Markers of Kidney Injury / Acute Kidney Injury (AKI)

DNB examination answer

Definition

• Rise in SCr by ≥0.3 mg/dL within 48 hours, or
• SCr ≥1.5 times baseline within 7 days, or
• Urine output <0.5 mL/kg/hour for 6 hours.

Why are novel biomarkers required?

• Rises late, often 24-48 hours after injury
• Influenced by muscle mass, age, sex, diet, volume status, liver disease, and drugs
• Does not distinguish prerenal azotemia from intrinsic tubular injury
• May remain normal in early or subclinical AKI

Classification of markers

Important biomarkers

1. Serum creatinine

• Conventional marker used to diagnose and stage AKI.
• Reflects reduction in GFR.
• Inexpensive and universally available.
• Late and nonspecific, hence poor for early detection.

2. Urine output

• An early bedside functional marker.
• Oliguria may occur before SCr elevation.
• Confounded by volume status, diuretics, obstruction, and hemodynamic alterations.

3. Cystatin C

• Low-molecular-weight protein produced by all nucleated cells.
• Freely filtered at glomerulus and nearly completely reabsorbed/catabolized in proximal tubules.
• Serum cystatin C detects fall in GFR earlier than creatinine in some settings.
• Less affected by muscle mass; may be influenced by thyroid disease, inflammation, steroid therapy, and malignancy.
• Urinary cystatin C indicates proximal tubular injury due to failed tubular reabsorption.

4. Neutrophil gelatinase-associated lipocalin (NGAL)

• One of the most studied AKI biomarkers.
• Measured in plasma or urine.
• Released from injured distal nephron and collecting duct after ischemic or nephrotoxic injury.
• Rises early after renal insult, often before SCr.
• Useful in cardiac surgery-associated AKI, sepsis, contrast-associated AKI, transplantation, and pediatric AKI.

• Raised in sepsis, systemic inflammation, infection, chronic kidney disease, malignancy, and neutrophil activation.
• Thus, it should be interpreted with the clinical setting.

5. Kidney injury molecule-1 (KIM-1)

• Type-1 transmembrane glycoprotein expressed in proximal tubular epithelial cells after injury.
• Urinary KIM-1 is a marker of proximal tubular injury.
• Useful in ischemic and nephrotoxic AKI, especially drug-induced nephrotoxicity.
• Has utility in assessing chronic tubular injury as well.

6. Interleukin-18 (IL-18)

• Pro-inflammatory cytokine produced by injured proximal tubular cells.
• Urinary IL-18 increases in ischemic acute tubular necrosis.
• May aid in differentiating ischemic ATN from prerenal azotemia.
• Has prognostic value in critically ill patients.

7. Liver-type fatty acid-binding protein (L-FABP)

• Cytoplasmic protein in proximal tubular cells.
• Urinary L-FABP rises in renal ischemia, oxidative stress, and tubular injury.
• May predict AKI following cardiac surgery, sepsis, and contrast exposure.

8. N-acetyl-β-D-glucosaminidase (NAG)

• Lysosomal enzyme from proximal tubular cells.
• Large molecular size prevents glomerular filtration.
• Urinary elevation reflects proximal tubular cell injury.
• Can rise in diabetes, hypertension, urinary infection, and CKD, hence it is nonspecific.

9. TIMP-2 and IGFBP7

• Reported as urinary [TIMP-2] × [IGFBP7].
• Rise early in ischemia, sepsis, major surgery, drug/toxin exposure, and critical illness.
• Identify patients at risk of developing moderate to severe AKI before creatinine increases.
• Particularly useful in high-risk ICU patients, after major surgery, and in sepsis.

• Optimize hemodynamics and volume status
• Avoid nephrotoxins and contrast where possible
• Monitor creatinine and urine output closely
• Adjust drug doses
• Seek and treat the cause of AKI

Other emerging markers

• Klotho: Decreases early in AKI; protective anti-aging renal protein.
• Osteopontin: Tubular injury and inflammation marker.
• Clusterin: Marker of tubular cell stress/injury.
• CCL14: May predict persistent severe AKI and poor renal recovery.
• Proenkephalin A: Functional marker related to GFR and AKI severity.
• Urinary Dickkopf-3 (DKK3): Marker of tubular stress and fibrosis risk.
• Urinary cell-free DNA / microRNAs: Investigational.

