Results of Two Cases of Pig-to-Human Kidney Xenotransplantation.

BACKGROUND: Xenografts from genetically modified pigs have become one of the most promising solutions to the dearth of human organs available for transplantation. The challenge in this model has been hyperacute rejection. To avoid this, pigs have been bred with a knockout of the alpha-1,3-galactosyltransferase gene and with subcapsular autologous thymic tissue. METHODS: We transplanted kidneys from these genetically modified pigs into two brain-dead human recipients whose circulatory and respiratory activity was maintained on ventilators for the duration of the study. We performed serial biopsies and monitored the urine output and kinetic estimated glomerular filtration rate (eGFR) to assess renal function and xenograft rejection. RESULTS: The xenograft in both recipients began to make urine within moments after reperfusion. Over the 54-hour study, the kinetic eGFR increased from 23 ml per minute per 1.73 m2 of body-surface area before transplantation to 62 ml per minute per 1.73 m2 after transplantation in Recipient 1 and from 55 to 109 ml per minute per 1.73 m2 in Recipient 2. In both recipients, the creatinine level, which had been at a steady state, decreased after implantation of the xenograft, from 1.97 to 0.82 mg per deciliter in Recipient 1 and from 1.10 to 0.57 mg per deciliter in Recipient 2. The transplanted kidneys remained pink and well-perfused, continuing to make urine throughout the study. Biopsies that were performed at 6, 24, 48, and 54 hours revealed no signs of hyperacute or antibody-mediated rejection. Hourly urine output with the xenograft was more than double the output with the native kidneys. CONCLUSIONS: Genetically modified kidney xenografts from pigs remained viable and functioning in brain-dead human recipients for 54 hours, without signs of hyperacute rejection. (Funded by Lung Biotechnology.).

Sexual bother and sexual activity before and after kidney transplantation.

Among patients awaiting kidney transplantation (KT), sexual dysfunction is common owing to end-stage kidney disease, but may improve post-KT. Leveraging a 2-center prospective study, 2422 adult KT candidates and 490 adult KT recipients (May 2014 to December 2023) were identified. Using the Kidney Disease Quality of Life Short Form, participants reported on the negative impact of sexual dysfunction due to end-stage kidney disease (ie, sexual bother) at KT evaluation, admission, and post-KT follow-ups. Using mixed-effect logistic regression models, we estimated odds and trajectories for odds of sexual bother. At evaluation, 46.1% of male and 29.6% of female candidates reported sexual bother; 39.0% and 34.5%, respectively, had been sexually active in the past 4 weeks. At admission, 53.8% male and 27.0% female recipients reported sexual bother; 41.6% and 41.8%, respectively, had been sexually active in the past 4 weeks. The estimated prevalence of sexual bother decreased during the first 3 years post-KT (odds ratio per year: 0.39; 95% CI: 0.25-0.60). Sexual activity increased and peaked 1-year post-KT. At 3 years post-KT, 48.9% of male and 50.0% of female recipients were sexually active. Sexual bother is common pre-KT and improves post-KT, and sexual activity increases post-KT. Sexual health is important and should be considered during KT management.

Dietary Restriction, Socioeconomic Factors, Access to Kidney Transplantation, and Waitlist Mortality.

INTRODUCTION: Dietary restrictions for patients with end-stage kidney disease (ESKD) are burdensome. Kidney transplantation (KT) candidates who lack neighborhood resources and are burdened by dietary restrictions may have decreased access to KT. METHODS: In our two-center prospective cohort study (2014-2023), 2471 ESKD patients who were evaluated for KT (candidates) reported their perceived burden of dietary restrictions (not at all, somewhat/moderately, or extremely bothered). Neighborhood-level socioeconomic factors were derived from residential ZIP codes. We quantified the association of perceived burden of the dietary restrictions with a chance of listing using Cox models and risk of waitlist mortality using competing risks models. Then we tested whether these associations differed by neighborhood-level socioeconomic factors. RESULTS: At evaluation, 18% of KT candidates felt extremely bothered by dietary restrictions. Those who felt extremely bothered were less likely to be listed for KT (adjusted hazard ratio [aHR] = 0.75, 95% confidence interval [CI]: 0.64-0.87); this association did not differ by neighborhood-level socioeconomic factors. Overall, the burden of dietary restrictions was not associated with waitlist mortality (p = 0.62). However, among candidates living in high food insecurity neighborhoods, those who felt extremely bothered had higher waitlist mortality (adjusted subhazard ratio [aSHR] = 2.07, 95% CI: 1.14-3.75, p[interaction] = 0.02). The association between dietary burden and waitlist mortality did not differ by neighborhood-level healthy food access. CONCLUSION: The perceived burden of dietary restrictions is associated with a lower chance of listing for KT, and higher waitlist mortality only among candidates residing in neighborhoods with high food insecurity. Transplant centers should identify vulnerable patients and support them with nutrition education and access to food assistance programs.

