From Saliva to Diagnosis: A Scoping Review of Conventional and Biosensor-Based Methods for Salivary Biomarkers in Chronic Kidney Disease

1. Introduction

Chronic kidney disease (CKD) is a progressive and irreversible condition marked by structural or functional abnormalities of the kidney lasting for more than three months [1]. Affecting over 850 million people globally—approximately 8–16% of the world’s population—CKD is a major public health challenge with a silent clinical course in its early stages [2,3]. Diagnosis often occurs only when significant renal impairment has already developed, limiting the effectiveness of early interventions. The disease is staged based on estimated glomerular filtration rate (eGFR) and albumin-to-creatinine ratio (ACR), as recommended by the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines [1,3].

Diabetes mellitus, particularly type 2, and hypertension are the leading causes of CKD, although a considerable number of cases remain of unknown etiology [4]. Without timely diagnosis and management, CKD progresses toward end-stage kidney disease (ESKD), requiring dialysis or kidney transplantation and contributing to high morbidity, mortality, and healthcare costs. By 2030, the number of individuals requiring renal replacement therapy is projected to reach 5.4 million, and by 2040, CKD is expected to be the fifth leading cause of death worldwide [5].

Historically, serum creatinine and urea have been the primary markers for assessing renal function, with blood sampling considered the gold standard [6,7]. While these methods are clinically validated and widely used, they are invasive, time-consuming, and dependent on laboratory infrastructure [8]. This imposes logistical and economic burdens, particularly in low-resource settings, and can negatively impact patient compliance with regular monitoring [8,9].

In this context, saliva has emerged as a promising noninvasive alternative for assessing renal function [8,10,11]. Saliva offers several practical advantages: it is easy to collect, minimally invasive, painless, and does not require specialized personnel or equipment, making it especially attractive for point-of-care and home-based testing. Saliva is considered a filtrate of blood, reflecting systemic biochemical changes through transcellular and paracellular transport pathways [12]. Consequently, biomarkers commonly used to assess kidney function—such as creatinine, urea, cystatin C, and various electrolytes—have been detected in saliva with varying degrees of correlation to serum and urine levels [13,14,15].

Recent studies have reported encouraging evidence of the diagnostic potential of salivary biomarkers, with some demonstrating strong correlations with their serum counterparts, particularly for creatinine and urea [14,16]. However, there remains considerable variability in results across CKD stages and patient populations, particularly in early disease detection and post-dialysis monitoring [17]. This inconsistency presents a significant knowledge gap regarding the diagnostic validity, sensitivity, and specificity of saliva-based assessments. Without a clearer understanding of these parameters, the widespread clinical adoption of salivary diagnostics remains limited.

The present scoping review addresses this gap by evaluating both traditional and emerging approaches to CKD biomarker detection, with a special focus on salivary diagnostics. It explores the integration of conventional biochemical assessments with advancements in biosensing technologies—such as electrochemical sensors, immunoassays, and microfluidic platforms—that aim to overcome current limitations in sensitivity and analytical reliability. These biosensor-driven strategies offer potential for rapid, low-cost, and decentralized diagnostics, which could transform CKD screening and monitoring, particularly in underserved populations.

By examining the diagnostic equivalence and clinical utility of salivary biomarkers compared to conventional fluids, this review aims to provide a comprehensive overview of the current evidence and guide future research in the development of accessible, noninvasive diagnostic tools for chronic kidney disease.

2. Materials and Methods

This scoping review explored existing studies assessing the diagnostic performance, specifically the accuracy, sensitivity, and specificity, of salivary biomarkers in detecting CKD. It also aimed to identify existing limitations in terms of standardization, clinical implementation, and technological innovation. A comprehensive literature search was carried out across multiple databases, including PubMed/MEDLINE, Scopus, Web of Science, and the Cochrane Library, without applying any publication year filters, up to 1st of July 2025. The search strategy followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses for scoping reviews (PRISMA-ScR) guidelines [18]. To ensure completeness, supplementary searches were conducted in gray literature sources, specialized Google search tools, relevant institutional or scientific websites, and the reference and citation lists of the selected publications. The research question was formulated using the PICOS framework (Population, Intervention, Comparator, Outcome, Study Design), as outlined in Table 1. The search utilized a combination of Medical Subject Headings (MeSH), keywords, synonyms, and free-text terms, combined with the Boolean operators “AND” and “OR”. The complete search strategy and the PRISMA-ScR checklist are provided in the Supplementary Materials.

