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Keywords:

  • HTK solution;
  • kidney preservation;
  • kidney transplantation;
  • UW preservation solution

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Single-center studies have reported equivalent outcomes of kidney allografts recovered with histidine-tryptophan-ketoglutarate (HTK) or University of Wisconsin (UW) solution. However, these studies were likely underpowered and often unadjusted, and multicenter studies have suggested HTK preservation might increase delayed graft function (DGF) and reduce graft survival of renal allografts. To further inform clinical practice, we analyzed the United Network for Organ Sharing (UNOS) database of deceased donor kidney transplants performed from July 2004 to February 2008 to determine if HTK (n = 5728) versus UW (n = 15 898) preservation impacted DGF or death-censored graft survival. On adjusted analyses, HTK preservation had no effect on DGF (odds ratio [OR] 0.99, p = 0.7) but was associated with an increased risk of death-censored graft loss (hazard ratio [HR] 1.20, p = 0.008). The detrimental effect of HTK was a relatively late one, with a strong association between HTK and subsequent graft loss in those surviving beyond 12 months (HR 1.43, p = 0.007). Interestingly, a much stronger effect was seen in African-American recipients (HR 1.55, p = 0.024) than in Caucasian recipients (HR 1.18, p = 0.5). Given recent studies that also demonstrate that HTK preservation reduces liver and pancreas allograft survival, we suggest that the use of HTK for abdominal organ recovery should be reconsidered.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Critical to the success of solid organ transplantation has been the development of methods to minimize injury from cold ischemia and reperfusion during organ recovery. The two most commonly used preservative solutions for abdominal organ recovery are University of Wisconsin (UW) solution and histidine-tryptophan-ketoglutarate (HTK) solution. UW solution has been considered the ‘gold standard’ for abdominal organ preservation fluid worldwide since 1987 (1). However, HTK use has nearly doubled in the United States since 2004 (2,3) because of lower cost, the reduced risk of reperfusion hyperkalemia, improved microvasculature perfusion due to lower viscosity and better cell preservation over a wider range of temperatures (4,5).

Conflicting data exist as to the impact of HTK preservation on the development of delayed graft function (DGF) and graft survival in deceased donor kidney transplants, particularly those with prolonged cold ischemia time (CIT). Several early reports came from collaborative studies by European centers evaluating the efficacy of HTK compared with UW in deceased donor kidney allograft outcomes. These initial reports found no statistically significant difference in graft survival (6–8). However, subsequent studies demonstrated a higher incidence of DGF and reduced graft survival with HTK preservation in allografts with a prolonged CIT (9–11).

Further clouding the picture are recent single-center reports from the United States. Agarwal et al. specifically studied the impact of HTK versus UW preservation in deceased donor renal allografts with prolonged CIT and found that there was improved graft survival and reduced DGF with HTK preservation (12). Similarly, in the largest single-center study to date of 317 HTK-preserved deceased donor kidney transplants, Lynch et al. found on multivariate analysis that HTK preservation was associated with reduced risk of DGF compared with UW solution, and that there was no statistically significant difference in graft survival (13).

To expand on previous reports, we analyzed a large national registry of deceased donor kidney transplants from 2004 to 2008. Because outcomes after kidney transplantation are, in general, very good, we hypothesized that differences between the preservation solutions might be subtle, especially with a short follow-up. The large sample size from a national registry provided the statistical power to detect subtle differences while maintaining a recent cohort to account for the advances in immunosuppression. Furthermore, because many centers are represented in the national registry, we were able to study and account for the effect of center-specific practices. We evaluated DGF and death-censored graft survival in the overall population as well as in specific subgroups based on the length of CIT, extended criteria donors (ECD) and recipient ethnicity.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Study design

