Relationship between the intrarenal resistive index and split renal function in patients with solitary kidneys

Introduction

The early detection of kidney disease and the accurate assessment of the extent of renal damage are crucial for developing effective renal protection strategies. The primary parameter for evaluating kidney injury is renal function, which is typically measured through laboratory tests such as blood creatinine levels and the glomerular filtration rate (GFR) (1). Renal function can be categorized into two types: total renal function and split renal function. Total renal function refers to the combined functionality of both kidneys, while split renal function refers to the functionality of either the left or right kidney specifically.

Blood creatinine levels and the GFR provide a comprehensive assessment of total renal function in most individuals, the majority of whom have two kidneys. However, in practical scenarios, if one kidney is injured while the other remains healthy, or if both kidneys experience mild or moderate injuries that allow for adequate compensation, the blood creatinine and GFR values may still fall within normal limits (2). This can lead to an incorrect assessment of split renal function and delay the implementation of effective renal protection strategies. Therefore, a reliable method for accurately evaluating split renal function needs to be established to guide clinical decision-making and protect split renal function in the early stages of injury.

Medical imaging plays a vital role in assessing split renal function, with nuclear medicine being considered the gold standard (3). Previous studies have shown that both contrast-enhanced magnetic resonance imaging (MRI) and contrast-enhanced computed tomography (CT) estimate split renal function with similar accuracy (2,4,5). However, the high costs and risks associated with anaphylaxis or radiation exposure limit the widespread use of these imaging methods in the evaluation of split renal function. Conversely, ultrasound (US) is a noninvasive, non-radiation, safe, and cost-effective imaging technique for this purpose (6-16). Previous studies assessed split renal function using US in patients with two kidneys (12,13). Due to compensatory mechanisms in renal function, significant unilateral injury to one kidney may be masked by the healthy contralateral kidney or by normal function in that kidney. These confounding factors might have affected the results of previous studies that used US to evaluate split renal function. Moreover, the resistive index (RI) of the intrarenal artery, measured by Doppler US and commonly used to assess total renal function (6-11,14-16), has not been adequately applied to evaluate split renal function. Therefore, the effectiveness of US, particularly the use of the intrarenal RI to assess split renal function, remains uncertain.

In patients with solitary kidneys, the split renal function equals the total renal function, which creates an optimal condition for using US to evaluate split renal function. Our study measured the intrarenal RI in solitary kidneys by US to assess split renal function. We also hypothesized that the sonographic RI criteria for diagnosing split renal function in non-solitary kidneys may not apply to solitary kidneys. Consequently, we investigated the correlation between the intrarenal RI measured by Doppler US and split renal function in solitary kidneys. This study aimed to (I) confirm that the intrarenal RI of a solitary kidney can serve as a reliable indicator of split renal function and to establish its cutoff value; and (II) assess the effectiveness of the intrarenal RI in predicting split renal function, using the estimated glomerular filtration rate (eGFR) as the reference standard for evaluating split renal function. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-932/rc).

Methods

Patients

This retrospective cross-sectional study was approved by the Institutional Review Board of The First Affiliated Hospital of Wenzhou Medical University (No. KY2024-R027). It was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The requirement of individual consent was waived due to the retrospective nature of the study.

From January 2004 to December 2023, the data of 78 patients who underwent US examinations at our hospital and were suspected of having solitary kidneys were retrospectively reviewed.

Patients were included in the study if they met the following inclusion criteria: (I) had a solitary kidney due either to a congenital condition or as a result of a unilateral total nephrectomy; (II) had undergone CT or MRI to validate the presence of a solitary kidney; (III) had undergone a Doppler US examination of the kidney; (IV) had an interval time between the Doppler examination and the renal function test not exceeding one week; and (V) had a solitary kidney that lacked any occupying lesions, such as tumors, and was free from vascular diseases, such as renal artery stenosis or hydronephrosis. Patients were excluded from the study if they met any of the following exclusion criteria: (I) had a severe heart disease characterized by a reduced ejection fraction, which would impair blood flow through the renal artery; and/or (II) had severe liver disease such as cirrhosis or severe intra-abdominal infection.