Immune response, phenotyping and molecular graft surveillance in kidney transplant recipients following severe acute respiratory syndrome coronavirus 2 vaccination.

BACKGROUND: Understanding immunogenicity and alloimmune risk following severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination in kidney transplant recipients is imperative to understanding the correlates of protection and to inform clinical guidelines. METHODS: We studied 50 kidney transplant recipients following SARS-CoV-2 vaccination and quantified their anti-spike protein antibody, donor-derived cell-free DNA (dd-cfDNA), gene expression profiling (GEP), and alloantibody formation. RESULTS: Participants were stratified using nucleocapsid testing as either SARS-CoV-2-naïve or experienced prior to vaccination. One of 34 (3%) SARS-CoV-2 naïve participants developed anti-spike protein antibodies. In contrast, the odds ratio for the association of a prior history of SARS-CoV-2 infection with vaccine response was 18.3 (95% confidence interval 3.2, 105.0, p < 0.01). Pre- and post-vaccination levels did not change for median dd-cfDNA (0.23% vs. 0.21% respectively, p = 0.13), GEP scores (9.85 vs. 10.4 respectively, p = 0.45), calculated panel reactive antibody, de-novo donor specific antibody status, or estimated glomerular filtration rate. CONCLUSIONS: SARS-CoV-2 vaccines do not appear to trigger alloimmunity in kidney transplant recipients. The degree of vaccine immunogenicity was associated most strongly with a prior history of SARS-CoV-2 infection.

Caregiver exposure to hepatitis C virus following transplantation with hepatitis C viremic donor organs: A case series.

Direct-acting antiviral (DAA) therapeutics have ushered in an era in which transplanting organs from donors infected with hepatitis C virus (HCV+) into recipients without (HCV-) is an increasingly common practice. Rare but potentially life-threatening events have been reported in recipients of HCV+ organs. Since 2018 at our institution, 182 HCV- patients have received HCV+ donor organs. Here, we retrospectively reviewed cases in which recipients' family member caregivers reported sustaining needlestick exposures at home following discharge of the transplant recipient from the hospital. Caregiver needlestick exposures were passively reported in three cases of HCV+ into HCV- transplants (1.64% of such cases at our center). In all instances, the exposed individuals were aiding in diabetic management and the exposure occurred via lancets or insulin needles. In one case, the recipient viral load was undetectable at the time of the exposure but in the other two, recipients were viremic, putting their family members at risk to contract HCV infection. Surveillance for the exposed individuals was undertaken and no transmissions occurred. For centers performing HCV+ into HCV- transplants, it is important that informed consent includes discussion of potential secondary risks to family members and caregivers. Further, protocols for postexposure surveillance and for the acquisition of DAA treatment in the event of a secondary transmission should be in place.

Safety and Efficacy of Drug Eluting Stents for Treatment of Transplant Renal Artery Stenosis.

