Educational Case: Diabetic nephropathy

Primary objective

The following fictional case is intended as a learning tool within the Pathology Competencies for Medical Education (PCME), a set of national standards for teaching pathology. These are divided into three basic competencies: Disease Mechanisms and Processes, Organ System Pathology, and Diagnostic Medicine and Therapeutic Pathology. For additional information, and a full list of learning objectives for all three competencies, see https://www.academicpathologyjournal.org/pcme.¹

Objective UTK5.4: Diabetic Nephropathy. Describe the pathogenesis of diabetic nephropathy and the associated clinicopathologic features. Competency 2: Organ System Pathology; Topic: Kidney (UTK); Learning Goal 5: Glomerular Disorders.

Secondary objective

Objective CHEM1.5: Renal Disorders. Discuss the efficient use of laboratory tests to identify and manage patients with renal disorders, to define the pathogenesis of these disorders, and to help distinguish among diseases in the differential diagnosis. Competency 3: Diagnostic Medicine and Therapeutic Pathology; Topic: Chemistry (CHEM); Learning Goal 1: Pathogenesis, Diagnosis, and Treatment of Common Disorders.

Patient presentation

A 55-year-old man presents to his primary care provider with 3 months of fatigue, burning pain in his toes, and gradually worsening lower extremity edema. He has a 20-year history of poorly controlled type 2 diabetes mellitus (T2DM), hyperlipidemia, obesity, and hypertension. His medications include metformin, amlodipine, and atorvastatin. Semaglutide and empagliflozin were also prescribed one year ago but were never taken due to high copays at the pharmacy. He has no known drug allergies.

Obesity and type 2 diabetes (T2DM) are present in his mother, maternal uncle, and brother. His father died of a myocardial infarction at age 66. He has no history of smoking, alcohol, or recreational drug use.

He reports no recent illnesses, fevers, weight loss, dysuria, or polyuria. He sleeps lying flat and reports no dyspnea or orthopnea. At his last primary care appointment 9 months ago, his hemoglobin A1c (HbA1c) was 8.9%. He has had no eye exams in the last 5 years.

Diagnostic findings, Part 1

The patient is 178 cm tall and weighs 130 kg. His body mass index (BMI) is 41 (class 3 obesity) with blood pressure of 148/96 mmHg, heart rate of 82 bpm, respiratory rate of 14 bpm, and temperature of 37 °C.

Cardiopulmonary exam reveals normal S1 and S2, no murmurs, and clear breath sounds. His abdomen is soft, nontender, and nondistended. He has 2+ pitting edema of the lower extremities without rashes or erythema. Fundoscopic exam shows bilateral microaneurysms and retinal hemorrhages without neovascularization. Neurologic exam shows intact cranial nerves 2–12 and 5/5 strength of flexion and extension in the bilateral upper and lower extremities. Pinprick and temperature sensation are decreased along the plantar surfaces of both feet without skin ulceration. Sensation is otherwise intact along the dorsal feet and bilateral hands. Finger-to-nose and heel-to-shin testing is normal.

A urine sample appears dark yellow with a foamy surface. Urine dipstick shows 2+ protein, no blood, trace glucose, negative leukocyte esterase, and negative nitrites.

Questions/discussion points, Part 1

What causes of proteinuria are in the differential diagnosis for this patient?

This patient has multiple risk factors for proteinuria secondary to chronic renal disease. The most likely cause of this patient's proteinuria is diabetic nephropathy (DN). This patient has significant risk factors for DN, including longstanding T2DM with poor glycemic control (HbA1c >7%), hypertension, and hyperlipidemia. He also has evidence of peripheral neuropathy. DN is most closely associated with the presence of retinopathy, evidenced by the microaneurysms and retinal hemorrhages seen on this patient's fundoscopic exam. DN is characterized by hypertension and proteinuria without hematuria or dysuria.²

Other risk factors for proteinuria secondary to chronic kidney disease include hypertension and obesity. In a 2011 retrospective cohort study of 43,000 patients, each 10 mmHg of systolic blood pressure greater than 120 mmHg was associated with a 6% greater risk of developing chronic kidney disease.³ Class 3 obesity puts him at risk for obesity-related glomerulonephropathy (ORG), a secondary glomerular disease occurring in individuals with BMI above 30 kg/m². Obesity-related glomerulonephropathy is characterized histologically by low glomerular density and glomerulomegaly, which can be accompanied by focal segmental glomerulosclerosis.⁴

How is proteinuria evaluated and classified?

