How important is it to have a clean catch in a 24 hour creatinine collection ? the lab didn't give me any instructions for how to collect the urine.

Clean catch is not: that important in a 24 hour collection. I may be repeating myself, for a 24 hour urine: On day one, go pee the toilet at 8 am, after that collect all the urine in the container given to you. On day two pee in the container at 8am to have 24 hour urine collection.

It isn't: For a 24 hour urine creatinine, they are not looking at infection. Therefore, a clean catch is not necessary to avoid bacterial contamination. The important things to avoid are unusual amounts of exercise or trauma to your muscles (both will artificially elevate your creatinine, throwing off the results). Hope this helps.

No worries : You don't have to be concerned about that in a 24 hour urine collection.