BACKGROUND: Transplant renal artery stenosis (TRAS) after renal transplantation is a common cause of graft dysfunction and failure. Endovascular intervention in the form of percutaneous transluminal angioplasty (PTA) and stenting has rapidly become the dominant treatment modality for the TRAS. There is a paucity of clinical data on the use of drug-eluting stent (DES) for TRAS. We investigated the outcomes of patients with clinically significant TRAS undergoing DES placement. METHODS: A retrospective review of patients with clinically significant TRAS undergoing PTA with DES placement from June 2014 to April 2021 was conducted. Patients treated for TRAS exhibited uncontrolled hypertension and/or unexplained allograft dysfunction. Patient demographics, procedural details, and follow-up outcomes were collected. Primary endpoints were the in-stent primary patency and graft survival. Secondary endpoints were freedom from reintervention, primary-assisted patency, and access-related complications. RESULTS: Thirteen TRAS in 12 patients with graft function alteration were treated with DES. The median age was 57 years (interquartile range (IQR), 48-63 years), and 9 (70%) patients were male. The median follow-up was 9 months (IQR, 4-52 months). The most common comorbidity was hypertension (100%), coronary artery disease (83%), and diabetes. The median time from deceased donor transplant to intervention was 5.8 months (IQR, 3.5-6.7 months). TRAS was most commonly found at the juxta-ostial segment (77%). The procedure was performed with carbon dioxide angiography with minimal amount of iodinated contrast (median, 3 mL) under local anesthesia in 9 (69%), and general anesthesia in 4 (31%) patients. The median stent diameter was 4.5 mm (IQR, 4-5 mm), and the median stent length was 15 mm (IQR, 15-18 mm). No intraoperative complications occurred. The rates of stenosis-free primary patency of the DES and graft survival were 76% and 100%, respectively. All 3 reinterventions for restenosis resulted from the kinking of the transplant renal artery proximal to the DES, which were treated by extending the stent more proximally 1-2 mm into the external iliac artery. There were no access-related complications. The median time to reintervention was 0.9 months (range, 0.23-2 months). Freedom from reintervention and primary-assisted patency were 76% and 100%, respectively. CONCLUSIONS: Our study demonstrates that DES is a safe and effective treatment modality in patients with TRAS at short to mid-term follow-up. As all reinterventions after DES were performed due to kinking of the transplant renal artery proximal to the stent, bridging of the DES 1-2 mm into the external iliac artery is recommended.

Pancreas transplantation from hepatitis C viremic donors to uninfected recipients.

Despite utilization of hepatitis C viremic organs for hepatitis C naïve recipients (HCV D+/R-) in other solid organ transplants, HCV viremic pancreata remain an unexplored source of donor organs. This study reports the first series of HCV D+/R- pancreas transplants. HCV D+/R- had shorter waitlist times compared to HCV D-/R-, waiting a mean of 16 days from listing for HCV-positive organs. HCV D+/R- had a lower match allocation sequence than HCV D-/R-, and this correlated with receipt of organs with a lower Pancreas Donor Risk Index (PDRI) score. All HCV D+/R- had excellent graft function with a mean follow-up of 438 days and had undetectable HCV RNA levels by a mean of 23 days after initiation of HCV-directed therapy. The rates of infectious complications, reoperation, readmission, rejection, and length of stay were not impacted by donor HCV status. A national review of potential ideal pancreas donors found that 37% of ideal HCV-negative pancreas allografts were transplanted, compared to only 5% of ideal HCV-positive pancreas allografts. The results of the current study demonstrate the safety of accepting HCV-positive pancreata for HCV-naïve recipients and advocates for increased utilization of ideal HCV-positive pancreas allografts.

Impact of the 2014 kidney allocation system changes on trends in A2/A2B into B kidney transplantation and organ procurement organization reporting of donor subtyping.

The current kidney allocation system (KAS) preferentially allocates kidneys from blood type A2 or A2B (A/A2B) donors to blood type B candidates. We used national data to evaluate center-level performance of A2/A2B to B transplants, and organ procurement organization (OPO) reporting of type A or AB donor subtyping, in 5-year time periods prior to (2009-2014) and following (2015-2019) KAS implementation. The number of centers performing A2/A2B to B transplants increased from 17 pre-KAS to 76 post-KAS, though this still represents only a minority of centers (7.3% pre-KAS and 32.6% post-KAS). For high-performing centers, the median net increase in A2/A2B to B transplants was 19 cases (range -2-72) per center in the 5 years post-KAS. The median net increase in total B recipient transplants was 21 cases (range -17-119) per center. Despite requirements for performance of subtyping, in 2019 subtyping was reported on only 56.4% of A/AB donors. This translates into potential missed opportunities for B recipients, and even post-KAS up to 2322 A2/A2B donor kidneys may have been allocated for transplantation as A/AB. Further progress must be made both at center and OPO levels to broaden implementation of A2/A2B to B transplants for the benefit of underserved recipients.

SARS-CoV-2 antibody responses in solid organ transplant recipients.

