Nephron number has been considered a major determinant of renal allograft function and outcome. It has been demonstrated that low birth weights give rise to a reduction in nephron number and low glomerular size in the postnatal kidney. The impact of birth weight on renal allograft function and outcome in living kidney donor recipients, however, hasn't been addressed so far.

To investigate the impact of donor birth weight, kidney volume, and kidney weight on allograft outcome and function, we collected data from 106 living donor and recipient pairs, who underwent living donor transplantation between 2003 and 2012. Samples were collected before nephrectomy, at +12, +36, and +60 months to assess eGFR and proteinuria. Birth weight was retrieved from donors' birth records. Kidney volume was measured from contrast-enhanced CT scans using the ellipsoid method. Kidney weights were collected immediately before grafting.

In univariate analysis, donor age, donor birth weight, kidney weight/BSA, and kidney volume were associated with eGFR (R=-0.267, p=0.015; R=0.336, 0.010; R=0.310, p=0.011; R=0.224, p=0.032). To identify independent factors that affect eGFR, multivariate analysis using a mixed model was applied. Donor age, birth weight, and and kidney weight/BSA (p<0.05) were independent factors that affected the eGFR. Renal function showed a positive correlation with donor birth weight in rejection-free KTRs until +36 and +60 months posttransplantation (R=0.199, p=0.048; R=0.290, p=0.025). Birth weight, however, does not affect allograft survival. In addition, low birth weight conferred greater risk for proteinuria and necessity for more antihypertensive drugs (p<0.05).

Based on our results, donor birth weight represents an important independent factor for short-term and long-term allograft function in rejection-free KTRs, changes in hypertension, and proteinuria. Due to the natural decline of renal function with age and limited compensation by renal functional reserve, markers of nephron mass are essential to predict allograft outcomes in particular from elderly donors. The impact of low birth weight on long-term allograft function and survival need to be addressed in upcoming studies.

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