Diabetic kidney disease

Diabetic nephropathy, now more appropriately encompassed within the broader concept of diabetic kidney disease (DKD), is the chronic renal manifestation of diabetes mellitus and one of the leading causes of chronic kidney disease (CKD) and end-stage kidney disease worldwide. The classic term “diabetic nephropathy” mainly denoted the form structurally attributable to typical diabetic glomerular injury, whereas the modern concept of DKD includes the entire spectrum of kidney involvement associated with diabetes, including classic albuminuric phenotypes, progressive reduction in glomerular filtration without significant albuminuria, and forms combining glomerular, tubulointerstitial, and vascular lesions. In clinical practice, diabetic nephropathy is therefore not a single linear morphologic entity, but an anatomic and clinical continuum in which diabetes drives hemodynamic, metabolic, inflammatory, and fibrotic injury.

Its importance is enormous because the diabetic kidney is not only a microvascular target organ but also an amplifier of systemic risk. When albuminuria and reduced glomerular filtration develop, the likelihood of cardiovascular events, heart failure, hospitalization, pharmacologic vulnerability, anemia, mineral and bone abnormalities, and progression to dialysis or transplantation rises markedly. Diabetic nephropathy should therefore not be regarded as a simple late complication, but as a true prognostic watershed in the natural history of diabetes. Early identification is also fundamental because many of the most effective therapies work best in the early or intermediate stages, before injury becomes fixed and scarred.

From an epidemiologic standpoint, kidney involvement affects a very large proportion of the population with diabetes. The most recent summaries indicate that CKD is present in approximately 20–40% of people with diabetes, with variation related to age, disease duration, type of diabetes, hypertension, obesity, cardiovascular disease, and the sensitivity with which albuminuria and reduced glomerular filtration are sought. This means that a substantial proportion of people with diabetes already have demonstrable kidney injury even without symptoms. Increasing longevity among people with diabetes, the global rise in type 2 diabetes, and recognition of nonalbuminuric phenotypes make a further increase in the overall clinical burden of this disease likely in the coming years.

The established etiologic cause of diabetic nephropathy is diabetes mellitus, meaning prolonged exposure of the kidney to a biologic environment dominated by hyperglycemia, insulin abnormalities, intrarenal hemodynamic dysfunction, and chronic inflammatory activation. The presence of diabetes alone, however, does not explain the wide clinical variability observed among patients. The risk and rate of progression are modified by well-documented factors including diabetes duration, poor glycemic control, arterial hypertension, genetic predisposition, visceral obesity, dyslipidemia, sex, smoking, ethnicity, age, reduced baseline nephron mass, recurrent episodes of acute kidney injury, and other microvascular complications. The relationship with diabetic retinopathy is particularly useful clinically because coexistence of the two complications strengthens the hypothesis of systemic diabetic microvascular disease, although absence of retinopathy certainly does not exclude diabetic nephropathy, especially in type 2 diabetes.

The first major pathophysiologic stage is glomerular hyperfiltration, a phenomenon particularly relevant early in the disease. In the diabetic kidney, proximal tubular reabsorption of glucose and sodium increases, partly through sodium-glucose cotransporter 2 activity. Less sodium therefore reaches the macula densa, tubuloglomerular feedback is altered, and the afferent arteriole dilates. In parallel, activation of the renin-angiotensin-aldosterone system increases efferent arteriolar constriction. The combined result is increased intraglomerular pressure. Initially compensatory, this condition becomes pathogenic over time because it subjects the glomerular tuft to chronic mechanical stress that accelerates damage to the filtration barrier.

Hemodynamic stress is compounded by metabolic injury from chronic hyperglycemia. Nonenzymatic glycation of proteins and lipids leads to formation of advanced glycation end products, which accumulate in the glomerular basement membrane, mesangium, and vascular wall, altering the structural properties of the extracellular matrix and activating proinflammatory receptors. Reactive oxygen species increase in parallel, protein kinase C-mediated intracellular signaling is altered, profibrotic pathways such as transforming growth factor beta and connective tissue growth factor are activated, and production of mediators promoting mesangial expansion, matrix deposition, and stiffening of the renal microcirculation is amplified. The diabetic glomerulus is therefore simultaneously subjected to biochemical injury and abnormal hemodynamic force.