Antibody responses among immunocompromised solid organ transplant recipients (SOT) infected with Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) may be diminished compared to the general population and have not been fully characterized. We conducted a cohort study at our transplant center to investigate the rate of seroconversion for SARS-CoV-2 IgG antibodies among SOT recipients who were diagnosed with Coronavirus disease 2019 (COVID-19) and underwent serum SARS-CoV-2 IgG enzyme-linked immunosorbent assay (ELISA) testing. The 61 patients who were included in the final analysis underwent initial SARS-CoV-2 IgG testing at a median of 62 days (Interquartile range 55.0-75.0) from symptom onset. Note that, 51 of 61 patients (83.6%) had positive SARS-CoV-2 IgG results, whereas 10 (16.4%) had negative IgG results. Six (60%) out of 10 seronegative patients underwent serial IgG testing and remained seronegative up to 17 weeks post-diagnosis. Use of belatacept in maintenance immunosuppression was significantly associated with negative IgG antibodies to SARS-CoV-2 both in univariate and multivariate analyses (Odds ratio 0.04, p = .01). In conclusion, the majority of organ transplant recipients with COVID-19 in our study developed SARS-CoV-2 antibodies. Further longitudinal studies of the durability and immunologic role of these IgG responses and the factors associated with lack of seroconversion are needed.

Outpatient management of kidney transplant recipients with suspected COVID-19-Single-center experience during the New York City surge.

Data describing the clinical progression of coronavirus disease 2019 (COVID-19) in transplant recipients are limited. In New York City during the surge in COVID-19 cases, a systematic approach to monitoring and triaging immunocompromised transplant patients was required in the context of strained healthcare resources, limited outpatient testing, and heightened hospital exposure risks. Public health guidance at the onset of the COVID-19 outbreak recommended outpatient monitoring of mildly symptomatic patients without specific recommendations for special populations such as transplant recipients. We developed and implemented a systematic monitoring algorithm for kidney transplant recipients at our transplant center who reported mild symptoms suggestive of COVID-19. We describe the outcomes of the first 44 patients monitored through this algorithm. A total of 44 kidney transplant recipients thought to be symptomatic for COVID-19 disease were followed for a minimum of 14 days. The majority of mildly symptomatic patients (34/44) had clinical progression of disease and were referred to the emergency department where they all tested PCR positive and required hospitalization. More than half of these patients presented with hypoxia requiring supplemental oxygen, 39% were intubated within 48 hours, and 53% developed acute kidney injury but did not require dialysis. There were 6 deaths. During surge outbreaks, kidney transplant patients with even mild symptoms have a high likelihood of COVID-19 disease and most will worsen requiring hospitalization for supportive measures. Earlier outpatient testing and hospitalization may improve COVID-19 outcomes among transplant recipients.

IdeS (Imlifidase): A Novel Agent That Cleaves Human IgG and Permits Successful Kidney Transplantation Across High-strength Donor-specific Antibody.

OBJECTIVES: The presence of a donor-specific positive crossmatch has been considered to be a contraindication to kidney transplantation because of the risk of hyperacute rejection. Desensitization is the process of removing hazardous preformed donor-specific antibody (DSA) in order to safely proceed with transplant. Traditionally, this involves plasmapheresis and intravenous immune globulin treatments that occur over days to weeks, and has been feasible when there is a living donor and the date of the transplant is known, allowing time for pre-emptive treatments. For sensitized patients without a living donor, transplantation has been historically difficult. SUMMARY OF BACKGROUND DATA: IdeS (imlifidase) is an endopeptidase derived from Streptococcus pyogenes which has specificity for human IgG, and when infused intravenously results in rapid cleavage of IgG. METHODS: Here we present our single-center's experience with 7 highly sensitized (cPRA98-100%) kidney transplant candidates who had DSA resulting in positive crossmatches with their donors (5 deceased, 2 living) who received IdeS within 24 hours prior to transplant. RESULTS: All pre-IdeS crossmatches were positive and would have been prohibitive for transplantation. All crossmatches became negative post-IdeS and the patients underwent successful transplantation. Three patients had DSA rebound and antibody-mediated rejection, which responded to standard of care therapies. Three patients had delayed graft function, which ultimately resolved. No serious adverse events were associated with IdeS. All patients have functioning renal allografts at a median follow-up of 235 days. CONCLUSION: IdeS may represent a groundbreaking new method of desensitization for patients who otherwise might have no hope for receiving a lifesaving transplant.

Staged Endovascular and Surgical Management of a Mycotic Pseudoaneurysm After Pancreas Transplant.