The review was guided by two central questions: (1) How do salivary biomarkers (e.g., urea, creatinine, ammonia, pH) compare to traditional blood and urine-based diagnostics in terms of accuracy, sensitivity, and specificity? (2) How can digital salivary diagnostic tools be incorporated into dental clinical workflows for early CKD screening and monitoring of systemic health?

The specific objectives of this review were:

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To evaluate the diagnostic accuracy, sensitivity, and specificity of salivary biomarkers in CKD.

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To compare salivary biomarkers with traditional blood and urine markers for CKD diagnosis and monitoring.

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To assess which salivary biomarkers demonstrate the highest diagnostic performance and are most suitable for guiding dietary management, continuous monitoring, and referral for medical intervention or dialysis.

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To explore how oral health factors and dental clinical workflows influence the reliability and integration of salivary diagnostics for CKD detection.

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To assess the technologies and methodologies used to detect CKD-related biomarkers in saliva, including biosensors, spectrophotometry, and microfluidic devices.

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To identify limitations and challenges in the clinical application of salivary diagnostics for CKD.

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To propose future directions and standardization strategies for the implementation of saliva-based diagnostics.

Studies were eligible for inclusion if they involved adult participants (≥18 years) with a confirmed diagnosis of CKD or at risk for CKD (e.g., due to diabetes or hypertension) and reported on salivary biomarkers relevant to kidney function. Eligible studies evaluated diagnostic accuracy metrics (e.g., sensitivity, specificity, Area Under the Curve -AUC) or reported correlations between salivary and serum or urine biomarker levels. Only studies comparing salivary findings with traditional blood or urine markers were considered. Additional inclusion criteria required a minimum of 20 CKD patients and the inclusion of a healthy control group. Furthermore, studies were required to use a validated reference standard for CKD diagnosis, such as glomerular filtration rate (GFR), serum creatinine (sCr), or blood urea nitrogen (BUN).

Excluded were pediatric studies, preclinical (animal or in vitro) investigations, and studies involving acute kidney injury or conditions known to severely alter salivary composition (e.g., Sjögren’s syndrome, salivary gland irradiation). To avoid redundancy with prior systematic reviews, studies focusing solely on changes in salivary biomarkers before and after hemodialysis in ESKD patients with or without a comparator healthy group were also excluded. This decision was made in light of the systematic review with meta-analysis by Rodrigues et al., which thoroughly evaluated this specific clinical context [7].

Eligible study designs included observational studies (cross-sectional, cohort, case-control), diagnostic accuracy studies, and clinical validation studies. Reviews, editorials, case reports, and non-comparative studies were excluded.

After removing duplicates, title and abstract screening was conducted in a blinded manner using Catchii.org by two independent reviewers (E.V.V. and E.D.B.), followed by full-text assessment of the eligible articles. Discrepancies were resolved through discussion or by a third reviewer (C.M.C.). Data were extracted using a structured Excel form capturing study characteristics, population details, salivary biomarkers assessed, comparator tests, analytical methods, diagnostic outcomes, and technological readiness.

3. Results

The initial search across the four databases yielded a total of 753 publications. After screening, 219 duplicates were removed, and 516 studies were independently assessed by the two reviewers based on title and abstract. A total of 449 studies were excluded during this stage. The full texts of the remaining 67 studies were retrieved and evaluated against the inclusion and exclusion criteria.

Out of these, 50 studies were excluded for the following reasons:

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2 were poster abstracts presented at the 49th Turkish Physiology Congress of the Turkish Society of Physiological Sciences in 2024 [19],

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15 were in vitro studies [16,20,21,22,23,24,25,26,27,28,29,30,31,32,33],

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14 were review articles [7,8,34,35,36,37,38,39,40,41,42,43,44,45], and

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19 for not meeting the inclusion/exclusion criteria (e.g., sample size fewer than 20 participants [46,47], absence of specific CKD patient cohorts [48], lack of a healthy comparator group [12,13,14,17,49,50,51,52,53,54,55,56,57], lack of validated kidney function assessment methods [58,59], or inclusion of pediatric populations [60]).