Practical DNB points

Biomarker-positive, creatinine-negative AKI

Biomarkers help in:

1. Early detection of AKI
2. Risk stratification in ICU, sepsis, cardiac surgery, and transplantation
3. Distinguishing functional reduction in GFR from structural tubular injury
4. Predicting AKI severity, dialysis requirement, renal recovery, and mortality
5. Monitoring drug-induced nephrotoxicity

Limitations

• No single biomarker is sufficiently accurate in every clinical context.
• Results are affected by timing of sampling, AKI cause, CKD, sepsis, inflammation, and assay availability.
• Cost and limited availability restrict routine use in many Indian centers.
• Biomarkers supplement, but do not replace, clinical assessment, urine microscopy, serial SCr, and urine-output monitoring.

Conclusion

NGAL, KIM-1, IL-18, L-FABP, NAG, cystatin C, and [TIMP-2]×[IGFBP7].

Pulmonary-Renal Syndrome

Definition

1. Diffuse alveolar hemorrhage (DAH), and
2. Rapidly progressive glomerulonephritis (RPGN)

Etiology / Causes

A. Pauci-immune small-vessel vasculitis: most common

1. Granulomatosis with polyangiitis (GPA)
   • PR3-ANCA, usually c-ANCA
   • Upper airway disease: chronic sinusitis, nasal ulcers, otitis, subglottic stenosis
   • Lung nodules, cavitation, DAH
   • Crescentic GN
2. Microscopic polyangiitis (MPA)
   • MPO-ANCA, usually p-ANCA
   • DAH with pauci-immune crescentic GN
   • No granulomatous upper respiratory tract disease
3. Eosinophilic granulomatosis with polyangiitis (EGPA)
   • Asthma, eosinophilia, rhinosinusitis, neuropathy
   • Renal disease and DAH are less common than in GPA/MPA.
4. Drug-induced ANCA vasculitis
   • Propylthiouracil, hydralazine, minocycline, levamisole-adulterated cocaine
   • May cause DAH and crescentic GN.

B. Anti-GBM disease

• Autoantibodies are directed against the noncollagenous (NC1) domain of the α3 chain of type IV collagen in glomerular and alveolar basement membranes.
• May follow smoking, hydrocarbon exposure, respiratory infection, or inhalational injury.
• Some patients have both anti-GBM antibody and ANCA positivity: “double-positive disease.”

C. Immune-complex mediated disorders

1. SLE and lupus nephritis
2. IgA nephropathy / IgA vasculitis
3. Cryoglobulinemic vasculitis, often associated with hepatitis C
4. Hypocomplementemic urticarial vasculitis
5. Post-infectious GN, rarely with DAH
6. Antiphospholipid syndrome
7. Rheumatoid arthritis, systemic sclerosis, and other connective-tissue diseases, rarely.

D. Nonimmune and mimic conditions

• Acute pulmonary edema due to renal failure, fluid overload, or cardiac failure
• Severe pneumonia with acute tubular necrosis
• Sepsis with ARDS and AKI
• Infective endocarditis with immune-complex GN
• Tuberculosis or fungal infection with renal dysfunction
• Pulmonary embolism with hemoptysis and AKI
• Coagulopathy, thrombocytopenia, anticoagulant-related bleeding
• Drug-induced lung injury and acute interstitial nephritis
• Rhabdomyolysis with pulmonary illness and AKI

Pathophysiology

1. Diffuse alveolar hemorrhage

Pulmonary capillaritis

• Neutrophils infiltrate alveolar septal capillaries.
• Capillary walls undergo necrosis.
• Red cells leak into alveoli.
• Recurrent hemorrhage produces hemosiderin-laden macrophages and may later cause fibrosis.