Mycotic pseudoaneurysms are a rare, life-threatening complication after pancreas transplant. There have been limited reports of endovascular treatment of mycotic pseudoaneurysms in pancreas transplant recipients. Herein, we report on a case of a mycotic pseudoaneurysm from Pseudomonas aeruginosa after pancreas transplant. A 53-year-old male recipient underwent an uneventful simultaneous pancreas and kidney transplant. He was readmitted 48 days posttransplant with fevers and rigors. Pan-cultures were performed and broad-spectrum antibiotics were initiated. Imaging studies demonstrated a large mycotic pseudoaneurysm arising from the right common iliac artery adjacent to the arterial Y-graft anastomosis of the transplant pancreas. Endovascular stent placement was used to exclude the pseudoaneurysm prior to transplant pancreatectomy. During pancreatectomy, the lateral wall of the common iliac artery was found to be necrotic with significant exposure of the endovascular stent. After ligation and excision of the common iliac artery, a femorofemoral bypass was performed to revascularize the lower extremity. This case report highlights the advantage of a staged endovascular and surgical management strategy for complex mycotic pseudoaneurysms after pancreas transplant.

Pig-to-human heart xenotransplantation in two recently deceased human recipients.

Genetically modified xenografts are one of the most promising solutions to the discrepancy between the numbers of available human organs for transplantation and potential recipients. To date, a porcine heart has been implanted into only one human recipient. Here, using 10-gene-edited pigs, we transplanted porcine hearts into two brain-dead human recipients and monitored xenograft function, hemodynamics and systemic responses over the course of 66 hours. Although both xenografts demonstrated excellent cardiac function immediately after transplantation and continued to function for the duration of the study, cardiac function declined postoperatively in one case, attributed to a size mismatch between the donor pig and the recipient. For both hearts, we confirmed transgene expression and found no evidence of cellular or antibody-mediated rejection, as assessed using histology, flow cytometry and a cytotoxic crossmatch assay. Moreover, we found no evidence of zoonotic transmission from the donor pigs to the human recipients. While substantial additional work will be needed to advance this technology to human trials, these results indicate that pig-to-human heart xenotransplantation can be performed successfully without hyperacute rejection or zoonosis.

Evaluation and Transplantation of a SARS-CoV-2 Seropositive Kidney Candidate.

The COVID-19 pandemic affected transplant center activity in areas with high number of cases such as New York City and prompted reevaluation of patients awaiting organ transplant diagnosed with SARS-CoV-2 infection. To resume safe transplantation at our center, we found it necessary to (1) identify transplant candidates with possible exposure to or history of COVID-19 infection, (2) outline a clinical and laboratory assessment to determine adequate clinical recovery from COVID-19 for transplantation, and (3) determine whether the possibility of perioperative COVID-19 transmission from the patient to staff would pose unacceptable risk. Here, we describe our center's approach to proceeding with transplantation in a SARS-CoV-2 seropositive living donor kidney transplant recipient and describe early posttransplant outcomes.

Clinical and Financial Implications of 2 Treatment Strategies for Donor-derived Hepatitis C Infections.

Transplanting hepatitis C viremic donor organs into hepatitis C virus (HCV)-negative recipients is becoming increasingly common; however, practices for posttransplant direct-acting antiviral (DAA) treatment vary widely. Protracted insurance authorization processes for DAA therapy often lead to treatment delays. METHODS: At our institution, 2 strategies for providing DAA therapy to HCV- recipients of HCV+ transplants have been used. For thoracic organ recipients, an institution-subsidized course of initial therapy was provided to ensure an early treatment initiation date. For abdominal organ recipients, insurance approval for DAA coverage was sought once viremia developed, and treatment was initiated only once the insurance-authorized supply of drug was received. To evaluate the clinical impact of these 2 strategies, we retrospectively collected data pertaining to the timing of DAA initiation, duration of recipient viremia, and monetary costs incurred by patients and the institution for patients managed under these 2 DAA coverage strategies. RESULTS: One hundred fifty-two transplants were performed using HCV viremic donor organs. Eighty-nine patients received DAA treatment without subsidy, and 62 received DAA treatment with subsidy. One patient who never developed viremia posttransplant received no treatment. Subsidizing the initial course enabled earlier treatment initiation (median, 4 d [interquartile range (IQR), 2-7] vs 10 [IQR, 8-13]; P < 0.001) and shorter duration of viremia (median, 16 d [IQR, 12-29] vs 36 [IQR, 30-47]; P < 0.001). Institutional costs averaged $9173 per subsidized patient and $168 per nonsubsidized patient. Three needlestick exposures occurred in caregivers of viremic patients. CONCLUSIONS: Recipients and their caregivers stand to benefit from earlier DAA treatment initiation; however, institutional costs to subsidize DAA therapy before insurance authorization are substantial. Insurance authorization processes for DAAs should be revised to accommodate this unique patient group.

