Understanding Your Kidney Function Tests: What Creatinine and eGFR Mean

If you've had routine bloodwork in the last few years, there's a good chance your results included two numbers you may not have thought much about: creatinine and eGFR. They sit quietly on the lab report, usually without much explanation, until something flags them as high or low. Here's what they actually measure, why the eGFR formula changed a few years ago, and what we look at before deciding whether a number is worth worrying about.

What creatinine actually tells us

Creatinine is a waste product made by your muscles as part of normal, everyday activity. Your kidneys filter it out of your blood and send it out in urine. Because the amount your muscles produce stays fairly steady, how much creatinine builds up in your blood is a decent proxy for how well your kidneys are filtering.

The catch is that creatinine alone is a blunt tool. Muscle mass, age, sex, and even what you ate recently can shift it a bit. A bodybuilder and a sedentary older adult can have very different "normal" creatinine levels with equally healthy kidneys. That's why creatinine by itself rarely tells the whole story, and why it gets plugged into a formula instead of read on its own.

eGFR: the number that actually matters more

eGFR stands for estimated glomerular filtration rate. It's a calculation, not a direct measurement, that combines your creatinine with your age and sex to estimate how many milliliters of blood your kidneys filter per minute. This is the number we actually pay attention to.

Generally, an eGFR of 90 or above reflects normal kidney function. Numbers in the 60s to 80s can still be normal, particularly as you get older, since kidney function naturally declines with age even in healthy people. Research on primary care risk stratification describes chronic kidney disease as reduced eGFR and/or increased albuminuria together, not either one in isolation (https://doi.org/10.1080/00325481.2026.2715889). That combination matters, which is why we don't react to a single low eGFR without more context.

What we're really watching for is a trend. One eGFR of 58 means very little on its own. An eGFR that's dropped from 85 to 70 to 58 over three years means a great deal, even though each individual number might look unremarkable in isolation.

Why your eGFR number might have shifted even though nothing changed

If you've had kidney labs drawn over several years, you may have noticed your eGFR value change slightly even when your creatinine stayed the same. In 2021, laboratories began adopting an updated calculation that removed race as a variable from the eGFR equation. Research evaluating the newer CKD-EPI and EKFC formulas found that the updated equations perform with comparable accuracy and more consistency across different population groups than the versions they replaced (https://doi.org/10.1681/ASN.0000000000000227). If your lab report shows a different eGFR than a result from several years ago using an identical creatinine value, this update is almost certainly why. It's not a sign anything went wrong with the earlier test.

The number labs often skip: albuminuria

Creatinine and eGFR tell you about filtering capacity, but they can miss early kidney changes. That's where a urine test called the albumin-to-creatinine ratio, or UACR, comes in. It measures whether your kidneys are letting a protein called albumin leak into your urine, which is often one of the earliest detectable signs of kidney stress, sometimes showing up before eGFR drops at all.

This test is especially important if you have diabetes or high blood pressure. A real-world study of primary care patients with both conditions found that UACR testing was substantially underused: only 32% of patients had it checked, compared to 76% who had eGFR tested and 86% who had an A1C done (https://doi.org/10.1016/j.jcjd.2026.08.185). Among those who were tested, 11% already had meaningful albumin leakage, and patients with higher blood pressure had notably higher rates of it. If you have diabetes or hypertension and can't remember your last UACR result, it's worth asking about at your next visit.

When a low number is a false alarm, and when it isn't

A single off kidney value is common and usually not a crisis. Dehydration, a recent intense workout, a high-protein meal, or certain medications (including over-the-counter pain relievers like ibuprofen) can temporarily shift creatinine and eGFR. Before acting on an abnormal result, we typically want to see it repeated, ideally when you're well-hydrated and off anything that might skew it.

What does prompt closer attention: a result that's low and stays low on repeat testing, a clear downward trend over months or years, or a low eGFR paired with albumin in the urine. High blood pressure deserves particular attention here, since uncontrolled hypertension is one of the most common drivers of gradual kidney decline, which is part of why the US Preventive Services Task Force continues to recommend routine blood pressure screening for all adults as a foundational piece of preventive care (https://doi.org/10.1001/jama.2021.4987). Chronic kidney disease is also more common in people with diabetes, in older adults, and is often under-screened outside of diabetes care specifically, according to a recent primary care review of CKD identification (https://doi.org/10.1080/00325481.2026.2715889).

What we do with an abnormal result

If your kidney labs come back outside the expected range, the next steps depend on how far off they are and what else is going on. Often that means repeating the test in a few weeks, reviewing your medication list for anything that might be contributing, and adding a UACR if it wasn't already done. If a lower eGFR is confirmed and persistent, we'll talk through what might be driving it, whether that's blood pressure, blood sugar, a medication, or something else, and build a plan from there. Some situations call for referral to a kidney specialist, particularly when eGFR is significantly reduced or albuminuria is substantial, but most mildly abnormal results are managed well within primary care with monitoring and some adjustments.

The goal isn't to alarm you over a number that bounces around for ordinary reasons. It's to catch a real, persistent trend early, while there's still a lot that can be done about it.

If it's been a while since your kidney function was checked, or you've never had a UACR done despite having diabetes or high blood pressure, that's a reasonable thing to bring up at your next visit. Schedule a primary care visit to review your labs and build a plan around what they show. Most major plans are accepted, including Medicare.