Meanwhile, an additional search in other databases yielded 17 more records. After eligibility assessment, 12 of these were included, resulting in a total of 29 studies [2,3,5,9,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85] included in the scoping review.

The PRISMA flow diagram illustrating the study selection process is presented in Figure 1.

Table 2 summarizes the key data extracted from the 29 included studies. The table presents information on study design, population characteristics, biomarker(s) investigated, methods of saliva collection and analysis, and main findings.

The 29 studies included in this review were conducted across 16 countries, with a strong representation from Asia, notably India with 8 studies, China, Japan, Thailand, Vietnam, as well as contributions from Europe, the Middle East, South America, and Africa, reflecting the global interest in salivary biomarkers for chronic kidney disease detection. Figure 2 illustrates the global distribution of the studies included in this review.

Regarding the biomarkers tested, creatinine and urea remain the most validated salivary markers, showing strong correlations with serum levels and CKD stage. The diagnostic performance of salivary creatinine and urea across studies is summarized in Table 3.

Creatinine and urea symbols retrieved from Wikimedia Commons (CC0 license).

In addition to conventional well tested biomarkers, creatinine and urea, several studies explored novel salivary markers such as TMAO, cystatin variants, specific amino acids, and proteomic profiles. Table 4 summarizes these exploratory findings, which may contribute to improved non-invasive diagnostics pending further clinical validation.

4. Discussion

CKD represents a major global health burden, affecting millions of individuals and contributing to over 1.4 million deaths and more than 40 million disability-adjusted life years annually [1]. The increasing prevalence of CKD is largely driven by aging populations and the widespread incidence of hypertension and diabetes [87]. Current diagnostic practices rely heavily on serum and urine analyses, such as eGFR and albumin-to-creatinine ratios, which, while effective, often present logistical and economic challenges—particularly in resource-limited settings [88]. Despite advancements in clinical protocols, there remains a critical unmet need for non-invasive, accessible, and cost-effective diagnostic tools that can facilitate early detection and continuous monitoring of CKD [89]. Developing alternative approaches that overcome the limitations of traditional testing is essential for improving clinical outcomes and reducing the overall burden of this chronic condition [90].

Saliva is increasingly recognized as a valuable biological fluid for non-invasive diagnostics, offering a practical alternative to blood and urine testing, particularly in point-of-care settings [37,91]. Its collection is simple, safe, and well tolerated, making it suitable for populations where venipuncture may be challenging, such as pediatric, elderly, or chronically ill patients [1]. The diagnostic potential of saliva is rooted in its rich and dynamic composition, which mirrors many of the biomolecules present in systemic circulation [91].

Physiologically, saliva is a viscoelastic and hypotonic fluid secreted primarily by the parotid, submandibular, and sublingual glands. Its production is tightly regulated by the autonomic nervous system, with parasympathetic stimulation favoring serous secretion and sympathetic activity enhancing mucous output [92]. On average, adults produce between 500 and 1500 mL of saliva daily, and its flow rate and composition are influenced by circadian rhythms, gland type, and external stimuli [93]. Saliva transitions from an isotonic fluid at the acinar level to a hypotonic one in the ducts due to ionic modifications, particularly the reabsorption of sodium and chloride [94].

The biochemical complexity of saliva is remarkable, with over 2000 proteins and peptides identified, including enzymes (e.g., α-amylase), mucins, antimicrobial peptides, hormones, and immunoglobulins. It also contains electrolytes such as sodium, potassium, calcium, and bicarbonate, the latter contributing significantly to its buffering capacity and maintenance of oral pH [95,96]. Many salivary components—such as cortisol, creatinine, urea, and albumin—have diagnostic relevance, as their concentrations reflect systemic physiological and pathological states. In fact, around 27% of salivary proteins are shared with blood, supporting the feasibility of saliva-based diagnostics for systemic diseases [37].

