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Original Clinical Science—General

Prospective Validation of Prediction Model for Kidney Discard

Zhou, Sheng MD1; Massie, Allan B. PhD1,2; Holscher, Courtenay M. MD1; Waldram, Madeleine M.1; Ishaque, Tanveen MD1; Thomas, Alvin G.1; Segev, Dorry L. MD, PhD1,2,3

Author Information
doi: 10.1097/TP.0000000000002362
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Increasing numbers of new patients are added to the kidney waiting list every year; between 2015 and 2016, this increase was 1.9%.1 Based on Organ Procurement and Transplantation Network (OPTN) data, as of April 2018, over 95 000 candidates were on the kidney waiting list. The increasing demand for organs far exceeded the supply of available kidneys, with only 19 849 waitlist candidates receiving a kidney transplant in 2017. Because of this organ shortage, over 16% candidates waited over 5 years for a transplant, and the number of patients who were removed for death or deteriorating medical conditions reached above 9000 in 2016.2 Moreover, the number of living donor transplants has declined in the past decade, exacerbating the shortage.2,3

Despite the severe organ shortage and numerous studies showing a survival benefit from transplantation even with marginal kidneys,4-7 the discard rate rose from 2127 (14.9%) in 2006 to 3631 (20%) in 2016 and remained high after the introduction of Kidney Donor Profile Index (KDPI) and implementation of the Kidney Allocation System (KAS).1-3,7-12 The increase in discard was mostly explained by the underlying changes in donor characteristics.7 In Europe and the United Kingdom, allocation schemes such as a “rescue” allocation policy and Kidney Fast-Track Scheme have been enacted to reduce unnecessary kidney discard.13-15 In the United States, although there has been discussion of policies that might reduce discard, none currently exist.16 Identifying kidneys at high risk of discard could help facilitate development of a “rescue” allocation policy.

In an effort to help operationalize prioritized allocation of kidneys more likely to be discarded, in 2010 we created a model to predict the probability that a kidney will be discarded or transplanted with cold ischemia time exceeding 36 hours (Probability of Discard/Delay [PODD]).17 This model seemed to function well but has never been validated and seems more needed than ever in light of markedly increased discard rates. Additionally, the mechanisms of kidney discard might have changed under KAS, further emphasizing the need for validation. Finally, the benefit of a “rescue” allocation policy requires that there be variation in center-level willingness to transplant an organ labeled likely to be discarded by the specified index. That is, there must be variation in rates of acceptance of high-PODD kidneys which can be used to direct such kidneys to a center that might be more willing to transplant them.17,18

We conducted a national registry study in the KAS era to prospectively validate the accuracy of PODD in predicting kidney discard. Additionally, we compared PODD with other scores, including a reduced PODD score (PODD with terms for biopsy, glomerular sclerosis, and use of machine perfusion removed), KDPI, and a model developed by Marrero et al19 for predicting risk of discard. Finally, we characterized the variation in utilization of hard-to-place kidneys predicted by PODD among centers.


Data Source

This study used data from the Scientific Registry of Transplant Recipients (SRTR). The SRTR data system includes data on all donors, waitlisted candidates, and transplant recipients in the US, submitted by the members of the OPTN, and have been described elsewhere.20 The Health Resources and Services Administration and US Department of Health and Human Services provides oversight to the activities of the OPTN and SRTR contractors.

Our study was classified by the Johns Hopkins Institutional Review Board as “exempt”—not human subjects research (NA_00042871).


