Creatinine is a nonenzymatic breakdown product of creatine and creatine phosphate, compounds that store and release energy in skeletal muscle. Biochemically it is a small organic acid measured in blood and urine; because production is relatively constant for a given muscle mass, it has long been used as an indirect indicator of renal excretory function. Early biochemical work in the 19th and early 20th centuries identified creatinine as a distinct metabolic product, and clinical labs adopted serum and urine measurements as routine tools to assess elimination and muscle-related variation. Physiologically, creatinine is filtered by the glomeruli and to a smaller extent secreted by renal tubules, so its concentration reflects the balance of generation and clearance. Because muscle mass, activity, diet, fluid status, and certain medications can influence levels, creatinine is most informative when interpreted together with urine measurements, estimated clearance calculations, and other markers of metabolic and vascular health.

Elevated creatinine levels often point to reduced clearance of metabolic waste relative to production; this can be associated with lower kidney filtration, reduced effective blood flow to the kidneys, dehydration, or higher muscle mass and recent intense activity. Certain drugs and rapid changes in hydration or muscle breakdown can also raise values. Low creatinine levels can reflect lower muscle mass, pregnancy, overhydration, or reduced creatine intake; lab variability and sampling timing also play a role. Always interpret high or low results in the context of the full Cardio Zoomer panel, including urine creatinine, metabolic markers, and clinical information before drawing conclusions.

Cardio Zoomer includes creatinine to characterize renal clearance and muscle-related metabolic load, which affect circulating metabolites, drug handling, and vascular stress markers. Paired with urine creatinine and other domains (metabolism, inflammation, endothelial function), it helps clinicians distinguish whether lab patterns stem from elimination differences, muscle mass, or systemic processes—supporting more precise, personalized assessment.