The study population consisted of 21 626 patients undergoing deceased donor kidney transplantation between July 1, 2004, and February 28, 2008, as reported to the United Network for Organ Sharing (UNOS). The patients were excluded if they were <18 years of age, had a live donor or underwent a multiorgan transplant, if the allograft underwent pulsatile preservation, if a solution other than HTK or UW was used for organ recovery or if no information was available about the preservative solution. The patients were divided into two groups: UW—organ recovery with UW solution (n = 15 898) and HTK—organ recovery with HTK solution (n = 5728). The primary outcomes were DGF, defined as requirement for dialysis within the first week post transplant, and death-censored graft survival, defined as postoperative time alive with a functioning allograft, censored for patient death from other causes, loss to follow-up or administrative end of study. Patient death information was supplemented by linkage with the Social Security Master Death File.

Multivariate models for graft loss and delayed graft function

The risk factors for DGF were determined by logistic regression, and the risk factors for graft loss were determined using Cox proportional hazards models. Multivariate models were adjusted for the following clinically relevant confounders: recipient age, gender, ethnicity, body mass index (BMI), hypertension, diabetes mellitus, etiology of renal failure, peak panel reactive lymphotoxic antibodies (PRA) and prior transplant; donor age, gender, ethnicity, BMI, hypertension, diabetes mellitus, terminal creatinine, cause of death and donation after cardiac death and transplant CIT and HLA mismatch. To adjust for center-specific effects, models were also designed that incorporated transplant center volume and clustered variance estimates to account for the correlation among patients and practice patterns by transplant centers.

Statistical analysis

Unless otherwise specified, all tests were 2-sided, with statistical significance set at α= 0.05. The comparisons of donor and recipient characteristics between UW and HTK subgroups are reported using the unpaired 2-tailed t-tests for continuous covariates and chi-squared tests of independence for categorical variables. For the Cox models, proportional hazards assumptions were confirmed by inspection of complementary log-log plots. Data were missing for 0–2% of the following covariates: recipient age, gender, ethnicity, diabetes mellitus, etiology of renal failure and prior transplant and donor age, gender, ethnicity, hypertension, diabetes mellitus, terminal creatinine, cause of death, donation after cardiac death and HLA mismatch. Data were missing for 2–5% of the following covariates: recipient BMI, donor BMI and PRA. Finally, data for recipient hypertension were missing in 9% of cases, and data for CIT were missing in 13% of cases. Missing data were handled by casewise deletion, with the exception of CIT. For observations where CIT was missing, it was imputed (14,15) based on the mean CIT for that observation's share type (local, regional or national); no observations were missing information about share type. All analyses were performed using Stata 10.0/MP for Linux (StataCorp, College Station, TX).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The use of HTK preservative solution in the recovery of deceased donor kidney allografts nearly doubled from 15.9% in 2004 to 31.6% in 2006, with annual decreases thereafter to 25.1% in 2008 (Table 1). During the study period, 50% of kidney transplants were performed by 50 transplant centers; however, there was no correlation between transplant center volume and HTK utilization (Figure 1).

Table 1.  Preservative solution for deceased donor kidney allografts
 UW (n = 15 898)HTK (n = 5728)
  1. UW = University of Wisconsin; HTK = histidine-tryptophan-ketoglutatrate.

200484.1%15.9%
200575.0%25.0%
200668.4%31.6%
200771.4%28.6%
200874.9%25.1%
image

Figure 1. Distribution of deceased donor kidney transplants and HTK use by transplant center. (A) Cumulative proportion of deceased donor kidney transplants performed by transplant centers with decreasing volumes. Note that 50% of deceased donor kidney transplants were performed by 50 centers. (B) Average proportion of HTK use among centers by deciles of deceased donor kidney transplant volume.

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Compared with UW-preserved allografts, HTK-preserved allografts had slightly longer CIT (17.6 h vs. 17.0 h) and were more likely to be allocated for national sharing (26.6% vs. 22.9%; Table 2). Recipients of HTK-preserved allografts were more likely to be Caucasian (53.2% vs. 46.7%) and have a higher PRA (19.9% vs. 18.1%), whereas donors of HTK-preserved allografts were less likely to be hypertensive (21.8% vs. 23.4%) or ECD (11.9% vs. 14.1%) and more likely to be recovered from a Caucasian donor (76.7% vs. 66.5%; Table 2).

