Kidney Function Tests: Creatinine, eGFR, and What the Numbers Mean

If you've moved to Pattaya, or you fly in for a check-up between countries, the kidney line on your blood panel is easy to skim past. Creatinine, eGFR, maybe a urine test you didn't know you'd given — a couple of numbers, rarely flagged, rarely explained. Yet this small corner of the report is quietly one of the most useful, because the kidneys give almost no warning when they start to struggle. This is a plain-language guide to what creatinine and eGFR actually measure, what the stages mean, and where the numbers can mislead. It's general education, not a diagnosis; your own results are interpreted by a doctor who knows your history.

What is creatinine, and why is it on the test?

Creatinine is a waste product. Your muscles break down a little of their fuel every day as part of normal activity, and creatinine is one of the by-products — produced at a fairly steady rate and then filtered out of the blood by healthy kidneys and passed in your urine ⁴. Because that clearance is the kidneys' job, the level of creatinine left circulating in your blood becomes a window onto how well they're working: when the kidneys filter less effectively, creatinine builds up, so a rising blood creatinine suggests the kidneys are filtering less well ¹⁴.

On its own, though, a raw creatinine value is hard to read — what's "normal" depends a lot on your body. That's why labs rarely stop there.

What is eGFR — and why is it the headline number?

The kidneys' core function is to filter blood, and the gold-standard measure of that is the glomerular filtration rate (GFR) — how many millilitres of blood your kidneys clean per minute. Measuring it directly is cumbersome, so labs estimate it instead, which is why you see eGFR (estimated glomerular filtration rate). An equation takes your creatinine and adjusts it for your age and sex, producing a single figure that estimates how much blood your kidneys filter per minute ¹³.

That adjustment is what makes eGFR so useful. By folding in age and sex, it turns a raw creatinine into something comparable across people — which is why eGFR, not creatinine alone, is the single best overall number for kidney function ¹. It's reported in mL/min/1.73m², and broadly, higher is better.

What is chronic kidney disease, and how is it staged?

Chronic kidney disease (CKD) is the gradual, long-term loss of kidney function. The formal definition has two routes in: kidney damage, or an eGFR under 60, that persists for at least 3 months ². The "3 months" matters — it's what separates a temporary dip from genuine chronic disease, and it's a big part of why a single low reading isn't a diagnosis.

CKD is then divided into stages by eGFR, from G1 (normal filtering) down to G5 (kidney failure) ²:

The single most important thing to know about CKD is how quietly it develops. It's often silent until advanced — there may be no symptoms at all in the early stages — which is exactly why it's usually picked up by blood and urine tests rather than by how you feel ¹².

What about the urine test?

Blood eGFR is only half the picture. The other half is a urine test for albumin, a protein that healthy kidneys keep in the blood. When the kidney's filters are damaged, small amounts of albumin leak into the urine — so its presence is an early marker of kidney damage, often appearing before eGFR falls. The usual measure is the urine albumin-to-creatinine ratio (uACR), and a result over 30 mg/g is considered a marker of damage ¹. It's why the full kidney check is a blood test and a urine test, not one alone — and it's part of how CKD can be diagnosed even while eGFR still looks normal ².

Where the numbers can mislead

Creatinine is convenient, but it has real limits worth understanding before you read too much into one result.

The biggest is muscle mass. Because creatinine comes from muscle, a very muscular person naturally produces more of it and can show a higher creatinine — and therefore a falsely "worse" eGFR — without any kidney problem at all. The reverse also happens: an elderly, frail, or low-muscle person can have a normal-looking creatinine despite genuinely reduced kidney function ¹. The number has to be read against the body it came from.

Creatinine is also nudged by everyday things: hydration, a recent high-meat meal, certain medications, and acute illness can all shift it, and it's least reliable when results sit near the normal cut-off ¹². For all these reasons a single value should be confirmed before diagnosing chronic disease — usually by repeating it after some weeks, which is also why the definition demands that 3-month persistence ². And one limit no equation can fix: eGFR tells you the kidneys are filtering less, not why — it doesn't identify the cause, which needs the urine test, history, and sometimes imaging to work out ¹.

One more thing has changed recently and is worth knowing if you compare reports across years or countries. In 2021 a US task force from the National Kidney Foundation and the American Society of Nephrology recommended removing the Black-race coefficient that older eGFR equations had used, and the race-free 2021 CKD-EPI creatinine equation is now the standard ⁵. If an old result and a new one differ slightly for no clear reason, the underlying equation may simply have been updated.

What we see at the clinic

A lot of the people we meet in Pattaya have never had their kidney numbers explained — they've been between health systems, feeling completely well, and CKD gives no symptoms to prompt a check. The two leading causes worldwide are diabetes and high blood pressure, so we pay particular attention to kidney markers in anyone managing diabetes or raised blood sugar, where keeping an eye on eGFR and the urine albumin test over time genuinely matters. We don't treat kidney disease or diagnose it from a single line on a report — anything that needs follow-up means seeing a doctor, and where appropriate a nephrologist. What we do is establish an honest baseline and explain it, which is especially worth doing here because CKD is far more common than people expect: the Thai SEEK study found an overall CKD prevalence of 17.5%, yet only 1.9% of those affected were aware they had it — a striking illustration of how silent this disease can be ⁶.

Common questions

My eGFR is "normal" — do I still need the urine test? Often, yes. Albumin can leak into the urine before eGFR drops, so the uACR can flag early kidney damage while your blood number still looks fine — which is part of why CKD can be diagnosed from urine alone ¹². The two tests answer different questions.

My creatinine came back a little high — does that mean kidney disease? Not on its own. Creatinine is swayed by muscle mass, hydration, a recent high-meat meal, medications and acute illness, and it's least reliable near the normal cut-off — so a single raised value is confirmed (usually by repeating it) before anyone draws a conclusion ¹².

Why might my eGFR be lower just because I'm very muscular? Because more muscle produces more creatinine, which the equation reads as reduced filtering even when your kidneys are fine ¹. It's a known limitation, and it's exactly the kind of context a doctor factors in rather than taking the number at face value.

Does eGFR tell me what's wrong with my kidneys? No — it estimates how well they filter, not why filtering has changed ¹. Finding a cause needs the urine test, your history, and sometimes imaging, which is why the number is a starting point rather than an answer.

Why are my results different from a test I had abroad? Partly because labs use slightly different methods and ranges, and partly because the eGFR equation itself was updated to a race-free version in 2021 ⁵. For tracking a trend it helps to use the same lab where you can and to keep your old reports.

Key takeaway

The kidney line on a blood panel deserves more attention than it usually gets, precisely because chronic kidney disease is silent until late and surprisingly common. Learn the two numbers that matter — creatinine, the muscle waste product your kidneys clear, and eGFR, the age- and sex-adjusted estimate of how well they filter ¹⁴ — and remember the line at 60 that, when it persists for 3 months, defines CKD ². Read both alongside a urine albumin test, treat any single value as something to confirm rather than a verdict, and let a doctor interpret it with your history. If you have diabetes or high blood pressure, this is one part of the regular check worth not skipping.