In total, 57 patients were included in the study. Patients were excluded from the study for the following reasons: contralateral kidney atrophy (n=2), a contralateral kidney that had undergone heminephrectomy (n=1), the lack of a renal function test (n=9), and an interval of more than one week between the kidney US examination and the renal function test (n=9).

Renal function was calculated using the Chronic Kidney Disease Epidemiology Collaboration equation, which is based on serum creatinine values adjusted for age and gender (17). Split renal function was defined as the absolute split eGFR of either the left or right kidney, and was calculated as follows: split renal function = total eGFR × split renal function percentage (as described previously) (13). Because in the present study, all the patients had a solitary kidney (either left or right), the split renal function (total eGFR × 100%) was equivalent to the total renal function. This condition provided an optimal setting for using US to evaluate split renal function.

An eGFR of 60 mL/min per 1.73 m² or higher indicated normal or mildly reduced renal function; an eGFR of less than 60 mL/min per 1.73 m² indicated that the patient was at risk for moderate renal dysfunction (RDF) (14). For the purposes of this study, an eGFR less than 60 mL/min per 1.73 m² was classified as RDF. The study cohort was categorized into two groups based on the eGFR results: the RDF group (eGFR <60 mL/min per 1.73 m²), and the non-RDF group (eGFR ≥60 mL/min per 1.73 m²). Meanwhile, based on the cause of the solitary kidney (i.e., congenital absence or surgical removal by unilateral nephrectomy), the patients were classified into three groups for further analysis: those with a congenital solitary kidney, those who underwent nephrectomy before puberty, and those who underwent nephrectomy after puberty (i.e., post-pubertal, over 14 years old). The patients’ demographic data, clinical histories, and outcomes were meticulously gathered from the hospital’s electronic medical records.

US examinations and imaging analysis

The US examinations were conducted by an experienced radiologist (S.P.C.), who had 30 years of experience in kidney US. The patient was positioned supine, and standard sonographic equipment was used, including the Philips IU 22 (Philips Medical Systems, Bothell, WA, USA), Acuson Sequoia 512 (Siemens Medical Solutions, Mountain View, CA, USA), GE Logiq E9 (GE Healthcare, Wauwatosa, WI, USA), and Hitachi HI-VISION Avius (Hitachi Aloka Medical, Tokyo, Japan). A 2–5-MHz convex probe was used for the procedure.

Initially, grayscale US imaging was performed. The kidney was visualized in a longitudinal section with the patient in a prone position, and the bipolar length of the kidney, defined as the largest distance between the upper and lower poles, was measured. Following this, color Doppler US was employed to assess the main trunk and branches of the renal arteries. The interlobar artery was identified using color-flow imaging, and the blood flow profile in the artery was monitored through spectral analysis. This method provided the best Doppler signal for assessing the quantity of flow and ensuring the correct angle (14,18).

For each patient, blood flow parameters were measured in the middle interlobar arteries of the kidney. These parameters included the minimum blood flow velocity (Vmin), maximum blood flow velocity (Vmax), and RI, which was calculated as follows: RI = (Vmax – Vmin) / Vmax. Measurements were taken only when at least three consecutive similar waveforms were observed. The renal artery waveform was captured with an angle of 60 degrees or less. The US imaging interpretations were determined through consensus by three radiologists (S.P.C., Z.Z., and M.G.; Z.Z. and M.G. had 10 years of experience in conventional US each). All three radiologists were blinded to the clinical diagnostic results at the time of the interpretation of the US imaging.

Statistical analysis

The statistical analyses were performed using the software SPSS 27.0 (IBM Corp., Armonk, NY, USA) and MedCalc 12.0 (MedCalc Software, Ostend, Belgium). The continuous variables are reported as the mean ± standard deviation, or the median with the interquartile range (IQR), while the categorical variables are expressed as the percentage. To compare groups, the unpaired Student’s t-test was used for the normally distributed continuous variables, the Mann-Whitney U test for the non-normally distributed continuous variables, and the chi-square test or Fisher’s exact test for the categorical variables. A statistical significance threshold was set at P<0.05.

