Kidney Week

Abstract: SA-PO0548

Validation of a Simple Oral Protein-Loading Protocol for Glomerular Functional Reserve Assessment

Session Information

• CKD: Clinical - Therapies, CKM Health, Special Populations, and More
  October 24, 2026 | Location: Exhibit Hall A, Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: CKD (Non-Dialysis)

• 2202 CKD (Non-Dialysis): Clinical, Outcomes, and Trials

Authors

• Athavale, Ambarish, University of California San Diego, La Jolla, California, United States

Ambarish Athavale, MD, MBBS

• Castro, Erick Orlando, University of California San Diego, La Jolla, California, United States

Erick Orlando Castro

• Ix, Nicole Illice, University of California San Diego, La Jolla, California, United States

Nicole Illice Ix

• Ix, Joachim H., University of California San Diego, La Jolla, California, United States

Joachim H. Ix, MD, FASN

Background

Renal glomerular functional reserve (RFR) reflects the kidney's capacity to increase glomerular filtration rate (GFR) in response to protein or amino acid challenge, and may uncover early impairments not detected by static GFR assessment, but existing protocols are cumbersome. A simple out-patient protocol would enhance use in research and clinical care.

Methods

We evaluated five healthy adults (3 men, 2 women; ages 46–52) with no known comorbidities. Following a baseline blood and urine collection, participants received an oral protein load based on body weight (LiquaCel®, 1.2 g/kg). Blood and urine samples were obtained every 30 minutes for 150 minutes. eGFR was calculated at each timepoint using the CKDEPI 2021 race free equations based on serum creatinine (eGFR_creat), cystatin C (eGFR_cys), and a combined C equation (eGFR_comb). RFR was defined as peak minus baseline eGFR for each method. The same experiment was repeated after 2 weeks, and stability was assessed by within individual coefficients of variation (CVs).

Results

All participants demonstrated a measurable increase in eGFR following protein loading, with peak eGFR at 120 minutes after protein load. Mean RFR differed by eGFR biomarker: +5.21 ± 4.2 mL/min/1.73 m² using eGFR_creat, +8.09 ± 4.5 using eGFR_cys, and +7.73 ± 4.4 using eGFR_comb. Comparing the two RFR tests 2 weeks apart, the mean within individual CVs were similar with eGFR_cys (mean CV 30.45%) and with eGFR comb (mean CV 30.22%) and greatest with eGFR_creat (mean CV 41.12%).

Conclusion

In healthy individuals, RFR is measurable over 2 hours using oral protein load, and standard eGFR measurements. The magnitude of RFR was larger using eGFR_cys and eGFR_comb, perhaps because of enhanced creatinine secretion. RFR changed considerably over 2 weeks, but had less variability using combined creatinine-cystatin c equation. A simple oral protein tolerance test (OPTT), akin to an oral glucose tolerance test, may be useful to guage RFR in persons with or at risk for kidney disease.