The level of renal functional reserve(RFR)indicates subclinical renal function loss,early stages of chronic kidney disease,and susceptibility to acute kidney injury(AKI).Assessing the patients susceptibility to kidney damage may help develop personalized treatments and prevent AKI.At present,we commonly give oral protein and intrave-nous amino acid infusion to stimulate and measure RFR.These methods are complex,time-consuming,and invasive,espe-cially when diagnostic time is limited or a large population needs to be screened.Therefore,developing simple and conven-ient methods to evaluate RFR is particularly urgent.Recent studies have reported a good correlation between intraparenchy-mal renal resistance index variation(IRRIV)and RFR.And,the IRRIV can be measured with color Doppler ultrasound.So,this is a safe,repeatable,inexpensive,and convenient method that can be used to evaluate RFR in healthy subjects.