Living kidney donor assessment: Kidney length vs differential function

ARTICLE HIGHLIGHTS

Research background

Potential candidates for kidney donation must have sufficient renal function post donation in order to minimize future risks when living with a single kidney. Currently most transplant units use split function between the kidney pairs in addition to other factors to make a decision on which kidney to donate. However, isotope differential renal function is not uniformly performed in all transplant centres, with many relying on kidney size measurements. Such an approach is supported by the BTS/RA Living Donor Kidney Transplantation Guidelines (2018) which state that differential kidney function, determined by ^99mTechnitium dimercaptosuccinic acid (^99mTcDMSA) is recommended where there is > 10% variation in kidney size or significant renal anatomical abnormality. It further states that, “A difference in size of 2 cm or more indicates the possibility of a significant difference in GFR between the two kidneys”. Hence this study.

Research motivation

The key question in living kidney donor assessment is how best to determine the contribution of each kidney to overall renal function and guide selection of which kidney to donate, ensuring safety of procedure and good outcome for both recipient and donor. With many units, particularly in the United Kingdom adopting the use of kidney length in the decision making process, there is risk of making wrong or harmful decisions with respect to living kidney donors unless it can be demonstrated that there is strong correlation between kidney length and split function.

Research objectives

This study aimed to determine the effect of using kidney length to decide which kidney to donate in a retrospective cohort of potential donors. Realisation of this objective would confirm the new approach as safe and reliable otherwise alternative approaches would need to be adopted such as use of kidney volume measurements and where indicated isotope differential renal function.

Research methods

All potential living kidney donors who completed assessment over a ten years period were retrospectively evaluated. Donor assessment was performed as per UK guidelines. This study is unique in presenting the results of a retrospective “what if” analysis of prospectively reported kidney length measurements that were not used in the decision-making process as to which organ to donate. During the study period, decisions were made on the basis of divided function and vascular anatomy. Analysis of a large number of potential donors in this way provides a useful tool in validating the use of kidney size alone in making decisions about which kidney to donate.

Research results

The key findings in this study include the following: Equal sex distribution among potential donors whose mean ages and GFRs were comparable; weak correlation between difference in length and divided function of kidney pairs (CT-measured kidney length provided a stronger correlation than US-measured length); the proportion of donors with a difference in length above 1 cm (34/285) was statistically significantly different from those with a differential function of 10% or higher (73/285); and of 73 with a split function greater than 10, 18 (24.7%) had no difference in kidney length; 54 (74%) had a difference of < 2 cm and only one of > 2 cm. Furthermore, using a difference in length of 2 cm as cut off for performing split function would lead to false reassurance in 72 patients (25%).

Research conclusions

This study highlights the significant potential for making wrong/harmful decision (removing the significantly better functioning kidney) if kidney length alone is used for decision making. A wrong decision would be made possible in 65/285 (23%) if the trigger for measuring split function were a difference in length of 1 cm; and in 72/285 (25%) if 2 cm were used in the presence of significantly different divided function. Length difference between kidney pairs alone is not sufficient to replace measurement of divided function. The findings of this study have important practical implications for clinical practice in avoiding potential harm to living kidney donors. This issue requires a randomised controlled trial to resolve it.

Research perspectives

This study has shown unequivocally that kidney length alone is not sufficient to determine which kidney to donate. It raises the question about the role of kidney volume measurement. The literature suggests that renal volume is the most precise predictor of kidney size. It has been shown by other workers that renal volume calculation using the ellipsoid method (length x antero-posterior diameter x lateral diameter x π/6) compares favourably with volume determined using volumetric software. CT based volume measurements of kidneys, (particularly cortical volumetry) correlates well with split renal function, raising the possibility that CT volumetric measurement of kidney size could replace the need for split function assessment. This issue is of vital importance and requires a randomised controlled trial to resolve whether CT-measured split cortex volume, for example is equivalent to MAG3-measured split renal function.