Complex sentence: The markers appear to be short-lived in the urine after AKI has occurred and may be normal in patients who have already manifest AKI by functional criteria (e.g., serum creatinine).
Split into simpler sentences: The markers appear to be short-lived in the urine after AKI has occurred. This may be normal in patients who have already manifested AKI by functional criteria (e.g., serum creatinine).