Creatinine

A 2025 review calls serum creatinine the most used clinical marker for estimating kidney function as the glomerular filtration rate (GFR), because the test is simple, fast and inexpensive [1]. It has limits. The review lists factors that can influence creatinine levels, including advanced age, physical activity, protein-rich diets, male sex, medications and ethnicity [1]. Serum cystatin C, alone or combined with creatinine, may serve as an alternative because these factors do not affect it [1].

Most creatinine is made in muscle, so the same review describes creatinine as a useful marker of lean body mass as well [1].

Creatine and creatinine are different molecules, but they are linked. A pharmacokinetic review explains that the body clears creatine partly through the kidneys and partly by breaking it down to creatinine [2]. A 2025 meta-analysis of creatine and kidney function says concerns about kidney health persist largely because creatine intake is associated with higher serum creatinine [3]. Its authors found a small rise in serum creatinine, which they attribute to metabolic turnover rather than kidney impairment, and no significant difference in GFR [3].

So a creatinine result depends on muscle, diet and supplements as well as on the kidneys. That is why the 2025 meta-analysis also looked at GFR [3], and why the creatine page reports the kidney evidence in its safety section.

• Creatine: the compound page grades the trial evidence and covers kidney safety.
• Lean mass: a 2025 review describes creatinine as a marker of what it calls lean body mass [1].
• Meta-analysis: the study design behind the kidney-function findings above.