Successful A2 to B Deceased Donor Kidney Transplant after Desensitization for High-Strength Non-HLA Antibody Made Possible by Utilizing a Hepatitis C Positive Donor.

Desensitization using plasma exchange can remove harmful antibodies prior to transplantation and mitigate risks for hyperacute and severe early acute antibody-mediated rejection. Traditionally, the use of plasma exchange requires a living donor so that the timing of treatments relative to transplant can be planned. Non-HLA antibody is increasingly recognized as capable of causing antibody-mediated renal allograft rejection and has been associated with decreased graft longevity. Our patient had high-strength non-HLA antibody deemed prohibitive to transplantation without desensitization, but no living donors. As the patient was eligible to receive an A2 ABO blood group organ and was willing to accept a hepatitis C positive donor kidney, this afforded a high probability of receiving an offer within a short enough time frame to attempt empiric desensitization in anticipation of a deceased donor transplant. Fifteen plasma exchange treatments were performed before the patient received an organ offer, and the patient was successfully transplanted. Hepatitis C infection was treated posttransplant. No episodes of rejection were observed. At one-year posttransplant, the patient maintains good graft function. In this case, willingness to consider nontraditional donor organs enabled us to mimic living donor desensitization using a deceased donor.

Is it cerebral or renal salt wasting?

Cerebral salt-wasting (CSW), or renal salt-wasting (RSW), has evolved from a misrepresentation of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) to acceptance as a distinct entity. Challenges still confront us as we attempt to differentiate RSW from SIADH, ascertain the prevalence of RSW, and address reports of RSW occurring without cerebral disease. RSW is redefined as 'extracellular volume depletion due to a renal sodium transport abnormality with or without high urinary sodium concentration, presence of hyponatremia or cerebral disease with normal adrenal and thyroid function.' Our inability to differentiate RSW from SIADH lies in the clinical and laboratory similarities between the two syndromes and the difficulty of accurate assessment of extracellular volume. Radioisotopic determinations of extracellular volume in neurosurgical patients reveal renal that RSW is more common than SIADH. We review the persistence of hypouricemia and increased fractional excretion of urate in RSW as compared to correction of both in SIADH, the appropriateness of ADH secretion in RSW, and the importance of differentiating renal RSW from SIADH because of disparate treatment goals: fluid repletion in RSW and fluid restriction in SIADH. Patients with RSW are being incorrectly treated by fluid restriction, with clinical consequences. We conclude that RSW is common and occurs without cerebral disease, and propose changing CSW to RSW.

Normal fractional urate excretion identifies hyponatremic patients with reset osmostat.

BACKGROUND: Reset osmostat (RO) occurs in 36% of patients with syndrome of inappropriate antidiuretic hormone secretion (SIADH) and is not often considered when evaluating hyponatremic patients. Patients with RO are not usually treated, but recent awareness that symptoms are associated with mild hyponatremia creates a therapeutic dilemma. We encountered patients with hyponatremia, hypouricemia and high urine sodium concentration (UNa), who had normal fractional excretion (FE) of urate and excreted dilute urines that were consistent with RO. We decided to test whether a normal FEurate in nonedematous hyponatremia irrespective of UNa or serum urate would identify patients with RO. METHODS: We determined FEurate in nonedematous hyponatremic patients. A diagnosis of RO was made if urine osmolality (Uosm) was <200 mOsm/kg in a random urine. We performed a modified water-loading test in patients with a normal FEurate whose random Uosm was >200 mOsm/kg. RESULTS: All nonedematous hyponatremic patients with FEurate of 4%-11% had RO, as determined by Uosm <200 mOsm/kg on a random urine collection in 8 patients, or after a modified water-loading test in 6 patients. Plasma antidiuretic hormone (ADH) in 4 patients was undetectable at <1 pg/mL during water-loading. Nine patients had baseline concentrated urine, 12 had UNa >20 mmol/L, 9 were hypouricemic, yet all had a normal FEurate. Comorbidities were similar to those reported in RO. CONCLUSIONS: RO, a benign form of SIADH, occurs commonly. A normal FEurate in a nonedematous hyponatremic patient is highly suggestive of RO. Determining FEurate is superior to serum urate. The therapeutic dilemma for RO must be resolved.