We included 50207 adult deceased donor kidneys recovered for transplantation between January 1, 2015, and March 1, 2018. Informed by the previous study, double/en-bloc kidneys were considered as 1 observation.17 For each recovered kidney, we calculated the PODD score as described in the previous study.17 Predictors of PODD included female, age spline at 40 years, blood type AB, donor body mass index (BMI, kg/m2), BMI spline at 23, cancer, smoking, hypertension, history of myocardial infarction, diabetes, diabetes over 10 years, diabetes requiring insulin, donation after circulatory death (DCD), death by cerebrovascular accident (CVA), death by head trauma, human T lymphocyte virus (HTLV) positive, cytomegalovirus (CMV) positive, hepatitis B core antibody (HBcAb), HBcAb and surface antigen (HBsAg), hepatitis C virus (HCV), CDC high-risk donor, machine perfusion, glomerulosclerosis over 20%, and serum creatinine (mg/dL). Because this study is an external (temporal) validation of the previous study, we did not refit the PODD model; rather, we kept the coefficients of the original PODD score and calculated the probabilities of discard/delay.21 Candidate comorbidities were assumed to absent in the case of missing or unknown data. There were <0.1% missing data for continuous predictors for BMI and serum creatinine.

Comparing PODD With Other Indices of Discard

For comparison, we tested the validity of several other indices in predicting discard. The Marrero score was calculated based on the odds ratios of the predictors and intercept described elsewhere.19,21 Predictors included age over 50, biopsy of either kidney, CMV status, DCD, CVA, donor height (cm), donor weight (kg), tattoos, either kidney pumped, HBcAb, HBsAg, HCV, history of cigarette use, history of diabetes, history of drug use, history of hypertension, terminal lab creatinine over 1.5 (mg/L), blood type, and donor race. KDPI was calculated using 2017 as the reference year.

Both PODD and Marrero score might have an advantage in predicting discard since they include factors that are not typically known at time of initial allocation (glomerulosclerosis, use of machine perfusion, and whether biopsy was done). We calculated reduced scores for PODD (r-PODD) and Marrero score (r-Marrero). For r-PODD, we fit the model with glomerulosclerosis and machine perfusion removed. To conduct a strict validation, we fit this model on the dataset used to fit PODD by Massie et al in 2010.17 We calculated r-PODD for our current study population using the coefficients from the model. For Marrero score, because of the lack of access to the original data, we calculated the reduced score (r-Marrero) with the coefficients for machine perfusion and biopsy removed. We examined the correlation between all the indices.

Accuracy in Predicting Discard

Area Under the Curve

Because PODD was developed to predict discard/delay while Marrero score was developed to predict discard only, we examined the validation on the same outcome–kidney discard. The fit of each model in predicting discard was evaluated and compared using C statistics and the area under the receiver operating characteristic curve (AUC).22-24

We calculated positive predictive value, negative predictive value, sensitivity, and specificity for each of the indices. Cutoffs were informed by our previous study: A stringent cutoff (0.6) was close to 90th percentile of PODD.17 This was applied to PODD, r-PODD, Marrero score, and r-Marrero. Although we aim to validate the discrimination of PODD, the calibration power of PODD might change over time. In other words, the proportion of kidneys that were more likely to be discarded for a certain cutoff might change. We also calculated the proportion of kidneys with PODD above other cutoffs (0.5 and 0.7) and the discard rate for kidneys with PODD above these cutoffs. We used 85% as the cutoff for high KDPI.11,25 Because KDPI was not designed to predict discard, we also explored the observed probabilities of discard of kidneys with KDPI over 90% and at 100%.

Net Reclassification Improvement Index

We quantified the value of each model using the Net Reclassification Improvement (NRI) index.26-28 In a perfect model, all kidneys discarded would be classified as high PODD and all kidneys transplanted would be classified as low PODD. The NRI index offers an intuitive way to measure improvement offered by 1 model over the other by describing how well the new model reclassifies subjects with events into high or low likelihood categories.29 In this setting, kidney discard may be considered an event, and cutoffs for high versus low likelihood of discard must be selected for each model. To make KDPI comparable with the other indices, we converted KDPI to predicted probability of discard using logistic regression and included KDPI as the only term as a linear predictor. Thus, each KDPI score corresponded to a probability of kidney discard. We used both a stringent cutoff (0.6) and a permissive cutoff (0.5), which was close to the 80th percentile of PODD, to evaluate the improvement from 1 index to the next.

