THE WEIGHT OF OLD HABITS: FROM RATIOS TO REALITY IN MEASURING KIDNEY GROWTH AFTER UNILATERAL NEPHRECTOMY

Live kidney donation is key to reduce wait times for patients with end-stage kidney disease. Most live kidney donors (LKD) thrive with a single kidney because the remaining kidney undergoes compensatory kidney growth (CKG) to restore near-normal function. Unfortunately, ~20% of LKD do not demonstrate CKG, putting them at risk of kidney dysfunction over time. It is crucial to gain a deeper understanding of why this occurs to improve LKD outcomes. In more than 90% of papers done on animal models, CKG after unilateral nephrectomy (UNX) is evaluated either 1) by comparing the average kidney-weight-to-body-weight ratios (KW/BW) of experimental and control animals (KW/BWother), or 2) by comparing the KW/BW between the nephrectomized and the remaining kidneys from the same animal KW/BWself. However, these methods are problematic because KW/BWother ignores intrinsic between-animal differences, and KW/BWself assumes that left and right kidneys are the same weight. We hypothesize that comparing kidney volumes obtained via MRI scans with both KW/BW approaches will demonstrate that they are unreliable to assess CKG after UNX.

We performed UNX or SHAM in ~8-week-old male mice (C57BL6). MRI scans were taken on post-operative days (POD) 0, 3, 7, 14, and 28. Kidney volumes were measured with ITK-SNAP, using manual outline of every other image in all 3 planes. All surgically removed kidneys were cleaned before weighing with a precision scale; all mice were weighed immediately before. KW/BW was calculated from values obtained on the same day.

We first evaluated the within-animal right-to-left variation in kidney weights at baseline (n=10). We found that the right kidney was larger (>10% heavier) in 20% of cases and was equal in the others. We did the same analysis using baseline MRI images for another 27 mice. We found that the right kidney volume was at least 10% larger in 9 (33%) of the mice, and equal in 18 (66%) of the mice.

For both KW/BWother and KW/BWself, we only observed a statistically significant difference in CKG at POD 28 (23.4% and 25.8%, respectively). Notably, these data were associated with high variance at all time points.

Longitudinal MRI scans showed that in most UNX mice, CKG is observed as early as POD 3 and continues to POD 28; the patterns of growth over time were quite variable between animals. The bulk of CKG occurred in the first 2 weeks. At POD 28, UNX and SHAM animals demonstrated 45% and 2% increase, respectively. Comparison of MRI and KW/BW data for individual animals reveals that KW/BWother and KW/BWself underestimated the degree of CKG in 6/9 (67%) of animals at POD28. One of these mice showed minimal CKG when assessed with  KW/BW methods but displayed significant CKG when using MRI data.

We found that the assumption of equal right-to-left kidneys is incorrect in ~20-30% of mice when combining both weight and MRI approaches. Our data suggest that KW/BW approaches routinely underestimate CKG when compared to volumetric data. Serial MRI revealed significant between-animal variation in growth trajectories; therefore, the interpretation of KW/BW data from early time points (common in most studies) may not accurately reflect the true CKG potential. Since each animal acts as its own control, fewer animals are needed to assess CKG with the MRI method. We conclude that serial MRI is superior to KW/BW approaches to assess CKG.