The first successful simultaneous liver and kidney transplantation (SLKT) was performed in 1983.¹ Thereafter, SLKT has been established as the treatment of choice for patients with concurrent end‐stage liver and end‐stage kidney diseases.^1,2 Currently, more than 400 SLKTs are performed annually in Europe and the United States.^2-4 In contrast, SLKT is still a rare procedure in Korea. Only 38 cases of SLKT from deceased donors were registered to the Korean Network for Organ Sharing (KONOS) database from 2000 to 2019. SLKT has been usually performed with different living liver and kidney donors in Korea. Since there are only a few case reports of SLKT from Korean transplant centers in the literature,^5-8 information on SLKT in Korea is very limited. The objective of the present study was to analyze the nationwide incidence of SLKT and the outcomes of SLKT in a high-volume transplant center in Korea.

A considerable proportion of patients with both end- stage liver and kidney diseases have undergone LT alone or SLKT worldwide. Recently, the proportion of SLKT among all LTs in the United States increased up to 9.9% in 2016.^3,4 Meanwhile, because of profound deceased donor shortage in Korea, around 1% of LT candidates had undergone deceased donor SLKT in the current analysis of the KONOS database. In our institution, only 0.2% of total LT cases underwent SLKT with deceased or living donors. To the best of our knowledge, our Case No. 1 was the first case of SLKT in Korea. Considering that KT was performed in 3 patients before LT and 34 patients after LT, the sum of combined LT and KT was 48, accounting for 0.7% of total LT cases. In addition, 51 patients are currently enrolled in the waiting list of DDKT. Considering that there are many other patients showing slow but progressive deterioration of renal function after LT, the demand for KT in LT recipients would be higher than we calculated.

Information on SLKT in Korea is very limited. There are only a few case reports of SLKT from Korean transplant centers in the literature.^5-8 A Korean single-center report on 3000 cases of KT has presented the performance of 16 cases of SLKT,⁹ however, detailed information on SLKT was not described. This was the primary reason that we carried out this study.

Renal disease has high impacts on LT candidates. Serum creatinine is included in the MELD scoring system because renal impairment is associated with a substantial increase in 30‐day mortality, especially if an LT candidate requires dialysis.^1,3 Renal impairment is a common occurrence in patients with liver disease. Some studies indicated that its prevalence is 15%-20% in LT candidates.¹⁰ The requirement to update MELD scores after the initial listing allows for allocation of a liver to those most in need.

Renal disease on LT recipients also has a high prognostic impact. The impact of kidney disease on LT candidates is not limited to pretransplant mortality. LT recipients who develop chronic kidney disease (CKD) following LT show significant increases of liver allograft failure and mortality compared to those who do not develop CKD after LT. Significant renal disease prior to LT is the most important risk factor for CKD after LT. Determining the cause of acute kidney injury in LT candidates can be challenging, which complicates the ability to assess potential renal recovery after LT. LT candidates on dialysis for more than 90 days with advanced CKD and prolonged acute kidney injury not from hepatorenal syndrome have a higher risk for significant renal impairment after LT.¹¹

After the introduction of MELD criteria for allocation of LT in the United States, there was a more than 400% increase in SLKT, from 2.5% in 2001 to 9.9% in 2016. This change was driven primarily by poor outcomes of LT recipients with posttransplant kidney disease, an inability to predict which candidates were likely to develop post‐LT CKD, and the lack of a mechanism to expedite KT in LT recipients who developed advanced CKD after LT. ¹¹ The introduction of MELD score to the KONOS liver allocation system and an increase in the number of deceased donors raised the proportion of deceased donor SLKT from 0.6% before the adoption of MELD score to 1.2% after the adoption of MELD score in the present study. ^12-14 Although this increase was statistically significant, the proportion of SLKT was still very low compared to that in the United States.^11,15

An analysis of United Network for Organ Sharing (UNOS)/Organ Procurement and Transplantation Network (OPTN) data has demonstrated a clear benefit of SLKT in LT candidates on dialysis for more than 2 months prior to LT and in those with a serum creatinine of greater than 2.5 mg/dl, in which SLKT recipients show improved survival than those who receive LT alone. ¹⁶ In addition, deceased donor SLKT can avoid immunological complexities of implanting two organs from different donors. Human leukocyte antigen tissue typing and compatibility have a much larger role in allograft survival in KT recipients. SLKT provides improved immunological outcomes in this setting. Based on these benefits of SLKT, a formal policy was enacted in 2017 in the United States with defined listing criteria for SLKT and priority for patients who had received LT and subsequently developed significant kidney disease after LT. ³

In the current KONOS allocation policy, multiple or two-organ transplantation does not have any priority for liver or kidney allocation. The primary background reasons of the increase in SLKT after introduction of MELD score in the KONOS allocation system might be the high MELD score per se from CKD in SLKT candidates and an increase of deceased donors at that time. However, considering that the daily cutoff MELD score for liver allocation is very high and the number of deceased donors is recently decreasing, it is difficult to expect that deceased donor SLKT would be continuously increasing afterwards.¹⁴ It is a dilemma to give a priority to SLKT candidates in the current Korean setting because separate allocation of kidney and liver organs can save two patients.

The majority of SLKTs in our institution were performed through a combination of LDLT and LDKT. It is usual to have two different donors for LDLT and LDKT. Although two-organ donation from a living donor has an immunologic benefit for LDKT,^15,17,18 there are ethical and medical issues, especially issues regarding donor safety. One study supports such two-organ donation for pediatric patients.¹⁵ However, right liver donation for adult LT recipients might carry higher risk than left liver donation for pediatric patients.

SLKT with different liver and kidney living donors has different aspects from that with one deceased donor. In reality, half of adult LDLTs in Korea have been performed for patients with HCC. These patients carry risk of posttransplant HCC recurrence.^19,20 In the present study, two of five patients died due to HCC recurrence. Sequential LDLT and KT with living or deceased donors would be beneficial to avoid risk of futile KT from early HCC recurrence after LT. We presume that it would be one of the underlying reasons to not prefer SLKT with living donors. As shown in results of this study, SLKT with living donors was not performed after 2011 in our institution. Sequential LT-KT has settled as a main modality of LT-KT combination.

This study has a notable limitation. It was a retrospective, single-center study with a small number of study patients. Further high-volume multicenter studies are needed to validate results of this study.

In conclusion, the results of this study revealed that the incidence of deceased donor SLKT was very low despite the introduction of MELD score. An increase of its incidence in the near future is not anticipated unless the number of deceased donors is markedly increased. Currently, sequential LDLT and KT with deceased or living donors is the main stay of transplantation rather than SLKT in our institution.