• Dyspnea
• Cough
• Hemoptysis, although it may be absent in up to one-third of patients
• Acute fall in hemoglobin
• Hypoxemic respiratory failure
• Bilateral diffuse alveolar infiltrates or ground-glass opacities

2. Rapidly progressive glomerulonephritis

• Hematuria with dysmorphic red cells
• Red-cell casts
• Proteinuria
• Rapid rise in serum creatinine
• Oliguria, hypertension, and edema

Immunopathological patterns

Clinical Features

Pulmonary manifestations

• Dyspnea, cough, tachypnea
• Hemoptysis, which may be absent
• Fever or constitutional symptoms
• Bilateral crackles
• Hypoxemia
• Acute anemia
• Diffuse bilateral alveolar infiltrates

Renal manifestations

• Hematuria, often microscopic
• Dysmorphic RBCs and RBC casts
• Proteinuria, usually subnephrotic
• Rising serum creatinine
• Oliguria or anuria
• Hypertension and fluid overload

Systemic clues to etiology

Diagnosis

1. Initial assessment and stabilization

• Respiratory distress and oxygen requirement
• Hemoptysis
• Hemodynamic instability
• Hemoglobin fall
• Oliguria/anuria
• Hyperkalemia, severe acidosis, pulmonary edema, uremia

2. Basic investigations

Blood tests

• Complete blood count: anemia, leukocytosis, thrombocytopenia, eosinophilia
• Serial hemoglobin
• Serum creatinine, urea, electrolytes
• Arterial blood gas
• Liver function tests
• ESR and CRP
• Serum albumin and LDH
• Coagulation profile
• Blood cultures if fever or endocarditis is suspected

Urine tests

• Urine routine and microscopy
• Dysmorphic RBCs
• RBC casts
• Proteinuria
• Urine protein-creatinine ratio or 24-hour urinary protein

3. Immunological tests

• ANCA
  • PR3-ANCA/c-ANCA: supports GPA
  • MPO-ANCA/p-ANCA: supports MPA or EGPA
• Anti-GBM antibody
• ANA
• Anti-dsDNA
• Complement C3 and C4
• Antiphospholipid antibodies when indicated
• Serum cryoglobulins
• Rheumatoid factor
• Hepatitis B, hepatitis C, HIV serology
• Eosinophil count and serum IgE if EGPA is suspected

4. Chest imaging

Chest radiograph

• Bilateral, diffuse, patchy alveolar infiltrates
• Usually central or perihilar
• May clear rapidly after cessation of bleeding

HRCT chest

• Bilateral ground-glass opacities
• Consolidation
• Septal thickening
• Nodules or cavities suggest GPA
• Pulmonary edema and infection must be excluded

5. Bronchoscopy with bronchoalveolar lavage

• Sequential lavage aliquots become progressively bloodier
• Hemosiderin-laden macrophages on cytology, often prominent after 48-72 hours
• Samples should be sent for bacterial, mycobacterial, fungal, and viral studies in immunocompromised or febrile patients.

6. Renal biopsy: diagnostic gold standard

• Confirms the type of GN
• Differentiates pauci-immune, anti-GBM, and immune-complex disease
• Assesses percentage of crescents and chronic damage
• Helps determine renal prognosis and treatment intensity

Characteristic renal biopsy findings

• AAV: focal necrotizing crescentic GN, pauci-immune IF
• Anti-GBM disease: crescentic GN with linear IgG staining
• SLE: “full-house” IF staining with IgG, IgA, IgM, C3, and C1q
• IgA nephropathy: dominant mesangial IgA deposits
• Cryoglobulinemia: MPGN pattern with immune deposits

Management

Principles

1. Stabilize airway, breathing, and circulation.
2. Confirm DAH and RPGN while excluding infection.
3. Send ANCA, anti-GBM, complement, and lupus serology immediately.
4. Obtain renal biopsy as early as possible.
5. Start cause-specific immunosuppression promptly in severe suspected immune-mediated PRS.