Beyond composition, the mechanisms facilitating the entry of systemic biomarkers into saliva further enhance its diagnostic value. Molecules reach saliva through transcellular diffusion, active transport, or paracellular ultrafiltration via salivary acini and gingival crevices [97,98]. This enables the detection of a broad range of analytes, including low-molecular-weight substances like urea and creatinine, which are particularly relevant in renal disease monitoring. Additionally, hormones and other small lipophilic molecules diffuse readily into saliva, allowing for hormonal profiling and stress assessment [99].

As a historical overview, salivary biomarker research in CKD has evolved significantly over the past four decades. Pioneering studies in the 1980s laid the foundation, Akai et al. (1983) [80], for instance, demonstrated that salivary urea nitrogen levels closely reflected serum values using a urease-based dry reagent strip read by a reflectance spectrometer, achieving a high correlation coefficient (r ≈ 0.93).

By the mid-1990s, researchers like Lloyd et al. (1996) [65] extended this concept to salivary creatinine, showing that creatinine concentration in saliva (about 10–15% of the serum level in healthy individuals) rises dramatically in CKD patients and correlates strongly with impaired renal function. Lloyd’s clinical validation achieved nearly 100% sensitivity and ~96% specificity for detecting elevated serum creatinine using a saliva cutoff, solidifying saliva’s potential as a noninvasive diagnostic fluid. In subsequent years, key methodological shifts enhanced the reliability of salivary tests: collection techniques became more standardized (e.g. fasting morning samples or swab-based collection to ensure consistency), and analytical technologies advanced from simple colorimetric assays to sophisticated platforms.

Today, portable biosensors enable rapid, on-site measurement of salivary biomarkers with impressive sensitivity. Nanomaterial-based electrochemical sensors, for example, can detect creatinine at concentrations far lower than those measurable by traditional Jaffé assays, addressing earlier limitations in detection thresholds. Spectroscopic techniques such as attenuated total reflectance FTIR (ATR-FTIR) [3] spectroscopy have also been introduced, allowing for non-reagent-based quantification of salivary urea and detection of broader biochemical signatures associated with uremia. In parallel, there has been a notable shift from single-analyte approaches toward multi-marker panels and omics-based strategies. Recent studies have demonstrated the feasibility of simultaneously measuring urea, creatinine, cystatin C, and additional metabolites to improve diagnostic precision and staging accuracy in CKD.

This historical progression, from early validations of urea and creatinine to the integration of biosensing, spectroscopy, and multiplex assays, underscores the growing analytical robustness and clinical relevance of salivary diagnostics in nephrology. An overview of key milestones and validation data is illustrated in Figure 3.

In addition to these diagnostic performance metrics, a strong and consistent correlation between salivary and serum levels of urea and creatinine has been observed across multiple studies. Lasisi et al. [9], Bagalad et al. [84], and Pham [76] all reported high correlation coefficients, indicating that salivary levels closely track serum elevations. Notably, Khursheed et al. [63] found that an electrochemical biosensor provided superior salivary creatinine recovery compared to the conventional Jaffe method. However, despite these encouraging findings, other studies such as Picolo et al. [2] and Wang et al. [77] suggest that integrating salivary biomarkers with systemic parameters, like age or diabetes status, may enhance predictive accuracy, particularly in early-stage CKD.

Among the investigated biomarkers, creatinine and urea remain the most validated and widely studied, consistently demonstrating robust performance. In addition, emerging markers offer potential for more refined monitoring and staging. For example, TMAO (trimethylamine N-oxide), as reported by Korytowska-Przybylska et al. [64], may help distinguish CKD stages and guide dietary interventions. Similarly, cystatin C and specific amino acids (e.g., arginine, valine, histidine, as described by Wang et al. [77]) have shown promise as indicators of nephropathy, particularly in diabetic populations. Spectroscopy-based markers, such as FTIR spectral bands, have also demonstrated excellent accuracy for detecting advanced CKD, with studies by Tangwanichgapong et al. [3] and Lin et al. [5] reporting AUC values of up to 1.0. These findings collectively support the development of multi-marker salivary panels for real-time monitoring, dietary guidance, and early clinical intervention.