The glomerular filtration barrier, composed of fenestrated endothelium, glomerular basement membrane, and podocytes, is a central target of the process. The endothelium loses part of its protective glycocalyx and becomes more permeable; the basement membrane thickens and remodels; and podocytes, which are essential to the integrity of the filtration slit diaphragm, undergo metabolic stress, cytoskeletal rearrangement, detachment, and numerical loss. Podocyte loss is highly important because it promotes albuminuria and predisposes to progressive glomerulosclerosis. In patients with the classic phenotype, the clinical translation of this injury is persistent albuminuria, initially moderately increased and subsequently severely increased, although it is now clear that not every patient follows this linear trajectory.

In the mesangium, progressive expansion of the extracellular matrix reduces the capillary surface available for exchange and promotes development of the characteristic nodular lesions, the Kimmelstiel-Wilson nodules. On the vascular side, hyaline arteriolosclerosis of both afferent and efferent arterioles is common and is a histologic finding highly suggestive of chronic diabetic injury. Tubulointerstitial changes include tubular atrophy, chronic inflammation, interstitial fibrosis, and peritubular capillary rarefaction. This tubulointerstitial component is prognostically decisive because it often correlates closely with loss of kidney function and explains why diabetic nephropathy is not merely a glomerular disease.

Another essential aspect is the increasingly recognized importance of tubulointerstitial hypoxia. In diabetes, the kidney consumes more oxygen because of increased tubular reabsorptive work, while the microvasculature that should meet this demand is progressively damaged. The resulting imbalance between oxygen demand and availability accentuates mitochondrial dysfunction, oxidative stress, and fibrotic activation. This mechanism helps explain why injury may progress even when albuminuria is not marked and why a diabetic kidney disease phenotype exists with reduced glomerular filtration and absent or modest albuminuria. This nonalbuminuric phenotype is now widely recognized and has undermined the old model in which the diabetic kidney was assumed to progress obligatorily from microalbuminuria to proteinuria and then to reduced filtration.

Historically, the classic natural history was described by the Mogensen model, which proposed initial hyperfiltration, development of moderately increased albuminuria, subsequent persistent proteinuria, declining filtration, and finally advanced kidney failure. This model retains teaching value but is now considered incomplete. A substantial proportion of patients, especially those with type 2 diabetes, show reduced glomerular filtration without an evident albuminuric phase, probably because of a greater vascular and tubulointerstitial component or the effects of modern therapies that modify the clinical profile of disease. The pathophysiology of diabetic nephropathy should therefore be understood as a network of parallel processes rather than a single obligatory corridor.

As injury progresses, the final result is loss of functioning nephrons. The remaining nephrons attempt to compensate through secondary hyperfiltration, but this response once again promotes further glomerular injury. A vicious cycle of hyperfiltration, albuminuria, inflammation, and fibrosis develops, leading to progressive reduction in the estimated glomerular filtration rate (eGFR). At this point, the kidney loses not only its filtration capacity but also its ability to regulate extracellular volume, acid-base balance, potassium homeostasis, erythropoietin production, and mineral metabolism adequately, paving the way for the clinical manifestations and complications typical of advanced stages.

Diabetic nephropathy develops silently over years. In most patients, the early stages are completely asymptomatic, which is why the spontaneous history rarely leads to diagnosis by itself. Suspicion arises first from the context: long duration of diabetes, worsening metabolic control, hypertension, obesity, retinopathy, neuropathy, a family history of kidney disease, and progressive accumulation of cardiovascular risk factors. In other words, patients almost never report early “renal symptoms,” but they have a clinical profile that obliges the physician to actively seek kidney damage through targeted testing.

During a directed history, symptoms begin to emerge mainly as albuminuria increases or glomerular filtration becomes more markedly reduced. Some patients report foamy urine, an indirect sign of substantial proteinuria, or a modest increase in nocturia, especially when urinary concentrating ability declines. More often, however, the first perceived signs reflect the systemic consequences of impaired kidney function: greater fatigue, reduced exercise tolerance, mild exertional dyspnea in the presence of sodium and water retention or anemia, headache, and a sensation of elevated pressure in patients with poorly controlled hypertension. In intermediate or advanced stages, dependent ankle swelling, weight gain due to fluid retention, reduced appetite, nausea, a metallic taste, pruritus, cramps, and an overall decline in quality of life may occur.

