Q&A 013: What Creatinine Says About Your Kidneys

⏱ 10 min read

Dr. Hayes Q&A · No. 013

By Eleanor Hayes, M.D. · Hospitalist

The Question

“I’m 61, and I’ve lifted weights four days a week for more than twenty years. My annual physical flagged my creatinine levels: 1.4, with an eGFR of 55. My doctor said ‘let’s keep an eye on it.’ At home I looked it up, and eGFR 55 means stage 3 chronic kidney disease. Stage 3. I don’t drink, and my blood pressure is normal. That night I ended up reading about dialysis. Are my kidneys really failing? And is my doctor being too relaxed about this?”

— Name withheld

The Short Answer

Start with what creatinine actually is, because everything follows from it. Creatinine is not something your kidneys make. It is a waste product your muscles put out as they work, and the kidneys are the crew that clears it. So this number is always the result of two values, not one: how much gets produced, which is muscle, and how much gets cleared, which is kidney. A body built by twenty years of lifting produces more than average. The 5 grams of creatine you take every day adds to the pile. And eGFR is an estimate that translates your creatinine into kidney function while assuming a roughly average build, which means that in a frame like yours, the translation can read worse than the reality. One more thing. The label “stage 3 chronic kidney disease” is not attached by a single reading of 55. By definition, the finding has to persist for more than three months. Your doctor’s “let’s keep an eye on it” is not complacency. It is the definition, being followed. But there is a right way to keep an eye on it, and that is the rest of this article.

Why Creatinine Levels Aren’t a Simple Kidney Score

The production-and-clearance equation. Give two men the same kidney function, and the one carrying more muscle will post the higher creatinine. Turn it around, and a frail patient who has lost most of his muscle can post a gentle-looking number over kidneys that are considerably damaged. Before creatinine is a report card on the kidneys, it is a ledger between muscle and kidney, and it can only be read with both parties in view.

eGFR, the translator. The eGFR is a formula: it takes your creatinine, factors in age and sex, and estimates how well the kidneys filter. It was built to fit the population average, and the further a body sits from that average, the more the translation drifts. A heavily muscled 61-year-old sits exactly in that “far from average” seat. The formal definition of chronic kidney disease, per the National Kidney Foundation, is an eGFR below 60 persisting for three months or more, and when the translation itself is in doubt, there is a second dictionary. Cystatin C is a marker far less influenced by muscle mass, and an eGFR recalculated from it can strip away much of the illusion that muscle creates. It is a word worth taking to your next appointment.

The things that shake the number. Creatinine levels move more easily than people assume. A hard training session the day before the draw. A large, well-cooked meat meal beforehand (the creatine in the meat gets absorbed). Dehydration. As for creatine supplements, the repeated finding is a specific one: a recent meta-analysis of controlled trials concluded that supplementation modestly raises the creatinine number itself without any significant change in actual filtration. It inflates the ledger, not the damage. Ibuprofen and its NSAID cousins are different. That one is not an illusion in the number. It genuinely tightens blood flow through the kidney. And if you have read this far, you may have noticed something: two items from this list, the creatine and the ibuprofen, are sitting side by side in the basics you sent.

The weight of the word “stage.” The search page was right about one thing. Stage 3 is the name given to the eGFR band from 45 to 59. But the definition carries a condition the page put in smaller print: it has to persist beyond three months. One measurement is not a stage. It is a snapshot. And a stage is never read alone. Its required companion is the urine, specifically the protein in it (a test called uACR). Two men with the same eGFR but different urine results carry genuinely different outlooks. Holding a single 55 and reading about dialysis is like holding one frame of a film and predicting the ending.

The line that still holds. This is an article about reading a number correctly, not about ignoring it. If a recheck done under clean conditions still lands under 55 and keeps drifting down, or if protein shows up in the urine, the story changes, and a nephrologist is the right next room. Whether this is an illusion built by muscle or a signal sent by the kidney is not settled by feel. It is settled by procedure, and the procedure is below.

What I See in the Hospital

I have a job where I watch creatinine move every single day. An admitted patient’s number rises and falls between morning labs. Correct the dehydration and it is down 0.3 by sunrise. Add a contrast scan or the wrong medication and it is up 0.4. That is why hospital people do not read one creatinine value as a verdict. We read direction, and we read context.

The creatinine that genuinely frightens me wears a different face. It is the one that walks quietly, one direction, over years: 0.9, then 1.1, then 1.3. The emergency room does not catch that number. By the time it arrives through those doors, it is often late. What catches it is the trend. In the first article of this category I wrote that a single value is a photograph and a trend is the film, and no organ makes that sentence more literally true than the kidney.

Plain-Language Reference

What to Actually Do

· Clean up the conditions for 48 hours before the recheck. Hold the creatine, rest from heavy training, skip the large meat meals, and drink water properly. A number drawn under those conditions is finally talking about your kidneys.
· Ask for two things at the follow-up: a urine protein test (uACR), and if available, an eGFR calculated from cystatin C. Those two answers settle what the 55 actually was.
· Put the post-workout ibuprofen habit on the table, especially during any stretch when kidney numbers are being watched, and talk through alternatives with your doctor. Medications get their own category later in this series.
· Repeat the test in three months under the same conditions. What you are looking for is not a value but a direction. Does 55 stay 55, or does it keep walking down? That answer decides the next destination.

This article is for general information only and does not constitute medical advice, and reading it creates no provider-patient relationship. How an eGFR of 55 should be interpreted depends on individual factors this article cannot assess, and nothing here replaces the judgment of your physician or a nephrologist. If reduced kidney numbers come with swelling, a marked change in urination, blood in the urine, or new high blood pressure, seek medical evaluation promptly rather than waiting for a scheduled recheck.

Take care of yourself. Nobody else will do it for you.
— Eleanor Hayes, M.D.