The kidney test that misreads creatine: what every gym-goer should know before their next blood draw

A 25-year-old walks out of his GP's office with a phone call to make. He has just been told his kidneys may be failing. He has no symptoms, no family history, no diagnosis. The only thing on his health record that is even slightly out of the ordinary is two years of daily creatine.

His kidneys are fine. The test was wrong. Or rather, the test was right, but it was reading the wrong number.

This is a real published case (Williamson & New, 2014), and a version of it happens regularly to gym-goers in Germany who never hear about it. If you supplement creatine, the standard kidney test that your doctor ordered was not designed for you. Here is what is actually going on, and the one marker that fixes it.

Why the test reads wrong

Standard kidney panels are built around a substance called creatinine. Your body produces it constantly as a waste product when muscle uses creatine for energy. Healthy kidneys filter it out into urine, so the level in your blood reflects how well your kidneys are working. Higher creatinine in the blood, in most contexts, means slower filtration.

Creatine supplementation breaks this assumption. Take a daily 5-gram dose, and you are giving your body more of the raw material that becomes creatinine. More substrate, more product. Your blood creatinine goes up. Your kidneys are still doing their job perfectly.

An analysis of 21 studies confirmed exactly this: creatine supplementation raises serum creatinine, but actual kidney filtration is unchanged (Naeini et al., 2025). Same kidneys, same filtration, higher number on the blood test.

The eGFR, the calculated filtration rate that ends up on most reports, is built directly from creatinine. It inherits the same blind spot. A high creatinine produces a low eGFR, which can land you below the threshold flagged as chronic kidney disease even when nothing is wrong.

The marker that does not get fooled

There is a different way to measure kidney filtration. It is called cystatin C.

Every cell in your body with a nucleus produces cystatin C at a steady rate. Your kidneys filter it out, the same way they filter creatinine. The level in your blood tells the same story about filtration. The difference is that nothing in a gym-goer's lifestyle moves it. Not muscle mass. Not protein intake. Not creatine.

In a randomised controlled trial, healthy men took high-dose creatine for three months while training. Their creatinine-based numbers got worse. Their cystatin C actually got slightly better, suggesting their filtration had if anything improved (Gualano et al., 2008). Two markers, same kidneys, opposite stories. One of them was telling the truth.

One more thing: the same blind spot exists for liver enzymes

While we are here, briefly. The liver enzymes AST and ALT have a similar problem. Both rise after intense weight training, sometimes for up to a week, because they are also present in skeletal muscle (Pettersson et al., 2008). A blood test taken two days after a hard session can look like a liver problem when you simply lifted heavy on Thursday. GGT, the third liver enzyme, is the one that does not rise with training and gives a cleaner liver signal.

That is a different article. The principle is the same: standard panels were not designed with serious training in mind. Read the result with that in mind, or take 48 hours off before the draw.

What to actually do

This is simpler than the rest of the article suggests.

If you have ever been told your kidney numbers look off, and you supplement creatine, ask for cystatin C before you panic. If you have not been told anything, but you have been on creatine for a year or more, get a baseline. Knowing what your filtration actually is when read through a marker your stack does not distort is worth the price of one blood test.

You can also read our deeper guide on creatine beyond the muscle if you want context on what the supplement is actually doing in the rest of your body.