Missing Data

For PODD and r-PODD, we first performed complete case analysis excluding the <0.1% missing data for BMI and serum creatinine. We then performed a sensitivity analysis using multiple imputation to handle the missing data. We conducted multiple imputation by chained equations for 10 cycles.30,31 We checked the normality of BMI, serum creatinine, and their log transformation. We specified linear regression model for the log transformation of BMI and predictive mean matching for serum creatinine.32 Predictive mean matching is a linear regression approach appropriate for nonnormally distributed continuous variables. This approach imputes missing values by randomly selecting a nonmissing value from 1 of the 5 nearest neighbors after linear regression. In other words, it calculates the regression-predicted values for each observation, then fills in a value randomly chosen from the observed values from the 5 observations with the closest regression-predicted values. For all other indices, we performed complete case analysis.

Clustering of Kidney Utilization Among Centers

We assessed offer acceptance using match run data that were available to us through December 2016. Using match run data for each kidney recovered between January 1, 2015, and December 31, 2016, we determined the offer acceptance of kidneys in each transplant center. We defined “offer acceptance” as acceptance and subsequent transplantation of an offered kidney for any candidate on the center’s match run list. That is to say, if a center declined the offer for 1 or more candidates but eventually accepted it for another candidate, we consider this an acceptance.33 Kidneys that were accepted but eventually not transplanted were treated equivalent to being declined. Centers that received fewer than 100 kidney offers during the study period were excluded for statistical stability (38 out of 241 were excluded).

We defined a high-PODD kidney as one with a PODD >0.6 and low PODD kidney as one with a PODD of 0.6 or less. We examined the clustering among transplant centers in high-PODD versus low-PODD kidney utilization using Lorenz curves and the Gini coefficient.34 Lorenz curves showed cumulative distribution of kidneys by center, sorted from those who transplanted the fewest kidneys to those who transplanted the most kidneys. The diagonal line represents perfectly equal utilization among the centers and the curves represent the actual distribution of utilization. Curves that are further away from the diagonal line represent more clustering. Gini coefficient is the area between the diagonal line and the curve. The Gini coefficient is a dimensionless measure of the degree of clustering of kidney utilization. It ranges from 0 to 1. A Gini coefficient of 0 means that all centers transplanted the same number of kidneys, while a Gini coefficient of 1 means that 1 center transplanted all kidneys.

In addition, we examined correlation between center practices over time. We identified the top 15 centers that transplanted the most high-PODD kidneys in 2015 and compared with the top 15 centers that transplanted the most high-PODD kidneys in 2016. We assessed the correlation between these top 15 centers in 2015 and 2016. We repeatedly assessed the correlation between the top 20, 30, and 40 centers in 2015 and 2016.

Sensitivity Analysis

As a sensitivity analysis, we calculated Gini coefficients for high and low-PODD kidneys for all centers (N = 241).

Surplus Offers

Surplus offers were defined as offers (kidneys being offered to 1 or more recipients at a single transplant center) of high-PODD kidneys to centers that have never accepted a high-PODD kidney between January 1, 2015, and December 31, 2016.18 We plotted the distribution of surplus offers among centers that received at least 100 total offers during the study period.18

Statistical Analysis

We used Wilcoxon rank-sum test to compare differences in continuous variables. We used Pearson χ2 test to compare differences in categorical variables. We calculated Pearson correlation coefficients between different indices. The DeLong test was used to compare differences in AUCs.22 The Gini coefficients were calculated based on the INEQUAL7 module for Stata by Philippe Van Kerm. Lorenz curves were created based on sg30 module for Stata by Edward Whitehouse. We used Q-Q plot to check the normality of the distributions. We used κ coefficient to assess the correlation between the top 15, 20, 30, and 40 centers in different years.35 Confidence intervals were reported as per the method of Louis and Zeger.36 All analyses were performed using Stata/SE 14.1 for Windows (College Station, TX).