Table 2.  Demographics of deceased donor kidney transplant recipients
 UW (n = 15 898)HTK (n = 5728)p-Value
  1. Data shown as mean for age and BMI. ESRD = end stage renal disease; FSGS = focal segmental glomerulosclerosis; PCKD = polycystic kidney disease; CVA = cerebrovascular accident.

  2. *p-value = NS.

A. Transplant characteristics
Cold ischemia time (h)17.017.6<0.001
Share type (%)
 Local68.465.5<0.001
 Regional8.77.9 
 National22.926.6 
B. Recipient characteristics
Age (years)50.951.1*
Female (%)39.439.9*
Ethnicity (%)
 Caucasian46.753.2<0.001
 African American29.528.7 
 Hispanic14.512.3 
 Asian6.53.6 
 Other2.82.2 
BMI27.427.6*
Hypertension (%)83.683.9*
Diabetes mellitus (%)31.233.20.006
Prior transplant (%)12.913.7*
Etiology ESRD (%)
 Glomerulonephritis7.86.60.002
 IgA nephropathy4.03.40.036
 FSGS6.46.1*
 PCKD8.48.5*
 Diabetes mellitus24.826.90.002
 Hypertension23.423.1*
PRA18.119.9<0.001 
C. Donor characteristics
Age (years)37.136.40.002
Female (%)39.841.1*
Ethnicity (%)
 Caucasian66.576.7<0.001
 African American13.19.7 
 Hispanic16.511.2 
 Asian2.71.5 
 Other1.20.9 
Cause of death (%)
 Anoxia15.116.7<0.001
 CVA43.039.0 
 Head trauma41.543.9 
 Other0.40.4 
Hypertension (%)23.421.80.014
Diabetes mellitus (%)5.25.2*
Terminal creatinine (mg/dL)1.071.06*
Extended criteria donor (%)14.111.9<0.001 
Donation after cardiac death (%)3.84.0*

After adjusting for donor, recipient and graft factors, HTK preservation was not associated with an increased risk of DGF on multivariate analyses when compared with UW preservation (odds ratio [OR] 0.99, 95% confidence interval [CI] 0.91–1.07, p = 0.7). Similar results were seen after adjusting for center volume and center-level clustered variance estimates (OR 0.98, 95% CI 0.87–1.11, p = 0.8).

After adjusting for recipient, donor and graft factors, HTK preservation was independently associated with a 20% increased risk of graft loss on multivariate analyses when compared with UW preservation (hazard ratio [HR] 1.20, 95% CI 1.05–1.37, p = 0.008). Similar results were seen after adjusting for center volume and center-level clustered variance estimates (HR 1.19, 95% CI 1.03–1.38, p = 0.022).

The impact of HTK preservation began to appear approximately 1 year after transplant (Figure 2). This finding prompted us to examine how HTK preservation impacted the subsequent survival of those allografts that were 1-year survivors. In this cohort, HTK use had a 43% increased risk of graft loss, even after adjusting for donor, recipient and graft factors (HR 1.43, 95% CI 1.10–1.86, p = 0.007) as well as center volume and center-level clustered variance estimates (HR 1.43, 95% CI 1.05–1.96, p = 0.023; Table 3). Interestingly, HTK preservation was associated with a much higher risk of graft loss in ECD recipients (HR 2.09, 95% CI 1.07–4.09, p = 0.031) than in standard criteria donor (SCD) recipients (HR 1.37, 95% CI 1.03–1.84, p = 0.031). Furthermore, African-American recipients also had a notably increased risk of graft loss with HTK preservation (HR 1.55, 95% CI 1.06–2.27, p = 0.024) as compared with Caucasian recipients (HR 1.18, 95% CI 0.77–1.79, p = 0.5). However, we did not detect a statistically significant difference in graft survival associated with HTK preservation in kidneys with CIT ≥ 24 h (HR 1.37, 95% CI 0.80–2.33, p = 0.2), although we caution that this sample size was somewhat smaller (Table 3).

image

Figure 2. Kaplan–Meier death-censored graft survival curves for kidney transplants based on preservative solution.

