Unmet needs and challenges in the development of noninvasive diagnostics for kidney disease

Advances in biological understanding and technologies continue to open new avenues in nephrology. Among the most compelling is the ongoing effort to diagnose kidney disease without relying on invasive procedures such as renal biopsy. As our understanding of renal pathophysiology deepens and our ability to interrogate urinary biomarkers improves, the prospect of noninvasive diagnostics is no longer aspirational, but increasingly feasible.

Although kidney biopsy is the gold standard for diagnosing kidney disease, it is an invasive procedure and can pose risks for patients who are taking antiplatelet agents or anticoagulants. Therefore, when kidney biopsy is relatively contraindicated, alternative approaches to diagnosing kidney disease are required. In addition, kidney biopsy incurs extra costs and can place a burden on patients with kidney disease [1].

There are numerous emerging methods for diagnosing and predicting kidney diseases, including kidney tumors and glomerulonephritis (Table 1 [2–6]). Although conventional noninvasive biomarkers such as estimated glomerular filtration rate (eGFR) and urinary albumin are widely used, they have limitations, especially for the early detection of kidney damage and for disease-specific diagnosis. The ultimate goal is to identify highly sensitive and specific approaches that will enable effective management of kidney disease.

Table 1.

Noninvasive diagnostic techniques for kidney diseases

Classification	Techniques	Advantages	Limitations
Imaging-based analysis [2]	Radiomics [3]	Provides structural and functional information	High cost and requires specialized equipment
	Photocounting imaging	Detects subtle changes in renal tissue
	Diffusion tensor imaging
Body fluid-based analysis [4]	Proteome, metabolome, nucleic acid analysis	Easily accessible samples	High interindividual variability
	Extracellular analysis	Reflects cellular origin	No standardization of isolation or quantification method
	Graphene-based lab-on-a-chip [5]	Multiplex testing possible
Breath analysis [6]	Mass spectrometry, gas chromatography, electrochemical sensors	Potential to detect metabolic derangements	Influenced by extrarenal factors

Extracellular vesicles (EVs), which are nanosized membranous particles released by nearly all cell types, play a pivotal role in a variety of kidney diseases [7]. Every nephron cell releases EVs, which are involved in cell-to-cell communication and can exacerbate kidney damage. As the molecular profile of EVs often reflects that of their origin cells, EVs are considered valuable not only for diagnostic but also for therapeutic purposes [7]. Molecules carried by EVs are relatively stable and can be detected in various body fluids because EVs protect and enhance the stability of their biomolecular cargo [8].

Nephrotic syndrome (NS) is characterized by heavy proteinuria and peripheral edema. NS encompasses a variety of glomerular diseases [9]. Membranous nephropathy (MN), a common cause of NS in adults, is defined by subepithelial immune complex deposition in glomeruli. Although an invasive renal biopsy often remains necessary for a definitive diagnosis, the discovery of autoantibodies against the phospholipase A2 receptor (PLA2R) has provided a valuable noninvasive diagnostic tool. Serum anti-PLA2R testing is positive in about 55% of primary (idiopathic) MN cases and correlates with disease activity; however, it may be negative in some patients and is not universally available.

In this context, there is great interest in developing urine EV-based noninvasive biomarkers that directly reflect kidney pathology. Yun et al. [10] conducted a pilot study in which they isolated exosomes from the urine of five biopsy-proven MN patients and four healthy controls. Although the sample size was very small, limiting statistical power and generalizability, the study yielded intriguing findings. The investigators quantified not only anti-PLA2R antibody but also PLA2R antigen in both whole urine and the EV fraction, normalizing the values to urine creatinine (UCr). Urinary EV PLA2R/UCr levels were significantly higher in MN patients than in controls. Notably, PLA2R was more readily detected in the EV fraction than in bulk urine. Importantly, the approach is entirely noninvasive, requiring only a urine sample. It could lessen reliance on kidney biopsy for the diagnosis of MN and allow for easier monitoring over time. The study also demonstrated feasibility in terms of sample handling: exosomes were successfully analyzed from as little as 0.5 mL of urine, far less than the volume required by earlier protocols. Another noteworthy finding is that elevated EV PLA2R was observed in both primary and secondary MN, implying that the urine exosome assay can detect PLA2R-mediated podocyte injury.

However, the authors did not find a significant correlation between urinary EV PLA2R levels and glomerular PLA2R staining intensity, serum creatinine, or eGFR. These results underscore the need for larger studies to clarify the clinical relevance of EV PLA2R in MN. In addition, PLA2R is only one of several antigens implicated in MN. In a subset of patients, MN is driven by other antigens (such as THSD7A, NELL-1, and others) and would not be identified by a PLA2R-based assay [9]. The authors acknowledge this antigenic diversity; therefore, focusing on PLA2R alone will inevitably miss such cases.

For nephrologists and clinical researchers, noninvasive diagnostic techniques remain a major unmet need in clinical practice. With the rapid advancement of technologies such as artificial intelligence, the dream of innovative, promising tools is coming within reach. However, our hope for developing noninvasive techniques should be examined and validated scientifically. A balanced approach, one that fosters ongoing investigation while exercising appropriate caution in adopting new biomarkers, will be essential. The journey from a compelling pilot study to a reliable clinical test will require careful navigation, but the potential payoff—a truly noninvasive diagnostic and monitoring method for MN—makes the journey well worth it.