Importantly, the reliability of salivary diagnostics can be influenced by oral health status. Pham & Le [68] observed that CKD progression is associated with deteriorating oral conditions, including reduced salivary flow and a higher Decayed, Missing, and Filled Teeth (DMFT) index, which can affect biomarker concentration and stability. Salivary composition may be altered by xerostomia, uremic halitosis, or systemic acidosis, introducing variability. Additionally, differences in collection methods, such as passive drool versus swabbing, can impact reproducibility. Given their routine patient contact and procedural standardization, dental clinics may serve as optimal environments for implementing saliva-based CKD screening, particularly when oral health assessments are included.

From a technological standpoint, a wide array of analytical methods has been used to detect CKD-related biomarkers in saliva. Electrochemical sensors, like the DPV-based system employed by Khursheed et al. [63], offer ultra-sensitive detection for creatinine. Spectroscopic techniques such as ATR-FTIR, used by Lin et al. [5] and Tangwanichgapong et al. [3], provide high-resolution molecular fingerprinting. Mass spectrometry platforms (e.g., LC-MS/MS and UPLC-MS/MS) have enabled precise detection of protein and amino acid profiles, as demonstrated by Picolo et al.[2] and Wang et al. [77]. Colorimetric assays remain prevalent, especially in resource-limited settings, while biosensing probes and salivary conductivity devices, as seen in the studies by Lu et al. [66] and Lin et al. [5], offer point-of-care potential for indirect renal function assessment.

Despite these advancements, several challenges continue to impede the clinical adoption of salivary diagnostics for CKD. These include variability in collection protocols (e.g., stimulated vs. unstimulated saliva, time of day), interference from oral health conditions, and a lack of standardized cutoff values or reference ranges. Moreover, studies have not consistently accounted for population-specific biological variability, with limited representation across different racial and ethnic groups, such as Asian, African, or European populations, which may influence biomarker expression and diagnostic thresholds. The limited validation in early-stage CKD and pediatric populations, regulatory barriers for point-of-care devices, and the high cost of advanced technologies like mass spectrometry constrain broader clinical implementation.

To overcome these barriers, several key strategies should be pursued. These include developing standardized operating procedures (SOPs) for saliva collection, storage, and analysis; conducting large-scale validation studies across diverse populations (including pediatric, diabetic, and hypertensive groups); and integrating multi-analyte biosensors into wearable or chairside platforms with cloud-based data management. Additionally, incorporating oral health assessments into diagnostic workflows, establishing international calibration standards, and designing clinical trials to assess the impact of salivary monitoring on referral decisions and patient outcomes will be essential. Finally, cross-disciplinary training and collaboration among nephrologists, dentists, and primary care providers can facilitate the integration of saliva-based diagnostics into routine healthcare.

The limitations of the present scoping review include the exclusion of pediatric populations, restriction to studies published in English, and the requirement for a minimum of 20 participants and a control group, which may have led to the omission of smaller or non-comparative studies.

5. Conclusions

This scoping review highlights the growing body of evidence supporting the diagnostic utility of saliva in the detection and monitoring of CKD. Salivary biomarkers such as creatinine and urea consistently demonstrate strong correlations with serum levels and CKD staging, with several studies reporting high diagnostic accuracy (AUCs > 0.90) and sensitivities and specificities approaching those of traditional blood and urine tests.

The review also underscores the emergence of novel salivary biomarkers—such as trimethylamine N-oxide (TMAO), cystatin variants, and specific amino acids—which may enhance early-stage detection and disease stratification, particularly when integrated into multi-analyte panels.

Importantly, advances in biosensing technologies, including electrochemical sensors, ATR-FTIR spectroscopy, and portable point-of-care devices, offer promising solutions to current diagnostic limitations by enabling rapid, non-invasive, and decentralized monitoring. However, challenges such as variability in saliva composition, oral health influence, lack of standardized collection protocols, and limited validation in early CKD stages must be addressed before widespread clinical implementation.

To translate salivary diagnostics into routine clinical and dental workflows, future research should prioritize large-scale validation studies, development of standardized procedures, integration of oral health assessments, and regulatory approval of diagnostic platforms. Interdisciplinary collaboration between nephrology, dentistry, and biomedical engineering will be essential to advance saliva as a viable alternative to blood and urine in CKD care.