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Table 3.  Effect of HTK preservation on subsequent graft loss in 1-year survivors after deceased donor kidney transplant
 Multivariate—patient-level*Multivariate—patient- and center-level**
HR (95% CI)p-ValueHR (95% CI)p-Value
  1. *Adjusted for recipient age, gender, ethnicity, body mass index (BMI), hypertension, diabetes mellitus, etiology of renal failure, peak panel reactive lymphotoxic antibodies (PRA) and prior transplant; donor age, gender, ethnicity, BMI, hypertension, diabetes mellitus, terminal creatinine, cause of death and donation after cardiac death and transplant cold ischemia time and HLA mismatch.

  2. **Adjusted for same covariates as above and also adjusted for center volume, with clustered variance estimates to account for the correlation among patients and practice patterns by transplant center.

All deceased donor transplants (n = 2839 HTK and 8746 UW)1.43 (1.10–1.86)0.0071.43 (1.05–1.96)0.023
Standard criteria donor (n = 2436 HTK and 7278 UW)1.37 (1.03–1.84)0.0311.37 (0.96–1.96)0.081
Extended criteria donor (n = 308 HTK and 1190 UW)2.09 (1.07–4.09)0.0312.15 (1.02–4.55)0.044
African-American recipients (n = 809 for HTK and 2541 for UW)1.55 (1.06–2.27)0.0241.55 (1.01–2.37)0.044
Caucasian recipients (n = 1511 for HTK and 4187 for UW)1.18 (0.77–1.79)0.5  1.18 (0.76–1.81)0.5  
Cold ischemia time ≥ 24 h (n = 565 HTK and 1501 UW)1.37 (0.80–2.33)0.2  1.36 (0.77–2.40)0.3  

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

In this national study of deceased donor kidney transplants between 2004 and 2008, we found that the use of HTK solution for kidney preservation has increased significantly in recent years. HTK preservation did not increase the overall risk of DGF in deceased donor kidney allografts. Nonetheless, the multivariate analysis, adjusting for potential recipient, donor, graft and center-specific factors, demonstrated that the use of HTK solution is an independent risk factor for death-censored graft loss of kidney allografts as compared with UW solution. The detrimental effect of HTK was a relatively late one, with a stronger association between HTK and subsequent graft loss in those surviving beyond 12 months. Interestingly, a much stronger impact of HTK preservation was seen in African-American recipients as well as in recipients of ECD kidneys.

Interpretation of prior reports comparing HTK preservation to UW preservation of deceased donor kidney allografts has been challenging, given the varying results reported between single-center and multicenter studies, as well as the potential confounding from differing eras and immunosuppression protocols on data analysis. Both single-center (13,16) and multicenter reports (6–8) of no significant differences between UW and HTK preservation on kidney allograft survival have fostered widespread enthusiasm and fueled increased HTK utilization both in Europe (11) and in the United States (2,3). However, the large multicenter European experience (9–11) with HTK and UW preservation of deceased donor renal allografts suggests that the two preservative solutions are not equivalent but rather that HTK use increases DGF and reduces graft survival of renal allografts. The current study, which corroborates the European experience, is the first multicenter analysis that is composed of a large study population of both HTK- and UW-preserved transplants, all performed exclusively in the recent era of immunosuppression.

In our analysis of the UNOS database, we did not observe increased DGF in kidney allografts preserved with HTK. It is possible that the impact of the preservative solution on kidney transplant DGF is overshadowed by other more important donor-specific risk factors including donor terminal creatinine, donor hypertension, donation after cardiac death recovery and CIT (17–19).

