatin C alongside creatinine, which is unaffected by supplementation and settles the question. A high creatinine in a creatine user with a normal cystatin C eGFR and a clean urinalysis is a supplement effect, not kidney disease.

A single high creatinine is repeated before anything is concluded, typically within a few weeks and under standard conditions: normal hydration, no hard training for two days, no large meat meal, creatine paused if practical. A repeat in range with a normal eGFR closes the question for most people. A repeat that is still high leads to the standard kidney work-up: a urine albumin-to-creatinine ratio, a urinalysis, cystatin C if muscle mass or medications could be confounding the picture, and a review of every medication and supplement for anything that raises creatinine or injures the kidney.

Chronic kidney disease is diagnosed only when reduced eGFR or a marker of damage persists for more than 3 months, so the timeline is measured in months, not days, unless the rise is steep or accompanied by symptoms. When it is confirmed, the priorities that slow it down are well established: blood pressure control, glucose control in diabetes, avoiding NSAIDs and other nephrotoxins, and medicines such as SGLT2 inhibitors that protect the kidney, prescribed and adjusted by the treating clinician. Symptoms such as rapidly worsening swelling, very little urine, confusion or shortness of breath alongside a high creatinine need same-day medical care.