Used as a screen for proteinuria, urine dipstick is inexpensive and can be performed in the outpatient clinic. It has low sensitivity but high specificity for proteinuria.⁵ Proteinuria is reported as negative, trace, +1, +2, +3, or +4, each correlating to a protein concentration between 0 and 1000 mg/dL. Urine dipstick is a semiquantitative measure of urine protein, meaning that it provides an estimate of protein concentration within a particular range but does not provide an exact value. For example, +1 may correlate to a range of 30–100 mg/dL, while +4 represents a higher range, such as 500–1000 mg/dL. Because it is a measure of protein concentration, results are influenced by the time of void, recent exercise, and the patient's hydration status. It may be falsely positive for protein when vaginal secretions, semen, or certain medications are present. It may also be falsely negative for proteinuria when the patient is excreting nonalbumin proteins like immunoglobulins. Because it captures protein concentration at only one time point, urine dipstick does not correlate well with quantitative measures of urine protein. For example, a patient with +1 protein on UA may be excreting more protein in a 24-hour period than a patient with +4 protein.⁶

Quantifying urine protein is essential for diagnosing and monitoring kidney diseases, as it helps differentiate between nephrotic and nephritic syndromes. Nephrotic syndrome results from glomerular permeability to protein and is characterized by heavy proteinuria (greater than 3.5 g/day), hypoalbuminemia, edema, and hyperlipidemia.⁶ In contrast, glomerulonephritis is associated with hematuria, hypertension, and variable degrees of proteinuria, indicating inflammatory glomerular injury.⁷ Quantification of urine protein can be performed through 24-h urine collection. The patient is instructed to collect all urine over 24 hours and keep the collection refrigerated. This is the most accurate quantification of proteinuria because it is a measure of total protein excreted in 24 hours. Unlike a urine dipstick, it does not rely on the concentration of protein in the urine. Although it is the best measure of urine protein, it is cumbersome and error-prone. Patients frequently forget to collect all voids or store the specimen improperly.⁸

Because of the practical limitations of 24-h urine protein collection, a urine protein-to-creatinine ratio can be used to quantify 24-h urine protein output.⁶ Creatinine is a breakdown product of striated muscle and is excreted renally. It is produced at a near-constant rate based on muscle mass and is excreted in the urine at a rate that approximates glomerular filtration rate (GFR). By comparing the amount of protein in the urine to the amount of creatinine, the ratio normalizes protein excretion for variations in urine volume, which can be extrapolated to estimate 24 hour urine protein excretion. Multiple prospective studies have shown a high degree of correlation (Pearson correlation coefficients of 0.8–0.95) between 24-h urine protein collection and urine protein-to-creatinine ratio.^5,9

Because of the limitations of urine dipstick in detecting proteinuria below 10–20 mg/dL, annual screening for chronic kidney disease in patients with type 2 diabetes should be performed with serum creatinine-based estimated GFR (eGFR) and urine albumin-to-creatinine ratio (UACR) in a spot urine sample. Urine albumin-to-creatinine ratio is preferred to dipstick for its ability to detect small quantities of albuminuria and monitor kidney damage over time, as it adjusts for variations in urine concentration. It should be performed on a first- or second-morning void after the patient has avoided heavy exercise. Because urinary albumin excretion can fluctuate, this testing should be repeated. Two of three UACRs collected over a three month period should be elevated (30–300 mg albumin per 1 g creatinine) before diagnosing DN in the absence of decreased eGFR.^6,10

What additional diagnostic testing should be performed in this patient?

This patient's urine dipstick is positive for protein. Because this is a semiquantitative measure of proteinuria that can be falsely positive, his proteinuria and lack of hematuria should be confirmed by urinalysis. If positive, his urine protein excretion should be quantified. As discussed above, a 24-h urine collection or urine protein-to-creatinine ratio can be used to quantify proteinuria. Renal function should be evaluated by measuring serum blood urea nitrogen (BUN) and creatinine. Renal insufficiency can cause electrolyte disturbances, so blood electrolytes including Na, K, and Cl should be measured. Because hypoalbuminemia can cause edema, serum albumin concentration should be assessed.

In many adult patients with diabetes, target HbA1c is below 7%. However, target HbA1c should be individualized and consider parameters including life expectancy, disease duration, comorbidities, and risk of hypoglycemia.¹⁰ This patient's most recent HbA1c is 8.9%. When above 7%, HbA1c should be checked in this patient every 3 months to evaluate diabetic control.¹¹ This patient's last HbA1c was measured 9 months ago. In the presence of glucose, the N-terminus of hemoglobin A undergoes nonenzymatic glycosylation via a ketamine linkage, thereby forming a stable molecule of HbA1c.¹² The percentage of glycosylated hemoglobin is a marker of average blood sugar concentrations over the past 3 months, approximately the lifespan of an erythrocyte.¹³

