Single biomarkers

Biomarkers reflecting glomerular injury and filtration barrier dysfunction

Podocyte injury is a key early event in DKD, and increased excretion of podocyte-specific proteins in the urine can directly reflect damage to the slit diaphragm. Among them, urinary Nephrin levels increase with disease progression, with a diagnostic sensitivity of 100% and a specificity of 88% (Veluri and Mannangatti, 2022); Podocin mRNA in urine sediment increases as early as in the initial stages, and its ability to predict a decline in eGFR is comparable to that of urinary albumin (Fukuda et al., 2020); the urinary excretion of Podocalyxin is already significantly increased during the stage when urinary albumin levels are normal (Zeng et al., 2023). These markers make it possible to identify podocyte injury in the preclinical stage.

Thickening of the glomerular basement membrane (GBM) is a core early pathological feature of DKD, arising from an imbalance in the metabolism of the basement membrane and extracellular matrix (ECM) components (Adeva-Andany and Carneiro-Freire, 2022). Type IV collagen is the main scaffold protein of the GBM, and its urinary excretion increases even when urinary albumin levels are still normal, being associated with subsequent declines in eGFR (Katavetin et al., 2010). Elevated urinary laminin levels may precede the appearance of microalbuminuria (Yurchenco and Kulczyk, 2024). Damage to the basement membrane also leads to abnormal filtration of serum proteins. Transferrin (TRF) is extremely sensitive to damage to the charge barrier, and its urinary excretion can rise earlier than microalbuminuria (Kamel et al., 2022). Urinary excretion of immunoglobulin G (IgG) indicates structural barrier damage and shows good diagnostic performance even at the microalbuminuria stage (Abdou et al., 2022). The urinary excretion of ceruloplasmin (Cp) is also a signal of early damage to the charge barrier, and may participate in disease progression by interfering with local metal metabolism (Jiayi et al., 2024). These biomarkers (Type IV collagen, laminin, etc., which directly reflect ECM remodeling; TRF, IgG, Cp, etc., which reflect loss of filtration function) change even before abnormalities in traditional urinary albumin indicators, providing important evidence for the early identification of DKD.

Glomerular endothelial cell injury and activation are the key initial events in DKD microvascular lesions. Among related biomarkers, the upregulation of vascular endothelial growth factor (VEGF-A) can disrupt endothelial junctions and directly drive proteinuria (Nussdorfer et al., 2024); von Willebrand factor (vWF) increases as early as the microalbuminuria stage, which may be disease-specific and can promote microthrombosis formation (Ono et al., 2019); elevated levels of soluble adhesion molecules (sICAM-1/sVCAM-1) reflect an inflammatory state of the endothelium (Rania et al., 2024). These biomarkers reveal the mechanisms of endothelial injury from different perspectives and have potential for early diagnosis.

Biomarkers reflecting tubular injury and dysfunction

Biomarkers indicating damage to renal tubular epithelial cells are crucial for the early diagnosis of DKD. Neutrophil gelatinase-associated lipocalin (NGAL) rises rapidly after kidney injury; its urinary levels increase with the progression of DKD, showing a positive correlation with UACR and a negative correlation with eGFR (Swaminathan et al., 2025). Kidney injury molecule-1 (KIM-1) is specifically upregulated during tubular injury; its baseline levels can serve as an early warning for injury, with diagnostic specificity as high as 90% (Varatharajan et al., 2024). Liver-type fatty acid-binding protein (L-FABP) is associated with tubular oxidative stress. Its levels are elevated even at the normoalbuminuria stage and continue to rise as the disease progresses, correlating positively with UACR and negatively with eGFR (Ma et al., 2024).

β2-microglobulin (β2-MG) levels increase significantly in the early stages of DKD and escalate with worsening proteinuria, serving as a sensitive signal that appears before traditional markers show abnormalities (Yang et al., 2022). Retinol-binding protein 4 (RBP4) circulatory levels are markedly increased in T2DM patients with renal injury, show a positive correlation with UACR and negative correlation with eGFR, and demonstrate strong diagnostic efficacy (Cao et al., 2024). These biomarkers can reflect early tubular reabsorption dysfunction in DKD.

In DKD, biomarkers associated with pathological processes such as inflammation and oxidative stress are also of significant importance

Monocyte chemoattractant protein-1 (MCP-1) is a key factor in recruiting inflammatory cells; its urinary levels are markedly elevated in rapidly progressive DKD and serve as an efficient predictor of disease progression (Swaminathan et al., 2025). 8-Hydroxy-2′-deoxyguanosine (8-OHdG), a specific product of oxidative DNA damage, shows increased levels in both serum and urine of DKD patients, and is negatively correlated with renal function (Spoto et al., 2025). The soluble forms of tumor necrosis factor receptors 1 and 2 (TNFR1/TNFR2) are novel inflammatory markers; their serum levels rise before a significant decline in kidney function, with TNFR2 being significantly associated with decreased eGFR (Lousa et al., 2023).

Single-type biomarkers have multiple limitations in clinical application

Firstly, different studies often report contradictory findings and vary greatly in predictive efficacy. This is due both to the inherent biological characteristics of the biomarkers themselves (for example, NGAL and KIM-1 can be affected by a variety of systemic or non-specific renal injuries, leading to insufficient specificity), and to the lack of internationally standardized detection protocols, which impedes data comparability and clinical translation. Secondly, there is no clear consensus on the optimal source of biological samples. Finally, the performance of these biomarkers may vary depending on the type of DM and ethnic population, further increasing the complexity of widespread application. Table 1 systematically compares the specificity grades and evidence limitations of these biomarkers accordingly.