Interestingly, the preservation differentially impacted the risk of graft loss based on recipient ethnicity, as African-American recipients had a higher risk of graft loss attributable to HTK than did Caucasian recipients. Although the precise mechanism for this remains unclear, it is possible that the same signaling pathways that lead to overall poorer allograft outcomes and increased chronic allograft nephropathy in African-American kidney transplant recipients are impacted at the time of organ recovery and may involve differential sensitivity to cellular injury during ischemia-reperfusion.

Although the biological mechanisms by which HTK preservation versus UW preservation may impact kidney allograft survival have yet to be elucidated, in vitro and animal studies offer some mechanistic insights. Several studies of human endothelial cells found that the use of UW solution reduced cellular necrosis, mitochondrial dysfunction and intracellular ATP depletion during ischemia-reperfusion when compared with HTK solution (20,21). Likewise, UW preservation was associated with reduced apoptosis of rat hepatocytes after 24 h of cold storage as compared with HTK preservation (22). Recent animal models suggest that improved organ recovery may come from a combination of HTK and UW solutions. The reduced viscosity of HTK solution as compared with UW solution has been proposed to improve the removal of red blood cells from the microvasculature during organ recovery and preservation (23). In a rat model of liver transplantation, the combination of HTK for the initial flush followed by a backtable flush and cold storage with UW was far superior to the use of UW for all stages of organ recovery, as assessed by the measurement of liver enzymes and bile production upon reimplantation of the allografts (23). It remains to be seen if these results can be recapitulated in humans. Clearly, studies are required to better elucidate the signaling pathways impacted by preservative solution composition during the ischemia-reperfusion injury incurred during organ recovery and preservation. Such knowledge is critical to allow further refinements of preservative solutions for better protection from ischemia-reperfusion injury during abdominal organ recovery and transplantation.

As with any analysis of the UNOS database, our conclusions rely on the assumption that there is no systematic bias generated by either reporting error or missing data. Confounders that could potentially impact kidney preservation and/or subsequent function not captured in the UNOS database include: donor hemodynamics and vasopressor requirement at the time of organ recovery, surgeon technique at both organ recovery and transplantation, the volume of flush used at the time of organ recovery, adequacy of preservative flush to kidneys during recovery, warm ischemia time during transplant surgery, the number and severity of rejection episodes, immunosuppression protocols and postoperative patient compliance. However, these differences would only bias our conclusions if there were a substantial interaction between the preservative solution and these other variables such that these variables not only impacted death-censored graft survival and DGF but also influenced the effect of the preservative solution on graft survival and DGF. Finally, because causes of graft failure are poorly captured in the UNOS database (over 50% of the data are missing), a meaningful analysis of specific causes of graft loss in HTK- versus UW-preserved kidney allografts could not be performed.

Although we attempted to account for the center-level effects using center-level clustered variance estimates, we acknowledge that these analyses cannot account for latent confounding, such as differences in follow-up or patient compliance. Additionally, it is possible that there are center-specific practices with ECD or donation after cardiac death recipients versus SCD recipients that cannot be adjusted for in this national registry analysis.

In conclusion, our analysis of the national experience with HTK and UW kidney allograft preservation demonstrates a trend of increasing HTK use in abdominal organ recovery. However, HTK preservation is an independent risk factor for increased death-censored graft loss for all deceased donor kidney allografts, and in particular for allografts from ECDs and those transplanted into African-American recipients. These findings coupled with similar data demonstrating increased risk of graft loss in HTK-preserved liver (3) and pancreas (2) allografts suggest that UW solution should remain the ‘gold standard’ for abdominal organ recovery and preservation.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The UNOS National Data Registry is supported in part by Health Resources and Services Administration contract 231-00-0115. The analyses described here are the responsibility of the authors alone and do not necessarily reflect the views or policies of the Department of Health and Human Services, nor does the mention of trade names, commercial products or organizations imply endorsement by the US Government.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References