Rethinking CKD Evaluation: Should We Be Quantifying Basal or Stimulated GFR to Maximize Precision and Sensitivity?

Evaluation of serum galectin-3 concentrations in healthy cats and in cats with ureteral obstruction

OBJECTIVES

Serum galectin-3 (sGal-3) is a protein present in renal tubules and increases in experimental rodent models of acute kidney injury. The aim of this study was to compare sGal-3 concentrations in healthy cats and cats with ureteral obstruction (UO).

METHODS

This was a retrospective study. Banked serum was used for sGal-3 evaluations in 15 healthy control cats and 22 cats with UO. For the control cats, creatinine and symmetric dimethylarginine were within reference intervals and ultrasound showed minimal to no kidney changes. A feline-specific sGal-3 ELISA was used to determine sGal-3 concentrations. Samples were analyzed in duplicate, and results were included if the coefficient of variation between samples was <20%. Shapiro-Wilk testing was used to evaluate for normality and parametric statistics were performed. P <0.05 was considered significant.

RESULTS

Mean (±SD) sGal-3 was lower in healthy cats (274.3 ± 146.5 pg/ml) than in cats with UO (707.7 ± 223.3 pg/ml; P <0.0001). There was no difference in sGal-3 concentrations between cats with unilateral or bilateral UO (P = 0.24) and no correlation between sGal-3 and creatinine, body weight or age. With a cutoff of 500 pg/ml, sGal-3 had a sensitivity of 86% (95% confidence interval [CI] 0.67-0.95) and specificity of 100% (95% CI 0.79-1.00) for differentiating cats with UO from healthy cats.

CONCLUSIONS AND RELEVANCE

An early diagnosis of UO is critical, given its associated morbidity and mortality. Given the differences in sGal-3 concentrations in healthy cats and those with UO in this study, sGal-3 shows potential for an early UO diagnosis; however, further research is needed.

Hemodynamic Renal Reserve Response in Conscious Normotensive and Hypertensive Mice

INTRODUCTION

Renal function may be compromised following recovery from kidney insults. Renal functional reserve (RFR) is a measure of the difference between the kidney's maximum capacity and its baseline function, which helps identify any areas of the kidney with compromised function. Usually, RFR is evaluated using acute volume expansion (AVE), but this is typically done in anesthetized animals, which may not accurately represent the kidney's complete functional capacity. In this study, we have introduced a novel method that enables AVE to be conducted in conscious mice.

METHODS

We have implemented this innovative approach in two animal models representing either intact or impaired renal function, specifically utilizing a lower nephron hypertensive model. Mice were implanted with radio-transmitters for mean artery blood pressure (MAP) monitoring during the experiment. After recovery, half of the mice were induced hypertension by right kidney nephrectomy combined with the ligation of the upper branch of the left kidney. For the AVE, a volume equivalent to 5% of the mouse's body weight was administered via intravenous (IV) or intraperitoneal bolus injection. Subsequently, the mice were individually housed in cages covered with plastic wrap. Urine was collected every hour for a total of 3 h for the measurement of urine and sodium excretion.

RESULTS

The MAPs for all normotensive mice were consistent throughout the AVE, but it increased 5-16 mm Hg in the hypertensive mice upon AVE. Remarkably, conscious mice exhibited a significantly stronger response to IV-administered AVE when compared to anesthetized mice. This response was evident in the increase in urinary flow, which was approximately 170% and 145% higher in conscious normotensive and hypertensive mice, respectively, compared to their respective baselines. In contrast, anesthetized normotensive and hypertensive mice showed only around a 130% and 100% increase in urinary flow, respectively. Additionally, upon AVE, conscious normotensive mice excreted approximately 47% more sodium than conscious hypertensive mice. In contrast, anesthetized normotensive mice excreted only about 30% more sodium than their anesthetized hypertensive counterparts.

CONCLUSION

Performing a kidney stress test with a significant solution load in conscious mice seems to be a superior method for evaluating RFR compared to conducting the test under anesthesia. Assessing kidney clearance while the mice are conscious has the potential to enhance the precision of diagnosing and predicting both acute and chronic kidney diseases.