There are no formal guidelines outlining the indications for renal biopsy in diagnosing DN. Many cases of DN can be diagnosed clinically in patients with risk factors, including poor glycemic control, a longstanding history of diabetes, and evidence of microvascular complications. In a 2013 retrospective study of kidney biopsies performed on 371 men with diabetes, DN was identified in one-third of cases, nondiabetic kidney disease in one-third, and a combination of DN and non-diabetic kidney disease in the final third. This sample was biased as biopsies were performed on inpatient, and many patients had new acute kidney injury or a positive serologic test for antineutrophil cytoplasmic antibodies (ANCA), antinuclear antibodies (ANA), double-stranded DNA (dsDNA), or cardiolipin antibody.¹⁴ Nevertheless, this shows that DN can coexist with other forms of renal disease, which should be considered when deciding if renal biopsy is appropriate. Renal biopsy is recommended in patients with hematuria, nephrotic syndrome, or a less than 5-year history of T2DM.¹⁵ The potential benefits of renal biopsy in diabetic patients include identifying a secondary or alternative renal disease and the measurement of the severity of DN.¹⁶ This patient ultimately progressed to end-stage renal disease, and a renal biopsy was performed during a pretransplant evaluation.

Diagnostic findings, Part 2

Table 1 shows this patient's laboratory results. Both BUN and creatinine are elevated, consistent with a glomerular filtration rate of 33 mL/min/1.73m². This is consistent with stage 3b chronic kidney disease, which indicates moderately decreased kidney function.¹⁷ His 24-h urine total protein excretion is 450 mg, which corresponds to moderate proteinuria.⁶ His blood albumin is decreased, likely due to his proteinuria. His blood electrolytes are within normal limits.

Table 1.

Follow-up testing results from this patient.

HbA1c	9.0% (H)
BUN	38 mg/dL (H)
Creatinine	2.1 mg/dL (H)
Sodium	136 mmol/L
Potassium	4.8 mmol/L
Chloride	99 mmol/L
Calcium	9.4 mg/dL
Albumin	2.9 g/dL (L)
24 hour urine protein	450mg (H)

BUN: blood urea nitrogen.

Representative images from a different patient with early DN are shown in Fig. 1. This patient's renal biopsy is shown in Fig. 2.

Fig. 1.

Renal biopsy from a patient with early diabetic nephropathy, showing two glomeruli with diffuse mesangial expansion (arrows) without nodular sclerosis. Periodic acid-Schiff (PAS) stain, 40x magnification. This image is reproduced with permission from its creator, Dr. Steven P. Salvatore at Weill Cornell Medicine.

Fig. 2.

A. Renal biopsy from the patient showing globally sclerosed glomeruli (arrows), Kimmelstiel–Wilson nodules (arrowhead). Hematoxylin and eosin stain, 10x magnification; B. Renal biopsy from the patient showing Kimmelstiel–Wilson nodules (arrow). Hematoxylin and eosin stain, 20x magnification.

Questions/discussion points, Part 2

Describe the renal biopsy findings in this patient. What is your histological diagnosis?

The early stages of DN are marked by histologic changes, some of which are seen in Fig. 1. Thickening of the glomerular basement membrane (GBM) results from increased production and reduced degradation of matrix components. Worn-out GBM material accumulates in the mesangium, leading to mesangial thickening and mesangial hypercellularity. Leakiness of glomerular capillaries allows plasma proteins to exude, forming fibrin caps between the glomerular endothelium and the GBM. These changes, in addition to excessive capillary growth, cause glomerulomegaly. Exudates of plasma form lesions (subcapsular drops) in Bowman's capsule, which spread around the glomerular circumference and into the tubular system, eventually causing tubular obstruction. This progresses to tubular atrophy, interstitial fibrosis, and mesangial infiltration of monocytes/macrophages.¹⁸ A distinguishing feature of DN is hyaline sclerosis affecting both afferent and efferent arterioles, while hypertensive glomerulopathy primarily involves the afferent arterioles.¹⁹

This patient's renal biopsy shows global sclerosis of over half of the glomeruli (Fig. 2A). The nonsclerotic glomeruli are enlarged and show thickened capillary walls with prominent mesangial expansion by an eosinophilic material that stains positive with both periodic acid-Schiff (PAS) and Jones methenamine silver stain. Mesangial Kimmelstiel-Wilson (KW) nodules are abundant (Fig. 2B). The arterioles show prominent hyaline arteriolosclerosis. These findings are consistent with late-stage DN.^18,20

Diabetic nephropathy is staged using the 2010 American Society of Nephrology guidelines. Four staging categories are defined. Class 1 is characterized by thickening of the GBM on electron microscopy. Class 2 is defined as the presence of mesangial expansion without KW nodules. Class 3 is defined as mesangial expansion with the presence of at least one KW nodule. Class 4 also includes KW nodules and mesangial expansion, but at least 50% of glomeruli demonstrate global sclerosis.²⁰ This patient's histological findings correspond to class 4 DN.