Characteristics of Donors

Of the 50207 kidneys recovered during the study period, 10381 (20.7%) were discarded (Table 1). Compared with kidneys that were transplanted, kidneys that were discarded came from donors with a higher median age (54 y vs 38 y, P < 0.001) and BMI (28 vs 27, P < 0.001). Discarded kidneys were more likely to be from donors who were African American (17.5% vs 14.1%, P < 0.001), female (46.0% vs 38.2%, P < 0.001), DCD (20.2% vs 18.8%, P < 0.001), Extended Criteria Donor (ECD, 49.5% vs 13.2%, P < 0.001), died from CVA (45.3% vs 25.7%, P < 0.001), had a history of smoking (32.5% vs 19.0%, P < 0.001), hypertension (60.7% vs 27.4%, P < 0.001), diabetes (23.8% vs 6.7%, P < 0.001), myocardial infarction (7.7% vs 2.4%, P < 0.001), and a history of infectious disease (Table 1). Discarded kidneys were less likely to be from donors who died of head trauma (14.8% vs 32.6%) and labeled CDC increased risk donors (21.5% vs 25.1%, P < 0.001). Discarded kidneys were less likely to be machine perfused (27.3% vs 36.6%, P < 0.001) and more likely to have sclerosis >20% (23.2% vs 1.4%, P < 0.001, Table 1).

Characteristics of donors of all kidneys recovered for transplantation in the United States January 1, 2015, to March 1, 2018

Correlation Between Indices

Predictors of each index were listed in Table 2. Correlation was good between PODD, r-PODD, Marrero score, and r-Marrero, with all the correlation coefficients between 0.8 and 1.0 (Table 3). KDPI was correlated less well with the other indices, with all correlation coefficients between 0.7 and 0.8 (Table 3).

Predictors of indices
Correlation between indices

Accuracy in Predicting Discard

Of kidneys studied, 9.5% had PODD over 0.6, 13.0% had Marrero score over 0.6, and 9.3% had r-PODD over 0.6, 8.2% had r-Marrero over 0.6, 12.7% had KDPI over 85% (Table 4). The observed probability of discard increased from 63.6% for kidneys with KDPI of 85% or greater to 70.5% for kidneys with KDPI of 90% or greater and 92.0% for kidneys with KDPI of 100%. Among all the indices, PODD had the highest positive predictive value of 78.5%. This means if a kidney has a PODD >0.6, then there is a 78.5% chance that this kidney will be discarded. The simpler r-PODD still had a higher positive predictive value than the Marrero score, r-Marrero, or KDPI (Table 4). When looking at other PODD cutoffs, 13.7% had PODD over 0.5 and 71.7% of these kidneys were discarded; 6.2% had PODD over 0.7 and 85.2% of these kidneys were discarded. AUC was the largest for PODD (0.87), followed by r-PODD (0.85), Marrero (0.84), r-Marrero (0.82), and KDPI (0.82) (all P < 0.001) (Figure 1). Using 0.6 as the cutoff, NRI index was 15.6% from KDPI to PODD and 4.1% from Marrero to PODD. Net Reclassification Improvement index was 11.4% from KDPI to r-PODD and 11.9% from r-Marrero to r-PODD. Using 0.5 as the cutoff, NRI was 9.3% from KDPI to PODD and 4.2% from Marrero to PODD. NRI was 7.0% from KDPI to r-PODD and 14.7% from r-Marrero to r-PODD. In summary, by all 3 metrics—AUC, NRI, and positive predictive value—PODD predicted discard most accurately. After multiple imputation, the conclusions stayed the same.

Evaluation of indices
ROC curves showing accuracy in predicting discard. Curves closer to the diagonal line represent worse accuracy while curves closer to the upper left represent better accuracy in predicting discard. PODD have greater accuracy in predicting discard than all other scores (P < 0.001). AUC, area under the receiver operating characteristic curve; KDPI, Kidney Donor Profile Index; PODD, Probability of Delay or Discard; ROC, receiver operating curve.