To explore the correlation between the eGFR and other variables, nonparametric line correlation (Spearman’s method) was employed. A stepwise binary logistic regression analysis was conducted to identify the independent risk factors for predicting renal failure. Potential confounders (age, the RI, and kidney length) and candidate variables with a P value <0.1 from the univariate analysis were included in the multivariate model to assess the association between the eGFR and candidate variables. The diagnostic performance of the RI and kidney length in predicting RDF was evaluated using the area under the receiver operating characteristic (AUROC) curve. The accuracy of the RI and kidney length in predicting RDF was reported as AUROC values with 95% confidence intervals (CIs). Differences between the AUROC values for predicting RDF using the RI alone, kidney length alone, and a combination of the RI and kidney length were assessed using the Z-test.

Results

Among the 57 patients with solitary kidneys, 36 were male and 21 were female, with an average age of 60.05±14.21 years (range, 31–86 years). Of the 57 solitary kidneys, 27 were on the left side, and 30 were on the right side, with an average kidney length of 11.1±1.4 cm (range, 6.4–13.5 cm). The causes of solitary kidney are detailed in Table 1, with unilateral total nephrectomy due to renal tumors identified as the most common cause. All solitary kidneys were confirmed by CT or MRI examinations.

Among the 57 patients in this study, six were diagnosed with single kidney anomalies. These six patients had an average age of 52.16±13.70 years (range, 38–70 years), and three were male. The remaining 51 patients underwent unilateral total nephrectomy. These 51 patients had an average age of 60.98±14.11 years (range, 31–86 years), and 33 were male. At the time the nephrectomy was performed, the 51 patients had an average age of 51.93±16.89 years (range, 1–81 years). No significant differences were found between the patients with single kidney anomalies and those who underwent unilateral total nephrectomy in terms of age or gender (P=0.152 and P=0.668, respectively).

Among the 51 patients who underwent unilateral total nephrectomy, two patients underwent pre-pubertal nephrectomies. These two patients were aged 34 and 32, respectively (average age: 33.10±1.41 years), and one was male. Of these two patients, one patient underwent the procedure at 1 year old, and another at 14 years old. The remaining 49 patients underwent post-pubertal nephrectomies. These 49 patients had an average age of 62.12±13.17 years (range, 31–86 years), and 32 were male.

There were no significant differences between the patients who underwent pre-pubertal and post-pubertal nephrectomies in terms of gender (P=0.792). However, the patients who underwent post-pubertal nephrectomies were significantly older than those who underwent pre-pubertal nephrectomies (P=0.003). Additionally, there were no significant differences between the patients with single kidney anomalies and those who underwent post-pubertal nephrectomies in terms of age or gender, with P values of 0.087 and 0.697 for age and gender, respectively. Similarly, there were no significant differences between the patients with single kidney anomalies and those who underwent pre-pubertal nephrectomies in terms of age or gender, with P values of 0.112 and >0.999 for age and gender, respectively.

Unfortunately, the pathology results of six patients were lost due to a lengthy delay following the surgery. The average time interval between the initial US examination and the confirmation of solitary kidneys was approximately six years. The average interval between US and eGFR examinations was 1 day (IQR, 0–3 days; range, 0–7 days). Among the 57 patients, 24 (42.1%) were diagnosed with hypertension.

The average eGFR of these 57 patients was 67.1 mL/min per 1.73 m² (range, 41.7–88 mL/min per 1.73 m²; overall range, 8–112 mL/min per 1.73 m²). Based on whether their eGFR was lower than 60 mL/min, the patients were classified into the RDF group (n=21, eGFR <60 mL/min per 1.73 m²) and the non-RDF group (n=36, eGFR ≥60 mL/min per 1.73 m²). The baseline clinical characteristics, eGFR, and US findings of solitary kidneys between these two groups are summarized in Table 2. No significant differences were observed between the RDF and non-RDF groups in terms of sex or solitary kidney site (all P>0.05). However, the patients in the RDF group were older and had a higher prevalence of hypertension, shorter kidney lengths, and higher intrarenal RI values than those in the non-RDF group (all P<0.05).

