Creatinine

What it is

Creatinine is produced at a fairly constant rate from creatine in muscle and removed almost entirely by the kidneys. If filtration falls, blood creatinine rises, which is why it underpins the estimated glomerular filtration rate reported on every basic panel.

The 2021 CKD-EPI equation, which removed the race coefficient, is the current standard for that estimate in adults, and it is what our calculator uses.

The muscle mass problem

Because creatinine comes from muscle, production varies with how much muscle a person has. A heavily trained adult can show a creatinine that looks abnormal with entirely normal kidneys, and an older adult with little muscle can show a reassuring creatinine while filtration is genuinely reduced.

High protein intake, a large meat meal in the previous 24 hours, creatine supplementation, dehydration and intense exercise all nudge it upward. Some medications, including trimethoprim and cimetidine, raise it by blocking tubular secretion without changing filtration at all.

What to do when it looks off

Repeat it under standard conditions first: hydrated, no creatine for a couple of weeks, no heavy training or large meat meal the day before. Many surprises resolve there.

If it persists, cystatin C is the marker that does not depend on muscle, and KDIGO recommends it as the confirmatory test. A urine albumin to creatinine ratio answers a different and equally important question about kidney damage rather than filtration.

References

1. Inker LA, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. 2021.
2. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013.
3. Samra M, Abcar AC. False estimates of elevated creatinine. Perm J. 2012.