How serum creatinine in blood is brought to normal, if it was high when you had stomach flu?

Hydration: In most cases, the elevation in creatinine is caused by dehydration due to nausea and/or vomiting, so the answer is rehydration. Sports drinks or soup broth are more effective than water alone. Less commonly, the dehydration can cause more serious kidney injury that may not get better quickly with rehydration. Medical therapy is required in such cases.

Rehydration: Your kidney functions often worsen while sick, generally due to dehydration. Particularly if you are vomiting, having diarrhea, or both. Increasing your oral fluid intake will help dramatically to improve your renal function.