What is the pathophysiology and progression of diabetic nephropathy?

Diabetic nephropathy begins with structural kidney changes that can precede clinical markers like albuminuria or elevated creatinine. Hyperglycemia causes nonenzymatic glycosylation of type 4 collagen in the GBM and activates the polyol, hexosamine, and protein kinase C (PKC) pathways. The polyol pathway depletes cellular nicotinamide adenine dinucleotide phosphate (NADPH), disrupting nitric oxide bioavailability. The hexosamine pathway promotes transforming growth factor β1 (TGF-β1) production, leading to mesangial expansion through deposition of mesangial matrix. The cause of mesangial hypercellularity is hypothesized to be secondary to hyperglycemia and renovascular hemodynamic changes, although this is not well understood. Protein kinase C activation results from increased diacylglycerol (DAG) production, which alters renovascular hemodynamics.²¹

Hyperfiltration arises early in DN from increased glomerular capillary pressure due to decreased afferent and increased efferent arteriolar tone. One proposed mechanism involves circulating mediators, such as angiotensin II, thromboxane A2, and endothelin-1, which decrease nitric oxide bioavailability. Alternatively, tubular mechanisms may drive hyperfiltration by enhancing proximal sodium and glucose reabsorption, reducing distal sodium delivery, and triggering tubuloglomerular feedback.²¹ Concurrent activation of vascular endothelial growth factor (VEGF) promotes vascular proliferation and hyaline arteriosclerosis, compounding renal injury. Increased capillary leakiness and proliferation result in plasma exudation, tubular obstruction, and interstitial fibrosis, culminating in nephron dropout and eventual global glomerulosclerosis.^21,22

Although albuminuria had traditionally been considered an early marker of DN, it is now recognized that structural lesions and even impaired GFR can occur without elevated urine albumin levels. Historically, it was believed that hyperfiltration and albuminuria were the initial presenting signs of DN, which gradually worsened with a corresponding decrease in GFR, leading to end-stage renal disease. It is now known that the degree of albuminuria may change within a single patient and may even regress. The progression of DN varies significantly between patients, and not all patients progress to end stage disease.²³

How can the risk of developing diabetic nephropathy be decreased in patients with type 2 diabetes?

Patients with T2DM and HbA1c below 7% have reduced microvascular complications of T2DM, including DN and retinopathy,²⁴ especially with blood pressure below 140/90 mmHg. Patients with T2DM taking angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers have a slower rate of developing DN.^25,26 Some trials have shown decreased rates of DN in patients treated with GLP-1 agonists, although this finding has not been shown across all medications evaluated.^27,28 This discrepancy is likely due to the length of time of the study, with longer trials showing more renoprotective effects. The largest change appears to be a reduction in albuminuria. Further clinical trials are needed to establish the role and pathophysiology of GLP-1 agonists in slowing progression to DN. Managing patients’ diabetic risk factors and concurrent renal risk factors like hypertension and obesity is the most effective way to prevent DN.

Teaching points

• •Renal biopsy is rarely required to diagnose DN. It should be performed when an alternative diagnosis is suspected, such as in patients with hematuria, nephrotic syndrome, or a less than 5-year history of type 2 diabetes.
• •Diabetic nephropathy arises from hyperglycemia-driven pathways, including the polyol, hexosamine, and PKC pathways. These contribute to mesangial expansion, disrupted nitric oxide bioavailability, and renovascular hemodynamic changes.
• •Early findings in DN include GBM thickening, mesangial expansion, fibrin caps, glomerulomegaly, insudative lesions in Bowman's capsule, tubular obstruction, and interstitial fibrosis with mononuclear cell infiltration.
• •Late findings include KW nodules, global glomerulosclerosis, and hyaline sclerosis of both afferent and efferent glomerular arterioles.
• •Urine dipstick is a poor screening test for proteinuria because it does not detect urine albumin below 10–20 mg/dL. It has high specificity but low sensitivity for albuminuria.
• •Patients with diabetes should undergo yearly screening for albuminuria with serum creatinine-based eGFR and spot urine albumin-to-creatinine ratio.
• •Quantification of proteinuria is commonly performed by the urine protein-to-creatinine ratio because 24-h urine collection is cumbersome and error prone.
• •Early detection of DN through screening allows for the initiation of angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, which can delay progression to end-stage renal disease.

Funding

The article processing fee for this article was funded by an Open Access Award given by the Society of ‘67, which supports the mission of the Association for Academic Pathology to produce the next generation of outstanding investigators and educational scholars in the field of pathology. This award helps to promote the publication of high-quality original scholarship in Academic Pathology by authors at an early stage of academic development.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.