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Kidneys have an intrinsic reserve capacity to respond to a higher workload by increasing filtration in their nephrons, called renal functional reserve (RFR). Despite the high clinical relevance of RFR, the necessary dynamic measurements are rarely done in the clinical routine, due to time- and workload as well as lack of standardized protocols.
Our goal was to create a one-day RFR test to use routinely in an outpatient clinic.
We developed a novel RFR protocol using 99mTc-DTPA (DTPA Cl) before and after an oral protein load performed within one day. In addition we measured in parallel the creatinine and Cystatin C clearances.
RFR was defined as the difference of post-protein stimulation peak measured GFR (mGFR) to baseline mGFR in fasting state.
In a pilot study we measured RFR in 7 healthy participants. The results showed a high heterogeneity and allowed for trouble-shooting.
We modified the study protocol for the next 16 patients by (a) extending the time of measurements to 330 min post-stimulation and (b) increasing the frequency of activity measurements to every 30 mins. In addition, we used an increased protein load of 1.5 g/kg bwt of beef protein. These standardized measurements showed inter-individual time differences in reaching the peak GFR values post-protein load, the peaks detectable between 150 to 270 mins after the protein meal. The mean RFR (±SD) was 14 (±13) ml/min/1.73 m2 corresponding to 16 (±15)% (see Fig.).
RFR can be measured with a same day pre- and post-stimulation DTPA-clearance protocol. Using a high oral load of beef protein and a long, post-stimulation period of measurements demonstrates inter-individual differences in reaching the hyperfiltration peak and a significant, previously not appreciated, post-prandial drop in GFR. A short post-stimulation measurement period, a too low protein load and too long measurement intervals might not detect the true RFR.
Parts of the results were presented as a poster at the ERA congress in Milan, June 2023.