eGFR

What it actually measures

Your kidneys filter your entire blood volume many times a day, and glomerular filtration rate is the measure of how fast they do it.

Measuring it directly requires injecting a marker substance and tracking its clearance, which is accurate and impractical for routine use.

So it gets estimated instead, which is what the e in eGFR stands for.

The estimate is built from creatinine, a waste product your muscles produce at a fairly steady rate and your kidneys clear. If filtration slows, creatinine accumulates. So creatinine goes up as kidney function goes down, and an equation converts that into an estimated filtration rate using your age and sex.

That is a clever solution and it carries an assumption worth understanding: that your creatinine production is typical for your age and sex.

Creatinine comes from muscle. So the estimate is only as good as that assumption about your muscle mass, which is where most of the interpretive trouble lives.

What changed in 2021, and what still distorts it

For years the standard equations included a race coefficient, adjusting the estimate upward for Black patients on the basis of observed average differences in creatinine.

That practice came under serious scrutiny, both for the biological reasoning and for its clinical consequences, since a higher estimated GFR can delay referral and affect transplant eligibility.

A 2021 study in the New England Journal of Medicine developed new equations without a race coefficient, using development sets of 8,254 participants for creatinine and 5,352 for the combined equation, validated across 12 studies and 4,050 participants.

Two findings matter. First, the old race-based creatinine equation overestimated measured GFR in Black participants by a median of 3.7 mL/min/1.73 m squared.

Second, and more useful practically, equations combining creatinine and cystatin C were more accurate against measured GFR than race-free equations using either marker alone.

Those equations are now the standard, which means a result from 2019 and a result from 2023 may not be directly comparable.

The muscle problem remains, and it is worth understanding. Someone with substantial muscle mass produces more creatinine, which makes their eGFR look lower than their kidneys deserve. Someone with very low muscle mass, through age, illness or amputation, produces less, which makes their eGFR look better than it is.

Cystatin C is the answer to that. It is produced by nearly all cells rather than muscle specifically, so it is far less affected by body composition, which is exactly why combining the two performs better.

What moves it

Down: genuine loss of kidney function from any cause, which is the finding that matters. Diabetes and high blood pressure, the two leading causes globally. Some medications including NSAIDs taken regularly. Dehydration, which lowers it temporarily. And ageing, which reduces filtration gradually as normal physiology.

Up: not usually something to aim for, and a high eGFR is occasionally reported in early diabetes as hyperfiltration. More commonly the number rises because creatinine fell, which can reflect muscle loss rather than better kidneys. Recovering from dehydration raises it back to baseline.

The honest hedge

I am not a doctor and I am not diagnosing anyone, and kidney function is genuinely a doctor question.

The most useful thing I can offer is context for reading your own number rather than advice on changing it.

eGFR is an estimate carrying an assumption about your muscle mass. If you are unusually muscular or unusually low in muscle, that assumption is wrong for you, and a cystatin C-based estimate answers better.

A single low eGFR is also not a diagnosis. Chronic kidney disease is defined by a reduced rate persisting over months, alongside other findings, and a one-off value can reflect dehydration or a temporary insult.

The thing that genuinely deserves attention is a downward trend across several results, which is information a single report cannot give you.

One more marker worth knowing about: urine albumin to creatinine ratio picks up kidney damage before filtration rate falls, which makes it complementary rather than redundant. That is a doctor conversation and it is a reasonable thing to ask about if you have diabetes or high blood pressure.

And a practical note: creatine supplementation raises creatinine modestly without harming kidneys, which is worth mentioning to whoever reads your result.

So. What to actually do.

Tonight, at no cost. Line up your eGFR from every panel you can find, with dates. The trend across years is the information, and a single value rarely is.

Note which equation produced it. The standard changed in 2021 when the race coefficient was removed, so older and newer results are not always directly comparable.

If you are unusually muscular or unusually low in muscle, say so. Creatinine comes from muscle, so the estimate carries an assumption that may not fit you.

Ask about cystatin C if the estimate is questionable. The 2021 work found combining it with creatinine was more accurate than either alone.

Mention creatine supplementation. It raises creatinine modestly without harming your kidneys, and unexplained it looks like something else.

Ask about urine albumin if you have diabetes or high blood pressure. It detects damage before filtration rate falls.

Be careful with regular NSAIDs. Routine long-term use affects kidney function, and it is one of the more common avoidable contributors.

You are not a filtration rate. You are a body whose muscle mass quietly shapes the estimate, being compared to an equation built for someone average.

Your body is not broken. It is blocked. And here the useful move is reading the trend rather than the value, because the direction says more than any single point on it.

Go line up your old results with their dates.