The clinical sequence is not the same in every phenotype. In the classic albuminuric profile, the presentation may be dominated by progressively increasing proteinuria, edema, and hypertension. In the nonalbuminuric phenotype, by contrast, patients may have a slow decline in eGFR with few urinary findings, and disease is discovered only because creatinine rises or calculated eGFR falls on periodic testing. This difference is important because it shows that absence of overt albuminuria or edema certainly does not exclude clinically relevant kidney involvement.

Physical examination may be entirely normal in the early stages. As disease progresses, arterial hypertension, peripheral edema, weight gain from expansion of extracellular volume, and, at more advanced stages, signs of overload such as pulmonary crackles, jugular venous distention, or worsening concomitant heart failure become more common. Patients may appear pale if anemia due to reduced erythropoietin production has developed; in more advanced uremic stages, dry skin, excoriations from pruritus, marked fatigue, and general decline may occur.

During examination, the other manifestations of multisystem diabetic injury should be sought systematically. Diabetic retinopathy increases the likelihood that the kidney is affected by a similar microvascular process, while peripheral neuropathy, diabetic foot disease, and cardiovascular disease suggest a patient with a high overall burden of vascular injury. Conversely, some findings should prompt consideration of nondiabetic kidney disease or mixed disease: gross hematuria or a highly active urinary sediment, sudden nephrotic syndrome, rapid loss of filtration, absence of retinopathy in a patient with substantial proteinuria, short diabetes duration, or systemic signs suggestive of immunologic or hematologic disease.

In the final stages, the clinical picture increasingly overlaps with advanced CKD and uremia. Anorexia, persistent nausea, sleep disturbances, cramps, impaired concentration, pruritus, poorer pharmacologic glycemic control because of altered pharmacokinetics, and greater metabolic vulnerability develop. In some patients, reduced renal clearance of insulin or certain glucose-lowering drugs may increase the risk of hypoglycemia, creating an apparently paradoxical situation in which a patient with long-standing diabetes and advanced CKD becomes more unstable precisely because the kidney no longer participates appropriately in glucose and drug metabolism.

Diagnosis of diabetic nephropathy follows an orderly pathway beginning with systematic screening of the patient with diabetes and progressing to definition of the nature, stage, and likelihood that kidney damage is truly attributable to diabetes. The most recent guidelines indicate that renal assessment should be performed annually using a spot urinary albumin-to-creatinine ratio, preferably from a morning sample, and serum creatinine with estimated glomerular filtration rate. In type 2 diabetes, screening should begin at diagnosis, whereas in type 1 diabetes it should start after at least 5 years of disease, except in specific situations requiring earlier assessment.

According to KDIGO (Kidney Disease: Improving Global Outcomes) guidelines, diagnosis of chronic kidney disease in a patient with diabetes requires documentation of a renal abnormality persisting for at least 3 months. In practice, a single abnormal value is insufficient unless chronicity or persistence is demonstrated, especially for albuminuria, which is biologically variable and may be influenced by fever, physical exertion, urinary tract infection, heart failure, marked hyperglycemia, and transient hypertension.

According to KDIGO, chronic kidney disease in a patient with diabetes is confirmed when one of the following conditions has been present for at least 3 months

• reduced estimated glomerular filtration rate, eGFR <60 mL/min/1.73 m²
• persistent albuminuria with urinary albumin-to-creatinine ratio UACR ≥30 mg/g, confirmed in repeated samples
• other markers of kidney damage, such as urinary sediment abnormalities, tubular disorders, structural findings, or documented histologic abnormalities

To confirm persistent albuminuria, usual practice requires at least 2 of 3 samples collected over 3–6 months to be abnormal. Once CKD is confirmed, the patient should be classified using the CGA framework: cause, glomerular filtration category, and albuminuria category. Albuminuria categories are A1 for UACR below 30 mg/g, A2 for 30–299 mg/g, and A3 for 300 mg/g or higher. Filtration categories follow the G1–G5 grading system. This classification is not a formality because it integrates the risks of progression, cardiovascular events, hyperkalemia, need for nephrology referral, and preparation for kidney replacement therapy.