The linear correlation analysis revealed significant correlations between the RI and eGFR (R=−0.625, P<0.001) (Figure 1). Additionally, correlations were found between age and the eGFR (R=−0.53, P<0.001) and between the kidney length and eGFR (R=0.403, P=0.002) (Figures 2,3). A strong negative correlation was also found between hypertension and the eGFR (R=−0.77, P<0.001). Further, the multivariate regression analysis revealed that only the RI and solitary kidney length were independent predictors of RDF, with significance values of P=0.005 and P=0.029, respectively.

Figure 1 Linear correlation between the RI and glomerular filtration rate in patients with solitary kidneys (R=−0.625, P<0.001). Red dots indicate the RDF group (eGFR <60 mL/min per 1.73 m²) group; Blue dots indicate the non-RDF group (eGFR ≥60 mL/min per 1.73 m²). eGFR, estimated glomerular filtration rate; RDF, renal dysfunction; RI, resistive index.

Figure 2 Linear correlation between solitary kidney length and the glomerular filtration rate (R=0.403, P=0.002). Red dots indicate the RDF group (eGFR <60 mL/min per 1.73 m²) group; Blue dots indicate the non-RDF group (eGFR ≥60 mL/min per 1.73 m²). eGFR, estimated glomerular filtration rate; RDF, renal dysfunction.

Figure 3 A 70-year-old female was diagnosed with a single kidney anomaly via CT one year ago. Her left kidney measured 10.4 cm (A), slightly smaller than the cutoff value of 10.6 cm. The RI of the left kidney was 0.63 (B, equal to the cutoff value of 0.63), and the split renal function test showed an eGFR of 30.9 mL/min per 1.73 m², indicating renal dysfunction in the solitary kidney. A 48-year-old male had his left kidney removed two months ago due to cancer. His remaining right kidney measured 11.3 cm (C), larger than the cutoff value of 10.6 cm. The RI of his right kidney was 0.66 (D), above the cutoff value of 0.63. A split kidney function test showed an eGFR of 54.6 mL/min per 1.73 m², indicating impaired function. A 38-year-old male had a single kidney anomaly confirmed by a CT scan on the same day as an ultrasound examination. The remaining right kidney measured 12.0 cm (E), larger than the cutoff value of 10.6 cm. Its RI was 0.60 (F), below the cutoff value of 0.63. A split renal function test showed an eGFR of 96.9 mL/min per 1.73 m², indicating normal split kidney function. CT, computed tomography; eGFR, estimated glomerular filtration rate; RI, resistive index; Vd, end-diastolic velocity; Vs, peak systolic velocity.

The receiver operating characteristic (ROC) curve analysis showed that the AUROC values for the RI, kidney length, and combination of both were 0.815 (95% CI: 0.691–0.906, P<0.0001), 0.776 (95% CI: 0.646–0.876, P<0.0001), and 0.782 (95% CI: 0.653–0.880, P<0.0001), respectively, for diagnosing RDF in patients with solitary kidneys (Figures 4,5). Table 3 summarizes the diagnostic indices for the RI, kidney length, and their combination in diagnosing RDF.

Figure 4 The ROC curve analysis yielded an AUROC of 0.815 (95% confidence interval: 0.691–0.906, P<0.0001). Using an RI value of 0.63 as the cutoff value for predicting renal dysfunction in patients with a solitary kidney, the sensitivity was 85.71% and the specificity was 72.22%. AUROC, area under the ROC curve; RI, resistive index; ROC, receiver operating characteristic.

Figure 5 The ROC curve analysis yielded an AUROC of 0.776 (95% confidence interval: 0.646–0.876, P<0.0001). Using a kidney length cutoff value of 10.6 cm for predicting renal dysfunction in patients with a solitary kidney, the sensitivity was 61.9% and the specificity was 88.89%. AUROC, area under the ROC curve; ROC, receiver operating characteristic.

