Creatinine and eGFR: Why Muscle Mass Changes the Reading

What is creatinine, and why is yours different from someone else's?

Creatinine is a waste product from normal muscle turnover. The kidneys filter it out steadily, so the level in your blood reflects how well filtration is working. It appears on almost every routine blood panel.

Because production depends on muscle mass, women generally have lower values than men, and laboratory reference ranges are reported separately by sex for that reason.

The same logic works within a person, and this is the part that misleads. A very muscular woman can have a creatinine at the top of the range with entirely normal kidneys. A woman who has lost substantial muscle through illness, ageing or prolonged undereating can have a creatinine that looks reassuring while filtration has actually declined. Muscle mass is the hidden variable in both directions.

What is eGFR, and how is it staged?

Because raw creatinine is hard to interpret alone, laboratories calculate an estimated glomerular filtration rate from it using your age and sex. eGFR is reported in mL/min/1.73m² and is the number usually acted on.

A single reduced eGFR is not a diagnosis. Chronic kidney disease is defined by an abnormality persisting for at least three months, which is why the first response to an unexpected result is usually a repeat test rather than a referral.

What moves the number without any disease?

• Dehydration raises creatinine and lowers eGFR, sometimes markedly.
• A large meat meal shortly before the draw.
• Intense exercise in the preceding days.
• Creatine supplements, which raise measured creatinine without any change in kidney function.
• Some medicines, including trimethoprim and cimetidine, which block creatinine secretion without affecting filtration.

The creatine point deserves emphasis. Creatine is now widely and reasonably recommended to women in midlife for muscle and bone, and it can raise a creatinine result. Mention it before anyone investigates your kidneys, because the sequence of an unexplained result followed by a scan is avoidable.

Is there a test that ignores muscle?

Yes. Cystatin C offers a second estimate of filtration that does not depend on muscle mass, and it is useful in exactly the situations where creatinine misleads: a very muscular woman, a woman who has lost substantial muscle, someone taking creatine, or an eGFR result that does not fit the clinical picture.

It is not a routine test and usually has to be requested, but it exists and it is the right answer when muscle is the confounder. If a creatinine-based eGFR is about to change your treatment, it is worth asking whether cystatin C should confirm it first.

What is the other half of the assessment?

Filtration is only one measure of kidney health. The other is whether protein is leaking into the urine, measured as the urine albumin-to-creatinine ratio. A person can have a normal eGFR and significant albuminuria, and that combination carries real risk a blood test alone would miss.

If kidney function is being assessed properly, both are checked. An eGFR on its own is half the picture, and asking for the urine test is reasonable if only the blood one has been done.

What changes with age, and what does not?

Filtration declines gradually with age in most people, so an eGFR in the 60 to 89 band in a woman in her seventies, stable across years and with no protein in the urine, is often unremarkable. A falling trend is what matters, not a single number compared against a young adult's.

This is another argument for keeping your own record of results. A laboratory report shows today's value; only the sequence shows the direction.

Why does this decide your bone treatment?

Kidney function determines which medicines can be used and at what dose, and in bone health the effect is direct.

Bisphosphonates including zoledronic acid are restricted below a defined level of kidney function, and dehydration at the time of an infusion raises risk further. Denosumab can be used at lower filtration rates but carries a higher risk of low blood calcium there, which is why calcium and vitamin D status is checked rather than assumed. See calcium and vitamin D for bones.

If you are choosing between osteoporosis treatments, your eGFR is part of that decision rather than background information. Our page on osteoporosis medications compared sets out where each option sits.

Questions worth asking: "Was I well hydrated when this was drawn?", "Should this be repeated before we treat it as a trend?", "Does my creatine supplement affect this?" and "Has my urine albumin been checked as well?"

Why did the eGFR equation change?

Until recently, the most widely used equations included a race coefficient that reported a higher eGFR for Black patients from the same creatinine. That adjustment had no clear biological basis and, in practice, delayed referral and transplant listing for the patients it was applied to.

Newer equations remove it. If you are comparing an eGFR from several years ago with a recent one, part of any apparent change may be the equation rather than your kidneys, which is worth raising rather than assuming a decline.

What symptoms would you actually notice?

Usually none until filtration is substantially reduced, which is the whole reason this is picked up on routine blood tests rather than by how someone feels. When symptoms do appear, they are non-specific: tiredness, poor appetite, itching, swelling in the ankles, needing to pass urine more often at night.

Foamy urine is worth mentioning specifically, because it can indicate protein leakage and is something people notice and dismiss. Blood in the urine, visible or found on a dipstick, is a separate question that needs its own assessment rather than being folded into a kidney function conversation.

What protects kidney function?

The two largest modifiable drivers of kidney decline are blood pressure and blood glucose, and both are more relevant after menopause than before it. Blood pressure control does more for long-term kidney function than anything in the supplement aisle; see blood pressure in women and measuring it at home.

Two practical points that come up repeatedly. Regular use of non-steroidal anti-inflammatory drugs such as ibuprofen affects kidney function, and women in midlife often take them frequently for joint pain without mentioning it. And staying reasonably hydrated matters at the time of a blood draw specifically, because dehydration is the most common reason for a one-off reduced eGFR.

Protein intake is a common worry and mostly a misplaced one: higher protein intake is recommended for muscle and bone in midlife and does not damage healthy kidneys, though it does modestly raise creatinine. If you have established kidney disease, that is a conversation with your clinician rather than a reason to avoid protein by default.

What if your result changed after starting a new medicine?

Some drugs raise creatinine by blocking its secretion into urine without touching filtration at all. Trimethoprim and cimetidine are the classic examples, and the rise appears within days of starting and reverses on stopping.

Other medicines genuinely affect kidney function, including regular non-steroidal anti-inflammatories and some blood pressure drugs, where a small early rise can be expected and monitored rather than a reason to stop. The distinction between an artefact and a real change is exactly the kind of thing worth asking about rather than guessing at, particularly if the timing lines up with a new prescription.

How does this interact with menopause?

Less directly than lipids or bone, but not by nothing. The two biggest drivers of long-term kidney decline are blood pressure and blood glucose, and both tend to move unfavourably across the menopause transition. Blood pressure in particular rises in the same years the lipid panel shifts.

Muscle loss is the other link, and it runs the wrong way for interpretation. Sarcopenia accelerates after menopause, and less muscle means less creatinine, which means an eGFR that can look better than filtration actually is. A woman who has lost significant muscle and whose eGFR has stayed flat may not be as reassured as the number suggests, which is another argument for cystatin C where it matters.

Our pages on menopause and heart health and resistance training cover both halves of that.