Renal functional reserve (RFR) measures the difference between the stimulated glomerular filtration rate (GFR) and the baseline GFR to detect early signs of renal functional decline. The protein load test (RFR-T) is the gold standard for RFR assessment but is a complicated procedure. Renal intraparenchymal resistance index (RRI) variation test (DRRI-T) is a non-invasive method to measure renal function reserve using ultrasound. A saline bag is used to induce renal vasodilation, and the DRRI is found by calculating the difference between baseline (without weight) and stress RRI (with weight). Normal DRRI-T is greater than 0.05.

Our study compared RFR-T and DRRI-T in 50 patients with normal kidney function. We evaluated anthropometric parameters, cardiovascular risk factors, and performed blood and urine tests. Patients were over 18 years old with an estimated GFR (eGFR) CKD-EPI > 60 mL/min/1.73 m2. We excluded pregnant patients, those intolerant to milk protein, those with abnormal kidney ultrasound, or taking medication affecting intrarenal hemodynamics. We used Gwet's AC1 statistic to assess concordance between tests.

Our study found moderate concordance (0.545 coefficient value, p-value < 0.001) between preserved RFR-T (≥ 15 mL/min/1.73 m2) and DRRI (VN > 0.05). Preserved RFR had a significant association with baseline eGFR. Age and sex have an impact on RFR. RFR deteriorates with age, leading to a significant decrease (p = 0.0220), which is more pronounced in women than men (p = 0.0350).

The Ultrasound test (DRRI-T) can measure RFR in just 10 min. This contrasts the gold standard method for estimating RFR, which involves a protein load test, takes a long time to execute, and requires numerous blood and urine samples, making it challenging for large-scale use. While the DRRI-T demonstrated moderate concordance with the protein load test, it did not meet the criteria to be considered a new gold standard test. We posit that it could serve as a valuable initial screening test, warranting further exploration alongside the more elaborate protein load test. Our study suggests that there may be differences between men and women in RFR changes and with regard to age, warranting further investigation on larger populations through ad hoc studies. Our work is among the first to offer original real-life experience in this field.