Clustering of Kidney Utilization Among Centers

There were 203 centers that received at least 100 kidney offers over the study period. The median number of low-PODD kidneys transplanted by centers was 66 (range, 2–375; interquartile range [IQR], 36–119). However, the median number of high-PODD kidneys transplanted by centers during the study period was 0 (IQR, 0–1; range, 0–54). There was markedly greater clustering among centers in use of high-PODD kidneys as compared with the use of low-PODD kidneys (Gini = 0.84 vs 0.41, Figure 2). In total, 122 (60.1%) centers did not use a single high-PODD kidney, and the top 15 centers (ranked according to number of high-PODD kidneys accepted) used the majority (63.0%) of high-PODD kidneys (Figure 2). When including all 241 centers, Gini coefficients remained the same (0.84 vs 0.41). In addition, among the top 15 centers that used the most high-PODD kidneys in 2015, 13 remained to use the most high-PODD kidneys in 2016 (κ coefficient = 0.77). The consistency between 2015 and 2016 was less well among the top 20, 30, and 40 centers (κ coefficient = 0.61, 0.52, 0.38, respectively).

Clustering in kidney utilization among centers. Curves closer to the diagonal line represent more equity and those closer to the lower right indicate more clustering and less equity of distribution. The Gini coefficient denotes the degree of clustering. There was less equitably distributed utilization of high-PODD kidneys than for low-PODD kidneys (Gini = 0.84 vs 0.41). In total, 122 (60.1%) centers did not use a single high-PODD kidney and 15 centers have used the majority (63.0%) of high-PODD kidneys. PODD, Probability of Delay or Discard.

Surplus Offers

Among the 203 centers offered (“offer” was defined as a donor kidney being offered to 1 or more recipients at a single transplant center) at least 100 kidneys during the study period, 122 (60.1%) never accepted a high-PODD kidney. There were 11 684 surplus offers (35.0% of all 33 386 high-PODD offers) to these centers. Distribution of these surplus offers were shown in Figure 3. The median (IQR) number of surplus offers per center was 78 (27–125). Surplus offers per center ranged from 0 to 629.

Distribution of surplus offers. Surplus offers were defined as offers (kidneys being offered to 1 or more recipients at a single transplant center) of high-PODD kidneys to centers that never accepted a high-PODD kidney. This figure shows the distribution of surplus offers among 122 centers that never accepted a high-PODD kidney during the study period. In total, 11684 surplus offers were made to these centers; a median (IQR) of 78 (27–125) surplus offers were made per center. At maximum, 629 surplus offers were made to a center. IQR, interquartile range; PODD, Probability of Delay or Discard.


In this national study of kidney discard in the KAS era, we prospectively validated PODD in predicting discard of deceased donor kidneys. The accuracy of PODD was excellent and better than the other indices using several evaluation metrics, including C-statistic, PPV, and NRI. We also found substantial clustering in centers’ use of high-PODD kidneys: while 122 centers did not use a single high-PODD kidney, 15 centers used the majority of these kidneys. The centers that used the most high-PODD kidneys remained consistent over time. Of all high-PODD kidney offers, 35.0% were arguably unnecessarily made to centers that never accepted such kidneys.

