Check in what units the GFR is given. If e.g. in ml/min, then if multiplying GFR with plasma creatinine concentration you get amount of creatinime excreted in 1 ml of urine during one minute, not in 24 hours.

There are several models for estimating GFR (e.g. MDRD, CKD-EPI), each with its own assumptions and limitations. Check the link below for a description to help you chose the one most suitable to your subjects. Are your patients children? Are your estimated GFR results already normalized for body surface area?

Estimating Glomerular Filtration Rate:

https://www.niddk.nih.gov/health-information/communication-programs/nkdep/laboratory-evaluation/glomerular-filtration-rate/estimating

I would recommend the sale answer as Katerina Sebekova

Dear Katarina.

In fact, it is in mil/min.

But, if I have total volumen excreted in 24 hrs, can I calculate the total creatinine in 24 hrs?

thanks everyone

Once again: multiplying GFR in ml/min by plasma concentration, you get amount of crea excreated in 1 minute. thus, multiply this with 60*24 to get excretion in 24 h. Alternatively, if you convert the grf into ml/24h, and you know the 24h urinary volume, you have 3 variables out of 4 in gfr formula, thus you can calculate urinary crea concentration. but you still have to multiply it by the volume to get 24 h excretion - thus this approach logically will give same results. Simply, in either case calculation is a simple proportion.