After initial assessment with UACR and eGFR, the diagnostic pathway continues with confirmatory and characterization tests. Complete urinalysis with sediment examination is highly useful for identifying hematuria, casts, or other findings suggesting a different or superimposed nephropathy. Measurement of electrolytes, bicarbonate, albumin, complete blood count, lipid profile, calcium, phosphorus, parathyroid hormone, and vitamin D helps define the systemic impact of kidney disease. In selected cases, cystatin C can improve the accuracy of filtration estimates, especially when creatinine is unreliable because of extreme muscle mass, sarcopenia, severe obesity, liver disease, or other conditions affecting its interpretation.

Renal ultrasonography does not “diagnose” diabetic nephropathy in the strict sense, but is important for excluding or identifying concomitant or alternative causes such as urinary obstruction, cystic kidney disease, marked size asymmetry, stones, or structural features inconsistent with the clinical history. In patients with diabetes and advanced CKD, the kidneys may be normal or enlarged early and become smaller and fibrotic in end-stage disease, but this finding is nonspecific and must always be interpreted in context.

The most delicate diagnostic issue is distinguishing “clinical” diabetic nephropathy from nondiabetic kidney disease or a mixed form. A clinical diagnosis of diabetic nephropathy is highly likely when long diabetes duration, persistent albuminuria, relatively gradual progression, hypertension, and diabetic retinopathy coexist. It becomes less certain when the presentation is atypical. In these circumstances, kidney biopsy, the histologic diagnostic gold standard, should be considered.

According to the best nephrology literature, biopsy should be considered particularly when atypical features are present, such as absence of retinopathy despite substantial proteinuria, short diabetes duration, glomerular hematuria, active urinary sediment, rapidly developing nephrotic syndrome, accelerated eGFR decline, unexplained rapid deterioration, or clinical signs of nondiabetic systemic disease. The reason is straightforward: a substantial proportion of biopsied patients with diabetes have isolated or superimposed nondiabetic lesions, some of which have specific treatments that radically alter prognosis and therapeutic strategy.

When performed, biopsy also permits formal histologic classification of diabetic injury. The Renal Pathology Society classification proposed by Tervaert distinguishes class I, with glomerular basement membrane thickening visible primarily by electron microscopy; class II, with mild or severe mesangial expansion; class III, with Kimmelstiel-Wilson-type nodular glomerulosclerosis; and class IV, with advanced diffuse glomerulosclerosis. These glomerular lesions are accompanied by scores for interstitial fibrosis, tubular atrophy, arteriolosclerosis, and hyalinosis, which have strong prognostic value. In practice, however, biopsy is not required in most patients because the clinical diagnosis is often sufficiently robust and baseline kidney-protective treatment would be indicated regardless.

The differential diagnosis includes hypertensive nephrosclerosis, primary or secondary glomerulonephritis, immunoglobulin A nephropathy, membranous nephropathy, amyloidosis, tubulointerstitial nephropathies, multiple myeloma, ischemic nephropathy, and atheroembolic disease, as well as mixed forms in which diabetes coexists with another kidney disorder. The final judgment should therefore always arise from synthesis of the history, physical examination, laboratory findings, urinary sediment, imaging, and temporal pattern of injury.

An often underestimated step is assessment of target organs and overall cardiorenal risk. Once diabetic nephropathy has been identified, cardiovascular risk should be quantified simultaneously, signs of heart failure sought, retinopathy and neuropathy defined, blood pressure measured accurately, and the entire treatment regimen reassessed. In diabetic kidney disease, diagnosis and prognostic stratification largely coincide with the ability to view the patient as a system rather than as a simple sum of creatinine and albuminuria.

Treatment of diabetic nephropathy is now one of the most transformed fields in internal medicine and nephrology because the historic centrality of metabolic and blood pressure control has been joined by genuine cardiorenal organ-protection therapy. The goal is not merely to slow the rise in creatinine, but to reduce the rate of nephron loss, limit albuminuria, prevent end-stage kidney disease, and lower cardiovascular risk, which in these patients is often as important as strictly renal risk.

The foundation of every strategy remains multifactorial intervention on the biologic substrate driving disease. This means pursuing individualized glycemic control, correcting hypertension, reducing sodium load, treating dyslipidemia, promoting weight loss when indicated, stopping smoking, encouraging physical activity compatible with the clinical condition, and limiting exposure to nephrotoxic drugs. Nutritionally, patients with CKD should not be encouraged to follow high-protein diets: protein intake of approximately 0.8 g/kg/day is generally considered appropriate in nondialysis disease, whereas excess protein may increase the intraglomerular load.