Impaired hemodynamic renal reserve response following recovery from established acute kidney injury and improvement by hydrodynamic isotonic fluid delivery

Renal reserve capacity may be compromised following recovery from acute kidney injury (AKI) and could be used to identify impaired renal function in the face of restored glomerular filtration rate (GFR) or plasma creatinine. To investigate the loss of hemodynamic renal reserve responses following recovery in a model of AKI, rats were subjected to left unilateral renal ischemia-reperfusion (I/R) injury and contralateral nephrectomy and allowed to recover for 5 wk. Some rats were treated 24 h post-I/R by hydrodynamic isotonic fluid delivery (AKI-HIFD) of saline through the renal vein, previously shown to improve recovery and inflammation relative to control rats that received saline through the vena cava (AKI-VC). At 5 wk after surgery, plasma creatinine and GFR recovered to levels observed in uninephrectomized sham controls. Baseline renal blood flow (RBF) was not different between AKI or sham groups, but infusion of l-arginine (7.5 mg/kg/min) significantly increased RBF in sham controls, whereas the RBF response to l-arginine was significantly reduced in AKI-VC rats relative to sham rats (22.6 ± 2.2% vs. 13.8 ± 1.8%, P < 0.05). RBF responses were partially protected in AKI-HIFD rats relative to AKI-VC rats (17.0 ± 2.2%) and were not significantly different from sham rats. Capillary rarefaction observed in AKI-VC rats was significantly protected in AKI-HIFD rats. There was also a significant increase in T helper 17 cell infiltration and interstitial fibrosis in AKI-VC rats versus sham rats, which was not present in AKI-HIFD rats. These data suggest that recovery from AKI results in impaired hemodynamic reserve and that associated CKD progression may be mitigated by HIFD in the early post-AKI period.NEW & NOTEWORTHY Despite the apparent recovery of renal filtration function following acute kidney injury (AKI) in rats, the renal hemodynamic reserve response is significantly attenuated, suggesting that clinical evaluation of this parameter may provide information on the potential development of chronic kidney disease. Treatments such as hydrodynamic isotonic fluid delivery, or other treatments in the early post-AKI period, could minimize chronic inflammation or loss of microvessels with the potential to promote a more favorable outcome on long-term function.

Precision Medicine Approaches to Diabetic Kidney Disease: Personalized Interventions on the Horizon

Diabetic kidney disease (DKD) is a significant complication of diabetes that requires innovative interventions to address its increasing impact. Precision medicine is a rapidly emerging paradigm that shows excellent promise in tailoring therapeutic strategies to the unique profiles of individual patients. This abstract examines the potential of precision medicine in managing DKD. It explores the genetic and molecular foundations, identifies biomarkers for risk assessment, provides insights into pharmacogenomics, and discusses targeted therapies. Integrating omics data and data analytics provides a comprehensive landscape for making informed decisions. The abstract highlights the difficulties encountered during the clinical implementation process, the ethical factors to be considered, and the importance of involving patients. In addition, it showcases case studies that demonstrate the effectiveness of precision-based interventions. As the field progresses, the abstract anticipates a future characterized by the integration of artificial intelligence in diagnostics and treatment. It highlights the significant impact that precision medicine can have in revolutionizing the provision of care for DKD.

Low-Flow Acute Kidney Injury: The Pathophysiology of Prerenal Azotemia, Abdominal Compartment Syndrome, and Obstructive Uropathy

AKI is a syndrome, not a disease. It results from many different primary and/or secondary etiologies and is often multifactorial, especially in the hospitalized patient. This review discusses the pathophysiology of three etiologies that cause AKI, those being kidney hypoperfusion, abdominal compartment syndrome, and urinary tract obstruction. The pathophysiology of these three causes of AKI differs but is overlapping. They all lead to a low urine flow rate and low urine sodium initially. In all three cases, with early recognition and correction of the underlying process, the resulting functional AKI can be rapidly reversed. However, with continued duration and/or increased severity, cell injury occurs within the kidney, resulting in structural AKI and a longer and more severe disease state with increased morbidity and mortality. This is why early recognition and reversal are critical.