Adopting an optimal cutoff value of 0.63 for the RI in the diagnosis of RDF resulted in a sensitivity of 85.71%, a specificity of 72.22%, and an overall accuracy of 77.19%. Conversely, adopting a kidney length cutoff value of 10.6 cm resulted in a sensitivity of 61.9%, a specificity of 88.89%, and an accuracy of 78.94%. The combination of the RI and kidney length yielded a sensitivity of 61.9% and a specificity of 94.44%, resulting in the highest accuracy achieved of 82.46%. However, no statistically significant differences were found in the AUROC values of the RI, kidney length, or their combination (all P>0.05) (Table 4).

Discussion

The present study showed that the intrarenal RI is significantly associated with split renal function in patients with solitary kidneys. Further, the ROC curve analysis demonstrated that an RI of 0.63 had a high sensitivity (85.71%) and specificity (72.22%) for detecting split RDF in patients with solitary kidneys. To the best of our knowledge, this is the first time these associations and findings have been reported.

Our results align with previous studies (6-16), indicating that Doppler US parameters, particularly the RI, can effectively predict renal function in patients with either one or two kidneys. Most previous studies have reported that an RI of approximately 0.7 is a reliable indicator for predicting total renal function failure, the worsening of renal function after angiography, or the response to treatment in lupus nephritis patients in patients with two kidneys (6-12,14-16,19). Zhang et al. (12) reported that an RI of 0.73 predicted split renal function in 35 patients with unilateral kidney disease. Additionally, Yura et al. (13) found that one Doppler US parameter—the S/D ratio [the peak systolic velocity (S) divided by end diastolic velocity (D)]—was significantly and positively associated with split renal function. However, it is important to note that, except for one patient, all the individuals evaluated for split renal function in these studies had two kidneys.

Due to compensatory mechanisms in renal function, assessing the function of a single kidney (split renal function) using data from both kidneys may lead to skewed results. Our findings differ from those of a previous study (12), as our observed RI value of 0.63 was lower than that reported in earlier research for predicting split renal function. This discrepancy may be due to the compensatory mechanisms in the two kidneys, which could mask actual injury. For instance, if both kidneys are injured but each functions at 50%, the renal RI might be assessed as 0.7. Since each kidney has 50% function, the combined total renal function can exceed that of a single kidney, which may result in total function reaching 80% or higher (with the eGFR exceeding 60 mL/min per 1.73 m²), while the RI remains at 0.7 or less than 0.7 for both kidneys.

Further, previous studies support our findings, indicating that the RI for patients with two kidneys in the normal range of renal function is below 0.7 (20-26). Notably, several studies report that the RI in patients with normal renal function is often below 0.65 or even below 0.60 (24-26), which aligns with our results. However, in these studies, most patients with renal function below 0.7 were those with two kidneys.

The present study also showed that kidney length was significantly associated with split renal function in patients with solitary kidneys. Kidney length is an essential clinical parameter, as it provides insights into the underlying disease processes and the potential reversibility of kidney damage. The average kidney length is approximately 10 cm (27-31). Our findings align with previous research (27-31), which indicates that kidney length can be used to predict renal function in individuals with either two kidneys or a solitary kidney.

In a previous study (31) of kidney donors with a remaining solitary kidney, the kidney length associated with normal renal function was 10.2 cm. This finding is consistent with our study, which found that the kidney length indicating normal split renal function was 10.6 cm. Additionally, the ROC curve analysis revealed that a kidney length of 10.6 cm exhibited high sensitivity and specificity for detecting RDF in patients with solitary kidneys. Notably, the combination of the RI and kidney length provided the highest accuracy (82.46%) for predicting RDF in these patients. To our knowledge, these associations and findings have not been reported previously.

The study results have significant clinical implications. First, the early detection of split renal function injury is more crucial for patients with a solitary kidney than for those with two kidneys. When injury occurs, patients with only one kidney lack the compensatory function of a second kidney, leading to a significant compromise in their renal function. This situation not only endangers their kidney health but also poses a serious risk to their overall life. Second, an RI of 0.63 was used in the study; an RI of 0.7 was not considered, despite previous studies indicating its reliability for predicting split renal function in patients with solitary kidneys. This approach may aid in the early detection of split renal function injury in such patients. Third, US is a noninvasive, non-radiative, safe, and cost-effective imaging technique that is easy to use in routine kidney examination to predict renal function injury. The findings of the present study can be applied to bedside settings, intensive care units, and emergency situations to predict split renal function injury in both solitary kidney patients and those with two kidneys.

