[scientizscht (OP)]

A substance has a concentration of 20 umol/mmol creatinine in urine.

How is this converted to uM in urine?

Or can you calculate the average amount of the substance excreted by urine every day?

Thanks

[Bored chemist]

The creatinine correction gives a (very) rough correction for urine volume.
Why do you ask?
In most cases the concentration of a material in urine is essentially a measure of how much water they drink rather than the amount of the substance in the body.

[evan_au]

Creatinine excretion is used as a general marker of kidney health.

Creatinine Clearance ratio is calculated from the concentration of creatinine in the blood and in urine.
An estimate of the Glomerular Filtration Rate (GFR) can be made from a blood test, or (better) from a blood test + urine test.
For a more thorough analysis of kidney function (and water intake), a full urine collection will be made over 24 or 48 hours.

See: https://en.wikipedia.org/wiki/Creatinine#Biological_relevance