Real-time glomerular filtration rate: improving sensitivity, accuracy and prognostic value in acute kidney injury

PURPOSE OF REVIEW

Acute kidney injury (AKI) is common and associated with high patient mortality, and accelerated progression to chronic kidney disease. Our ability to diagnose and stratify patients with AKI is paramount for translational progress. Unfortunately, currently available methods have major pitfalls. Serum creatinine is an insensitive functional biomarker of AKI, slow to register the event and influenced by multiple variables. Cystatin C, a proposed alternative, requires long laboratory processing and also lacks specificity. Other techniques are either very cumbersome (inuline, iohexol) or involve administration of radioactive products, and are therefore, not applicable on a large scale.

RECENT FINDINGS

The development of two optical measurement techniques utilizing novel minimally invasive techniques to quantify kidney function, independent of serum or urinary measurements is advancing. Utilization of both one and two compartmental models, as well as continuous monitoring, are being developed.

SUMMARY

The clinical utility of rapid GFR measurements in AKI patients remains unknown as these disruptive technologies have not been tested in studies exploring clinical outcomes. However, these approaches have the potential to improve our understanding of AKI and clinical care. This overdue technology has the potential to individualize patient care and foster therapeutic success in AKI.

DNA damage response protects against progressive kidney disease

The pathophysiology of cellular injury and repair has been extensively studied in acute kidney injury (AKI) for more than 70 years. Although a great deal of knowledge has been generated, a debate over the importance of repairing damaged cells versus replacing them by proliferation remains. In this issue of the JCI, Kishi et al. demonstrate that following kidney epithelial cell injury, DNA repair, rather than cell proliferation, plays the central role in recovery and longevity by minimizing apoptosis, G2/M cell-cycle arrest, and subsequent fibrosis. This has important therapeutic implications and highlights the need for more sensitive techniques to evaluate functional, structural, and molecular recovery following injury.

Subclinical AKI: ready for primetime in clinical practice?

There has been considerable progress over the last decade in the standardization of the acute kidney injury (AKI) definition with the publication of the RIFLE, AKIN, KDIGO and ERBP classification criteria. However, these classification criteria still rely on imperfect parameters such as serum creatinine and urinary output. The use of timed urine collections, kinetic eGFR (estimated glomerular filtration rate), real time measurement of GFR and direct measures of tubular damage can theoretically aid in a more timely diagnosis of AKI and improve patients' outcome. There has been an extensive search for new biomarkers indicative of structural tubular damage but it remains controversial whether these new markers should be included in the current classification criteria. The use of these markers has also led to the creation of a new concept called subclinical AKI, a condition where there is an increase in biomarkers but without clinical AKI, defined as an increase in serum creatinine and/or a decrease in urinary output. In this review we provide a framework on how to critical appraise biomarker research and on how to position the concept of subclinical AKI. The evaluation of biomarker performance and the usefulness of the concept 'subclinical AKI' requires careful consideration of the context these biomarkers are used in (clinical versus research setting) and the goal we want to achieve (risk assessment versus prediction versus early diagnosis versus prognostication). It remains currently unknown whether an increase in biomarkers levels without functional repercussion is clinically relevant and whether including biomarkers in classification criteria will improve patients' outcome.

Haemodynamic or metabolic stimulation tests to reveal the renal functional response: requiem or revival?

Renal stimulation tests document the dynamic response of the glomerular filtration rate (GFR) after a single or a combination of stimuli, such as an intravenous infusion of dopamine or amino acids or an oral protein meal. The increment of the GFR above the unstimulated state has formerly been called the renal functional reserve (RFR). Although the concept of a renal reserve capacity has not withstood scientific scrutiny, the literature documenting renal stimulation merits renewed interest. An absent or a blunted response of the GFR after a stimulus indicates lost or diseased nephrons. This information is valuable in preventing, diagnosing and prognosticating acute kidney injury and pregnancy-related renal events as well as chronic kidney disease. However, before renal function testing is universally practiced, some shortcomings must be addressed. First, a common nomenclature should be decided upon. The expression of RFR should be replaced by renal functional response. Second, a simple protocol must be developed and propagated. Third, we suggest designing prospective studies linking a defective stimulatory response to emergence of renal injury biomarkers, to histological or morphological renal abnormalities and to adverse renal outcomes in different renal syndromes.