The high rate of kidney discard has been a concern for decades and has continued despite recent allocation changes in KAS.7,37-40 A “rescue” strategy has been proposed to reduce discard in which kidneys at high risk of discard are identified prospectively and preferentially offered to centers more likely to transplant such kidneys. Such a strategy requires a reliable method for identified kidneys at high risk of discard; the number of kidneys thus identified should be small enough to ensure minimal disruption to the overall allocation system. In this study, we have validated that PODD would be an appropriate tool for such a system. Only 9% of kidneys had PODD exceeding 0.6; over 79% of these kidneys were discarded. Over 13% of kidneys have PODD exceeding 0.5; 71.7% of these kidneys were discarded. Only 6.2% of kidneys had PODD exceeding 0.7; 85.2% of these kidneys were discarded. The OPTN could implement rescue allocation by allocating kidneys differently if PODD exceeded a certain threshold. The higher the threshold, the higher the chance that a “rescued” kidney would otherwise be discarded –but the fewer kidneys would be “rescued.” In addition, we have also validated the usefulness of reduced PODD, which does not depend on factors that may not be known at time of initial allocation and thus would be a more practical tool. Concerns have been raised that such a system might direct all “rescue” offers to a few centers, denying other centers the opportunity to show that they are willing to utilize these marginal kidneys. Applying “rescue” allocation only to kidneys refused at the local Organ Procurement Organization level, or allowing centers to petition for a temporary opportunity to prove themselves, could partially alleviate such concerns.

Our findings are consistent with previous reports that KDPI inadequately identifies kidneys at high risk of being discarded.17,19 While KDPI was designed to describe organ quality, it was not designed to predict kidney discard.41 The current findings were consistent with our initial report of PODD, showing that PODD was substantially better at predicting discard than KDPI with an improvement of 15.6%.17 The ability of PODD to discriminate between acceptance and discard has been consistently high, with an AUC of 0.83 in our prior study and 0.87 in our current validation study.17 After excluding machine perfusion and glomerulosclerosis as predictor variables, which are often not available at the time of initial organ offer, our reduced PODD (r-PODD) model remained more accurate in predicting discard than KDPI with an improvement of 11.4%. Compared with the Marrero score, for kidney discard prediction, PODD still performed significantly better with an improvement of 4.1%. Reduced PODD also performed significantly better than reduced Marrero with an improvement of 11.9%.

In considering center-level behavior, we found that the use of high-PODD kidneys varied widely in the US. This confirms several other reports of center-level variation in utilization of kidneys. Wey et al42 found that lower offer acceptance in a donation service area (DSA) was associated both with higher discard rates in that DSA and higher rates of export of organs from that DSA. Our group previously demonstrated that there was clustering of the utilization of suboptimal grafts among transplant centers.43 We also found the clustering of use of high-PODD kidneys over time were the most consistent among the top 15 centers. In addition, we found that 35.0% high-PODD kidney offers went to centers that have never accepted a single high-PODD kidney. This is also consistent with previous findings that marginal kidney offers were concentrated among a small amount of centers.18 Expedited offers of high-PODD kidneys to centers that are willing to accept them could be an effective way to reduce discard and improve access to transplantation.

Internationally, there have been efforts to target distribution of kidneys that were more likely to be discarded to those centers more likely to transplant them. In Europe, a policy was introduced in 2006 that transitioned organ offers to a center-oriented “rescue allocation” after a series of 5 rejections in a patient-oriented offer scheme.13 This was effective at decreasing discard rate,13 and after changes to the policy in 2013, the cold ischemia time of transplanted kidneys decreased as well.15 In another example, a policy was implemented in the United Kingdom in 2006 to allocate kidneys to centers agreeing to participate in the “Declined Kidney Scheme,” which then have the discretion to use the offered kidney locally for one of their patients, after a series of 5 rejections in the national allocation policy.44 Later in 2012, the “Kidney Fast-Track Scheme” further expanded upon this by allowing these kidneys to be offered simultaneously to multiple centers.14,45 Studies have not only shown decreased kidney discard following both policies, but perhaps more importantly have demonstrated posttransplant outcomes that are comparable to those of organs transplanted through standard allocation schemes.13,14,45-47

Our study validated the effectiveness of PODD to identify kidneys that are more likely to be discarded and demonstrated existing variation in utilization of kidneys with high PODD among centers. Use of such a validated index to improve the distribution of kidneys that are at higher risk of being discarded could decrease kidney discard rates and potentially improve candidate survival. In the setting of an ongoing dialogue about the high rates of kidney discard, we recommend developing a policy using a validated metric to improve distribution of kidneys likely to be discarded.


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