Blockade of the renin-angiotensin system with an angiotensin-converting enzyme inhibitor or angiotensin II receptor blocker remains a cornerstone, especially in patients with albuminuria and hypertension. These drugs reduce intraglomerular pressure, lower albuminuria, and slow progression. They should be titrated to the maximum tolerated dose, with monitoring of creatinine and potassium. A modest initial rise in creatinine may be an expected hemodynamic effect and does not automatically require discontinuation; by contrast, combining two renin-angiotensin system blockers is not recommended because it increases adverse events without a net benefit on hard outcomes.

In recent years, sodium-glucose cotransporter 2 inhibitors (SGLT2 inhibitors) have changed the prognosis of diabetic kidney disease. Their kidney-protective effect extends beyond glucose lowering and depends largely on correction of glomerular hyperfiltration, reduction of intraglomerular pressure, improvement of sodium balance, favorable effects on inflammation and energy metabolism, and associated cardiovascular protection. Modern guidelines recommend them for patients with type 2 diabetes and CKD even at relatively low eGFR values, provided the value is above the initiation threshold specified for the drug and by current recommendations, with continuation often possible until dialysis or transplantation begins. Clinicians must nevertheless understand the precautions: a possible initial fall in eGFR, genital fungal infections, risk of volume depletion in frail individuals, and, in selected settings, euglycemic ketoacidosis.

A second major advance is finerenone, a nonsteroidal mineralocorticoid receptor antagonist. This drug has been shown to reduce albuminuria, kidney disease progression, and cardiovascular risk in patients with type 2 diabetes and albuminuric CKD despite optimized renin-angiotensin system blockade. Its biologic rationale is strong: mineralocorticoid receptor activation promotes renal and cardiovascular inflammation and fibrosis, whereas selective inhibition attenuates these processes. Patient selection is fundamental because finerenone should be used only with an appropriate serum potassium level and close potassium monitoring owing to the risk of hyperkalemia.

Glucagon-like peptide-1 receptor agonists (GLP-1 receptor agonists) have an increasingly important place in patients with type 2 diabetes and diabetic nephropathy, especially when additional glycemic control, weight reduction, or cardiovascular protection is needed. For years, their renal benefit was observed mainly for albuminuria and secondary renal composite outcomes; more recently, dedicated data have strengthened the concept that some agents, particularly semaglutide, may also help protect against hard kidney outcomes. This makes them highly relevant in combination strategies, especially for patients with obesity, high cardiovascular risk, or a need for further reduction in albuminuria.

Control of blood pressure remains decisive. In the diabetic kidney, adequately lowering blood pressure reduces glomerular stress, proteinuria, and CKD progression. Many patients require combination therapy with a renin-angiotensin system blocker, a dihydropyridine calcium channel blocker, and a diuretic. Diuretic selection depends on kidney function and the pattern of sodium and water retention. In advanced disease, volume control is often as important as blood pressure control itself.

Treatment cannot stop at glomerular protection. CKD complications—anemia, metabolic acidosis, hyperkalemia, mineral and bone disorders, and volume overload—must be addressed actively. Correcting these conditions does not always directly slow fibrosis, but improves survival, quality of life, treatment tolerance, and the ability to maintain cornerstone kidney-protective therapies. Similarly, statin therapy has a structural role in reducing atherosclerotic risk in patients with diabetes and nondialysis CKD.

When disease progresses toward advanced kidney failure, preparation for kidney replacement therapy should begin early. This means discussing hemodialysis, peritoneal dialysis, and kidney transplantation with the patient while considering biologic age, comorbidities, family support, and preferences. In selected individuals with type 1 diabetes, combined kidney-pancreas transplantation may offer specific advantages. Early preparation is important because it reduces emergency starts, allows a more rational choice of therapy, and improves overall outcomes.