Augmented renal clearance

Adding to the complexity of caring for critically ill patients is the fact that many of them have a creatinine clearance that exceeds 130 mL/min/1.73 m². This phenomenon, termed augmented renal clearance (ARC), has only recently been widely recognized and its pathogenesis remains incompletely understood. However, ARC has been shown to result in increased dose requirements for drugs that are primarily eliminated by renal excretion, including many antimicrobial agents and enoxaparin. Recognition of ARC is hampered by the fact that the standard creatinine-based equations used to estimate renal function are not accurate in this clinical setting and the diagnosis is best established using both serum and urine creatinine measurements to calculate clearance. So a high index of clinical suspicion and awareness is usually required before this step is taken to confirm the diagnosis of ARC.

Real-time measurement of glomerular filtration rate

PURPOSE OF REVIEW

Measurement of glomerular filtration rate is an essential tool for determining the health or dysfunction of the kidney. The glomerular filtration rate is a dynamic function that can change almost instantaneously in response to stressors. Despite its central role in nephrology, there are no techniques available to the clinician for monitoring glomerular filtration rate in real time. Recent advances in technology to measure fluorescent compounds through the skin are providing a new approach for real-time monitoring of glomerular filtration rate. This review frames these technologies within how such measurements might be used in clinical medicine.

RECENT FINDINGS

Fluorescent molecules that act as ideal filtration markers are now available. Using transdermal sensors, the plasma disappearance rate of these exogenous markers can be measured rather than their steady state concentration. This eliminates the delay inherent in using an endogenous marker of filtration and permits continuous monitoring of GFR.

SUMMARY

These new technologies provide enhanced opportunities for diagnosis of kidney dysfunction and therapeutic monitoring. Accurate assessment of measured GFR will eliminate the erroneous diagnosis of chronic kidney disease (CKD) from many patients. Assessment of renal reserve will provide a new risk factor for progression of CKD. Real-time monitoring of GFR in critically ill patients will allow for earlier diagnosis of acute kidney injury and a dynamic metric to guide therapeutics. These are but a few of the many opportunities that this new technology will provide in both the clinical and research arenas.

A Novel Method for Rapid Bedside Measurement of GFR

Background Direct quantitative measurement of GFR (mGFR) remains a specialized task primarily performed in research settings. Multiple formulas for estimating GFR have been developed that use the readily available endogenous biomarkers creatinine and/or cystatin C. However, eGFR formulas have limitations, and an accurate mGFR is necessary in some clinical situations and for certain patient populations. We conducted a prospective, open-label study to evaluate a novel rapid technique for determining plasma volume and mGFR.Methods We developed a new exogenous biomarker, visible fluorescent injectate (VFI), consisting of a large 150-kD rhodamine derivative and small 5-kD fluorescein carboxymethylated dextrans. After a single intravenous injection of VFI, plasma volume and mGFR can be determined on the basis of the plasma pharmacokinetics of the rhodamine derivative and fluorescein carboxymethylated dextrans, respectively. In this study involving 32 adults with normal kidney function (n=16), CKD stage 3 (n=8), or CKD stage 4 (n=8), we compared VFI-based mGFR values with values obtained by measuring iohexol plasma disappearance. VFI-based mGFR required three 0.5-ml blood draws over 3 hours; iohexol-based mGFR required five samples taken over 6 hours. Eight healthy participants received repeat VFI injections at 24 hours.Results VFI-based mGFR values showed close linear correlation with the iohexol-based mGFR values in all participants. Injections were well tolerated, including when given on consecutive days. No serious adverse events were reported. VFI-based mGFR was highly reproducible.Conclusions The VFI-based approach allows for the rapid determination of mGFR at the bedside while maintaining patient safety and measurement accuracy and reproducibility.