This is a single-centre, prospective longitudinal follow-up study of 104 LKDs from March 2010 to January 2018 at National University Centre for Organ Transplantation (NUCOT) in National University Hospital, Singapore. Institutional Review Board approval was obtained with the reference number—2013/00497-SRF0002. All methods and protocols complied with institutional review board-based guidelines.

Potential donors undergo a pre-operative assessment by a multi-disciplinary team comprising of transplant surgeons, transplant nephrologists, psychiatrist and medical social worker. Assessment and selection of all LKDs was in accordance with the Amsterdam Forum [5]. Following medical assessment, potential donors were reviewed and approved by the institutional ethics review committee prior to donation. Donor nephrectomy was performed either using an open technique via a flank incision, or minimally invasive techniques which included total laparoscopic [6] or hand-assisted laparoscopic donor nephrectomy [7].

The choice of side of donor nephrectomy is generally based on surgeon’s discretion/preference after taking into consideration of factors such as renal vascular anatomy, presence of kidney stones, kidney size and split renal function. Generally, the smaller kidney will be selected for donation. However, the larger kidney may be selected in some cases. The side of nephrectomy and the rationale behind choosing it is explained clearly to the donor during the informed consent taking process.

All LKDs underwent contrasted multi-phasic computed tomography (CT) scans prior to donation. CT images were acquired on a multi-slice helical CT scanner using 3 mm cuts obtained in arterial, venous and non-contrast phases according to our hospital’s Department of Diagnostic Imaging protocol. Kidney volumes were measured using an adapted technique described for liver volumetry with propriety tissue segmentation software, ImageJ (NIH, Maryland) [8], which was previously validated for use for kidney volume measurement [9]. Using this method, left and right kidneys were measured separately.

Patients were subsequently divided into 2 groups depending on the donated kidney being ≥ 10% larger (Group 1) or < 10% larger (Group 2) than the remaining kidney, as directed by their SRV of bilateral kidneys based on computed tomography volumetry, taking 52.5% as cut-off. The calculation of SRV is performed using the following formula:

$$SRV = \frac} \times 100$$

Examples of calculation is shown in Table 1. Using this formula, any LKD with a SRV of ≥ 52.5% would have donated a kidney ≥ 10% larger than the remaining kidney. The cut-off of SRV ≥ 52.5% was used as it is widely acknowledged amongst renal transplant surgeons and based on a poll performed amongst renal transplant surgeons through the American Society of Transplant Surgeon (ASTS) CenterSpan forum.

After donor nephrectomy, patients were seen 6-weeks after for a post-surgical review, then at 6-months, 1-year, and then annually thereafter—unless there are clinical indications to deviate from this.

Patients’ demographic data such as age, gender, BMI, past surgical history were collected. In addition, individual kidney sizes, kidney function at various time intervals and comorbid conditions were also collected. For this study, kidney function is measured using estimate glomerular filtration rate (eGFR) calculated using the CKD-EPI creatinine 2021 formula [10], which is the default calculation used and reported together with serum creatinine by our laboratory and previously validated in our living donor population [9].

Descriptive statistics are presented in frequency and percentages for categorical variables, mean and standard deviation for continuous variables. Comparison across categorical variables were performed using Chi-squared test and Fisher’s exact test (when required). Means of continuous variables were analysed using Student’s T-test. For comparison across time intervals, repeated measures ANOVA test was used. All analysis was performed using IBM SPSS® statistics software, version 25. For all analyses, a p-value of < 0.05 is taken as statistically significant.