A. General supportive treatment

• Admit to ICU if hypoxemia, hemodynamic instability, massive DAH, or severe AKI
• Supplemental oxygen
• Noninvasive ventilation or mechanical ventilation when necessary
• Packed red-cell transfusion for significant anemia
• Correct coagulopathy and stop anticoagulants where appropriate
• Fluid balance, daily weight, strict input-output charting
• Avoid nephrotoxins: NSAIDs, unnecessary contrast, aminoglycosides
• Dose-adjust all drugs to renal function
• Treat hyperkalemia, metabolic acidosis, and pulmonary edema
• Dialysis for standard indications:
  • Refractory hyperkalemia
  • Severe acidosis
  • Fluid overload/pulmonary edema
  • Uremic complications
  • Persistent severe renal failure
• Pneumocystis jirovecii pneumonia prophylaxis during intensive immunosuppression, typically trimethoprim-sulfamethoxazole if suitable
• Screen for and treat infection aggressively

B. ANCA-associated vasculitis

Induction therapy for organ- or life-threatening disease

1. Glucocorticoids
   • IV methylprednisolone 500-1000 mg/day for 1-3 days in life-threatening DAH or RPGN
   • Followed by oral prednisolone with a structured taper
2. Rituximab or cyclophosphamide
   • Rituximab is preferred in relapsing disease, fertility concerns, or where cyclophosphamide toxicity is a concern.
   • Cyclophosphamide is often used in severe/new-onset disease, especially with very advanced renal dysfunction.
   • In fulminant presentations, rituximab and cyclophosphamide may occasionally be combined in expert centers.
3. Avacopan
   • May be used as a glucocorticoid-sparing option in selected patients with AAV, depending on availability and clinical context.

Plasma exchange in AAV

• Severe kidney failure or rapidly rising creatinine
• Dialysis-dependent disease
• Severe, hypoxemic DAH
• Coexisting anti-GBM antibodies

Maintenance therapy

• Rituximab, or
• Azathioprine, or
• Methotrexate only if renal function is adequate and there is no severe renal disease

C. Anti-GBM disease

Immediate treatment

1. Plasma exchange
   • Daily or alternate-day sessions
   • Removes circulating anti-GBM antibodies
   • Usually continued until antibody becomes undetectable or according to expert protocol
2. High-dose corticosteroids
   • IV methylprednisolone in severe disease, followed by oral prednisolone taper
3. Cyclophosphamide
   • Suppresses further autoantibody production

D. SLE-associated PRS / DAH

• Pulse IV methylprednisolone
• Cyclophosphamide or rituximab in refractory/severe disease
• Mycophenolate may be used depending on lupus nephritis class and clinical severity
• Plasma exchange may be considered for catastrophic DAH, antiphospholipid syndrome, or refractory disease
• Treat associated infection and thrombocytopenia if present

E. IgA vasculitis, cryoglobulinemia, and other immune-complex disorders

• High-dose corticosteroids for severe DAH/RPGN
• Cyclophosphamide or rituximab depending on cause
• For hepatitis C-associated cryoglobulinemia: antiviral therapy plus immunosuppression in life-threatening vasculitis
• Plasma exchange may be required in severe cryoglobulinemic vasculitis or hyperviscosity.

Differential Diagnosis

Exam-oriented summary

The three commonest causes

ANCA-associated vasculitis, anti-GBM disease, and SLE.

Key tests to order urgently

Urine microscopy, renal function, hemoglobin, chest imaging, ANCA, anti-GBM antibody, ANA/dsDNA, C3/C4, bronchoscopy with BAL if needed, and renal biopsy.

Key pathology

Pauci-immune GN in AAV, linear IgG in anti-GBM disease, and granular immune-complex deposits in lupus/immune-complex disorders.

Core treatment

Stabilize the patient, exclude infection, give high-dose glucocorticoids plus rituximab or cyclophosphamide for severe AAV, and use urgent plasma exchange plus steroids plus cyclophosphamide for anti-GBM disease.