Prognosis depends on the intersection of several variables. The two most powerful in clinical practice are albuminuria and eGFR: the higher the albuminuria and the lower the filtration rate, the greater the risks of progression, cardiovascular events, and death. The slope of filtration decline over time is also extremely important. A patient with moderately reduced but rapidly falling eGFR may be at greater risk than a patient with a lower but stable eGFR. Prognosis is worsened by persistent proteinuria despite treatment, resistant hypertension, concomitant retinopathy, heart failure, obesity, smoking, poor adherence, recurrent acute kidney injury, and advanced histologic lesions when known. Conversely, early identification and prompt combination of multifactorial interventions with modern cardiorenal therapies can substantially alter the natural history of disease.

The most intuitive complication of diabetic nephropathy is progression to end-stage kidney disease. This occurs because glomerular, tubulointerstitial, and vascular injury progressively reduces the number of functioning nephrons, and the remaining nephrons undergo compensatory hyperfiltration, perpetuating the vicious cycle of injury. When nephron reserve becomes insufficient, the patient requires dialysis or transplantation. Focusing exclusively on this final outcome, however, risks overlooking the fact that many of the most common and lethal complications arise long before dialysis.

The most important in terms of mortality is cardiovascular disease. Diabetic nephropathy greatly increases the risk of atherosclerosis, myocardial infarction, stroke, cardiovascular death, and especially heart failure. The mechanism is multifactorial: sodium retention, hypertension, systemic inflammation, endothelial dysfunction, oxidative stress, anemia, neurohormonal overactivation, and accumulation of uremic toxins all worsen hemodynamic and vascular status. In many patients with diabetes, nephropathy is therefore not merely a pathway to dialysis but a powerful multiplier of cardiac risk.

Among the most common clinical complications are arterial hypertension and volume overload. These develop because the diseased kidney loses the ability to excrete sodium and water adequately and because the renin-angiotensin-aldosterone system remains activated. The consequences are peripheral edema, pulmonary congestion, worsening heart failure, and greater difficulty controlling blood pressure. In advanced disease, even a small excess of volume may have a substantial symptomatic impact.

Another very common complication is hyperkalemia, which results from reduced renal potassium excretion and is promoted by the necessary use of renin-angiotensin system blockers and finerenone. Risk increases in advanced stages and with metabolic acidosis, a potassium-rich diet, dehydration, or concomitant use of other drugs interfering with potassium homeostasis. Hyperkalemia is clinically important because it may force reduction of fundamental kidney-protective therapies, creating a difficult balance between renal protection and immediate safety.

Metabolic acidosis develops when the kidney can no longer regenerate bicarbonate and adequately excrete fixed acids. Patients may experience fatigue, subjective dyspnea, worsening protein catabolism, and more rapid loss of muscle mass. In the long term, acidosis also contributes to bone demineralization and worsening inflammatory and nutritional status.

Anemia of CKD is particularly common in diabetic nephropathy and may appear relatively early. It is caused mainly by reduced erythropoietin production by the diseased kidney, but chronic inflammation, functional or absolute iron deficiency, blood loss, and malnutrition also contribute. Clinically, it causes fatigue, dyspnea, reduced physical performance, cognitive impairment, and greater cardiovascular vulnerability, especially in patients who already have ischemic heart disease or heart failure.

The advanced diabetic kidney also develops the typical CKD abnormalities of mineral and bone metabolism, with phosphate retention, reduced vitamin D activation, secondary hyperparathyroidism, and abnormal bone remodeling. This creates a risk of skeletal fragility, vascular calcification, and further worsening of cardiovascular risk. Although these complications are common to many chronic nephropathies, in diabetic nephropathy they are superimposed on an already strongly proatherogenic and inflammatory substrate.

An often underestimated complication is increased susceptibility to acute kidney injury. Patients with diabetic nephropathy tolerate dehydration, sepsis, contrast media, nephrotoxic drugs, and hemodynamic decompensation poorly. Each episode of acute kidney injury may leave residual loss of function and accelerate progression toward end-stage disease. Prevention of acute episodes, education on sick-day management, and periodic medication review are therefore integral parts of care.

Finally, advanced CKD alters the metabolism of insulin and glucose-lowering drugs, increasing the risk of hypoglycemia and adverse drug events. This clinical paradox is important: the patient may appear “improved” because blood glucose falls, when in reality reduced kidney function is decreasing insulin or drug clearance. Without careful treatment adjustment, diabetic nephropathy may therefore also become a metabolic complication of diabetes treatment itself.