The present study had several limitations. First, as a retrospective analysis, there may be selection bias in the patients chosen for the study. Second, the small size of the study cohort means that the results may not be representative of all patients with solitary kidneys. Therefore, a more extensive cohort study is needed to confirm our findings. Third, since all the patients involved had chronic RDF, caution should be exercised when applying the current results to predictions of acute RDF. Fourth, we did not perform renal biopsies or pathological examinations, as conducting a solitary renal biopsy may pose significant risks to patients. Fifth, as a previous study reported, the age at which an individual undergoes unilateral total nephrectomy can affect compensatory renal growth in animal models (32). Those with a congenital solitary kidney, those who undergo pre-pubertal nephrectomy, and those who undergo post-pubertal nephrectomy may have different compensatory renal growth. However, in our study, due to the limited number of cases, we divided the patients into three groups to compare their basic demographic data, including age and gender. In a subsequent study with a larger study population, we intend to categorize patients with solitary kidneys into three groups for further analysis and include additional data. Sixth, the size and ethnicity of patients can significantly influence kidney length, necessitating adjustments to measurements based on factors such as height, weight, body surface area, and race (33,34). In our study, all patients were of Asian descent, which minimized the impact of ethnicity on kidney length measurements. However, since this study was retrospective, specific data, including patient height, weight, and body surface area, were not available for analysis. Consequently, we were unable to adjust the kidney length measurements or findings based on these factors. Therefore, caution should be exercised when using kidney length to predict split renal function. Seventh, we defined split renal function as the absolute split GFR, which was calculated as the total GFR multiplied by the split renal function percentage. In nuclear medicine, split renal function is typically expressed as the relative percentage (i.e., the split renal function percentage, such as left kidney =45%, right kidney =55%); however, it can also be expressed as the absolute value (i.e., the split renal GFR, such as left kidney GFR =50 mL/min, right kidney GFR =40 mL/min). In this context, using the absolute split renal GFR can also provide a clearer reflection of the actual status of split renal function. Moreover, a previous study (13) reported that using an absolute split GFR as a measure of split renal function is associated with Doppler US parameters and carries significant clinical implications, supporting the definition of split renal function used in our research. Nonetheless, it is essential to be aware of the differing definitions of split renal function in US versus nuclear medicine when comparing results from these two imaging modalities. Additionally, the renal RI is influenced by several factors: (I) renal vascular impedance, which results from the interplay between pulsatility and vascular compliance; (II) changes in intrarenal perfusion; and (III) the impairment of both renal microvascular and macrovascular systems (35-38). Conditions that decrease vascular compliance, increase pulse pressure, alter intrarenal perfusion or damage renal microvascular or macrovascular structures are associated with an elevated intrarenal RI. Examples of such conditions include advanced age, smoking, hypertension, atherosclerosis, and chronic kidney disease, all of which contribute to an increased intrarenal RI. It is crucial to consider these factors when analyzing the implications of intrarenal RI for predicting split renal function. Further studies are necessary to explore the use of intrarenal RI in assessing split renal function, especially in patients with solitary kidneys and specific diseases that compromise renal function.

Conclusions

An intrarenal RI of 0.63 or higher and a kidney length of 10.6 cm or shorter indicate potential split kidney dysfunction in patients with solitary kidneys. However, further multicenter studies, either retrospective or prospective with larger sample sizes, are needed to validate the effectiveness of intrarenal RI or kidney length in predicting split renal function in this population.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-932/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-932/dss

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-932/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This retrospective cross-sectional study was approved by the institutional review board of The First Affiliated Hospital of Wenzhou Medical University (No. KY2024-R027). It was conducted in compliance with the Helsinki Declaration and its subsequent amendments. Individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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