Biomarker Category | Specific Biomarker | Proposed Mechanism/Reflection | Evidence in DKD | Specificity to DKD | Key Limitations/Notes | Ref
Podocyte/GlomerularFiltration Barrier Injury | Nephrin | Podocyte slit diaphragm injury; urinary excretion increases with DKD progression | Sensitivity 100%, Specificity 88% in DKD cohorts | ○Limited | Also elevated in other glomerular diseases (e.g., minimal change disease, FSGS); not pathognomonic for DKD | Veluri and Mannangatti (2022)
 | Podocin mRNA | Decreased podocyte expression, leakage into urine after injury | Early elevation in DKD; predictive of eGFR decline comparable to albuminuria | ○Limited | Limited DKD cohort validation; may reflect general podocyte stress | Fukuda et al. (2020)
 | Podocalyxin | Podocyte glycocalyx disruption; early urinary excretion before albuminuria | Elevated in normoalbuminuric stage | ○Limited | Preliminary evidence; specificity for DKD unestablished | Zeng et al. (2023)
 | Type IV Collagen | GBM thickening; ECM remodeling | Increased urinary excretion before albuminuria; correlates with future eGFR decline | ×Poor | GBM remodeling occurs in multiple chronic kidney diseases; not DKD-specific | (Adeva-Andany and Carneiro-Freire, 2022; Katavetin et al., 2010)
 | Laminin | Basement membrane and ECM component metabolism imbalance | Elevated before microalbuminuria onset | ×Poor | ECM changes are common to progressive kidney diseases regardless of etiology | Yurchenco and Kulczyk (2024)
 | TRF | Charge barrier dysfunction; highly sensitive to anionic barrier loss | Urinary excretion rises before microalbuminuria | ×Poor | Charge barrier injury occurs in any glomerular disease (e.g., membranous nephropathy) | Kamel et al. (2022)
 | IgG | Structural barrier compromise; size-selectivity loss | Good diagnostic performance in microalbuminuria stage | ×Poor | IgGuria present in various proteinuric kidney diseases; not DKD-specific | Abdou et al. (2022)
 | Cp | Charge barrier injury; local metal metabolism disturbance | Early signal of charge barrier damage | ×Poor | Limited DKD evidence; may reflect systemic copper metabolism rather than kidney-specific injury | Jiayi et al. (2024)
Glomerular Endothelial Injury | VEGF-A | Endothelial junction disruption; drives proteinuria via permeability increase | Upregulated in DKD; directly promotes proteinuria | ×Poor | VEGF-A dysregulation occurs in multiple vascular and renal pathologies (e.g., preeclampsia, cancer); not DKD-specific | Nussdorfer et al. (2024)
 | vWF | Endothelial activation; microthrombosis promotion | Elevated in microalbuminuria stage; possible disease specificity suggested | △Moderate | May have DKD specificity (as noted in original text), but requires validation in larger cohorts and comparison with non-DKD diabetic patients | Ono et al. (2019)
 | sICAM-1/sVCAM-1 | Endothelial inflammatory activation; leukocyte adhesion | Elevated levels reflect endothelial inflammation | ×Poor | Systemic inflammation markers; elevated in atherosclerosis, infection, autoimmune diseases | Rania et al. (2024)
Tubular Injury & Dysfunction | NGAL | Proximal tubular epithelial injury; rapid response to kidney damage | Increases with DKD progression; correlates with UACR and eGFR | ×Poor | Classic AKI biomarker; elevated in any acute or chronic tubular injury (e.g., ischemia, toxins, obstruction) | Swaminathan et al. (2025)
 | KIM-1 | Tubular epithelial injury; specific upregulation in damaged tubules | Baseline levels predict early injury; specificity ∼90% for tubular damage | ○Limited | Specific for tubular injury but NOT specific for DKD etiology; elevated in any cause of tubular damage | Varatharajan et al. (2024)
 | L-FABP | Tubular oxidative stress; fatty acid metabolism in proximal tubule | Elevated in normoalbuminuria stage; progresses with disease | ○Limited | Oxidative stress marker; may be elevated in other conditions with tubular hypoxia/oxidative stress | Ma et al. (2024)
 | β2-MG | Tubular reabsorption dysfunction | Early elevation in DKD; increases with proteinuria severity | ×Poor | Non-specific marker of decreased GFR and tubular dysfunction; elevated in any CKD stage | Yang et al. (2022)
 | RBP4 | Tubular reabsorption impairment; retinol transport dysfunction | Circulating levels elevated in T2DM with kidney injury; correlates with UACR and eGFR | Limited | May be influenced by systemic metabolic state (obesity, insulin resistance); limited DKD-specific validation | Cao et al. (2024)
Inflammation &Oxidative Stress | MCP-1 | Monocyte chemotaxis; inflammatory cell recruitment to kidney | Significantly elevated in rapid-progression DKD; efficient progression predictor | ×Poor | Inflammatory chemokine elevated in any chronic kidney disease and systemic inflammatory conditions | Swaminathan et al. (2025)
 | 8-OHdG | Oxidative DNA damage; marker of oxidative stress | Elevated in serum and urine; correlates negatively with renal function | ×Poor | Systemic oxidative stress marker; elevated in diabetes, aging, cancer, and any condition with increased ROS | Spoto et al. (2025)
 | TNFR1/TNFR2 | Soluble forms of TNF receptors; inflammatory signaling | Elevated before significant eGFR decline; TNFR2 correlates with eGFR decline | ○Limited | Emerging markers with promising data but limited large-scale DKD cohort validation; may reflect systemic inflammation | Lousa et al. (2023)