Acute Kidney Injury (AKI)

Definition

AKI is diagnosed when any one of the following is met:

• Serum creatinine (sCr) rises by ≥0.3 mg/dL within 48 hours, OR
• sCr rises to ≥1.5× baseline within 7 days, OR
• Urine output (UO) <0.5 mL/kg/h for ≥6 hours

• Sabiston Textbook of Surgery, p. 449
• Washington Manual of Medical Therapeutics, p. 478

RIFLE / AKIN / KDIGO Staging

• Brenner & Rector's The Kidney, p. 1245

Etiology: Pre-renal / Intrinsic / Post-renal

• Washington Manual of Medical Therapeutics, p. 478

Diagnostic Evaluation

Key lab tests distinguishing pre-renal vs. intrinsic vs. post-renal AKI:

• National Kidney Foundation Primer on Kidney Diseases, p. 347
• Sabiston Textbook of Surgery, p. 450

Novel Biomarkers (emerging)

• NGAL (neutrophil gelatinase-associated lipocalin): rises within 2-4 hours of injury
• KIM-1 (kidney injury molecule-1): tubular injury marker
• Cystatin C: sensitive for early GFR decline
• TIMP-2 × IGFBP-7: urinary cell-cycle arrest markers, approved by FDA for predicting AKI risk
• α1-microglobulin: proximal tubule dysfunction; urinary cystatin C and α1-microglobulin have AUC ~0.86 for predicting need for RRT in small studies
• Brenner & Rector's The Kidney, p. 1163

Clinical Features

• Symptoms: nausea, vomiting, fatigue, confusion (uremic encephalopathy), asterixis
• Signs: pericardial rub (uremic pericarditis), peripheral edema, pulmonary infiltrates (fluid overload), abnormal bleeding
• Oliguria: UO <400 mL/day; anuria: <100 mL/day (consider obstruction or bilateral cortical necrosis)

Management

Prevention (most effective strategy)

• Identify high-risk patients: age, CKD, diabetes, hypertension, obesity, high ASA class
• Optimize volume status and hemoglobin before surgery
• Maintain MAP >65 mmHg intraoperatively (>75-80 mmHg in hypertensives)
• Avoid/adjust nephrotoxic drugs (aminoglycosides, NSAIDs, contrast)
• Target intraoperative UO ≥0.5 mL/kg/h

General Treatment Once AKI Develops

1. Restore and maintain renal perfusion - correct hypovolemia, support MAP
2. Avoid further injury - stop nephrotoxins, adjust drug doses
3. Volume management - avoid both hypovolemia and volume overload; evaluate fluid responsiveness carefully
4. Diuretics - use to manage fluid overload, not to prevent or treat AKI per se; do not delay RRT if indicated

Contrast-induced AKI prevention:

• IV volume expansion is the most beneficial intervention (isotonic saline or NaHCO3 solution)
• Start 6-12 hours pre-procedure, continue 4-12 hours post-procedure
• N-acetylcysteine and vitamin C remain controversial

Indications for Renal Replacement Therapy (RRT)

• A - Acidosis (refractory metabolic acidosis)
• E - Electrolytes (refractory hyperkalemia)
• I - Intoxication (dialyzable toxins: lithium, methanol, salicylates)
• O - Overload (refractory pulmonary edema)
• U - Uremia (encephalopathy, pericarditis, bleeding)

• Sabiston Textbook of Surgery, pp. 449-450

AKI vs. AKD vs. CKD

Outcomes & Prognosis

• Repeated AKI episodes can lead to permanent kidney function deterioration and progression to CKD
• Stage 3 AKI carries significantly higher mortality than Stage 1
• ICU patients: up to 80% meet AKI criteria when UO criterion is added to sCr; most UO-only cases are Stage 1 with high recovery rates
• AKI is an independent risk factor for long-term cardiovascular events and mortality

Renal Replacement Therapy (RRT) in AKI

Indications for RRT

Absolute Indications ("AEIOU")

Relative Indications

• Progressive azotemia without overt uremic symptoms
• Persistent oliguria without response to optimization
• Impending acute respiratory failure from fluid overload

• Brenner & Rector's The Kidney, pp. 1245-1248
• Miller's Anesthesia, 10e, pp. 5702-5703

Principles of Solute Clearance

• Diffusion: Concentration gradient across a semipermeable membrane - best for small molecules (<1 kDa). This is the basis of hemodialysis.
• Convection (ultrafiltration): Hydrostatic pressure-driven solvent drag - clears molecules up to 50 kDa, ideal for fluid removal and middle molecule clearance. This is the basis of hemofiltration.
• Hemodiafiltration combines both mechanisms.

RRT Modalities

1. Intermittent Hemodialysis (IHD)

• Schedule: 3-6 sessions/week, 3-6 hours per session
• Mechanism: Diffusive clearance - blood and dialysate flow in opposite directions across a semipermeable membrane (countercurrent)
• Increasing clearance: Raise blood flow rate OR dialysate flow rate
• Best for: Hemodynamically stable patients
• Advantage: Time-efficient, allows scheduled patient mobility
• Disadvantage: Rapid fluid/solute shifts - risk of intradialytic hypotension (can prolong AKI and delay recovery)

2. Continuous RRT (CRRT)

• Replacement fluid is infused either pre-filter (predilution - reduces clotting, reduces efficiency slightly) or post-filter (postdilution - higher efficiency but more clotting risk)
• Dose: Target effluent flow rate of 20-25 mL/kg/h (prescribed dose should be ~25 mL/kg/h to account for downtime). Intensive dosing at 35 mL/kg/h showed no mortality benefit over 20 mL/kg/h in the ATN trial.

3. Hybrid / Prolonged Intermittent RRT (PIRRT / SLED)

• Also called SLEDD (Sustained Low Efficiency Daily Dialysis) or SLEDD-F (+ filtration)
• Schedule: 6-18 hours/day, daily or every other day
• Mechanism: Slower flow rates than IHD, longer treatment time
• Advantage: More hemodynamically stable than IHD, less resource-intensive than CRRT; when done daily it approaches CRRT in clearance
• No standard machine - protocols vary by center

4. Peritoneal Dialysis (PD)

• Catheter placed intra-abdominally (laparoscopic or bedside)
• Dextrose-containing dialysate infused and drained via osmotic gradient
• Clears solutes and fluid without extracorporeal circuit
• Limitations in AKI: Cannot achieve adequate clearance in the hypermetabolism of critical illness; contraindicated after recent abdominal surgery
• Role: Primarily outpatient ESRD; important in low/middle-income settings where CRRT is unavailable

Vascular Access for Acute RRT

Preferred access sites (KDIGO order of preference):

1. Right internal jugular vein - most direct path to superior vena cava, lowest dysfunction rate
2. Femoral veins - easy bedside insertion; acceptable if BMI is normal (infection risk increases in high BMI patients)
3. Left internal jugular vein - higher dysfunction rate due to tortuosity
4. Subclavian vein - last resort only (risk of pneumothorax on mechanical ventilation, high rate of central vein stenosis - this can preclude future AV fistula creation in the ipsilateral arm)

Catheter specifications:

• Internal jugular: 15-20 cm length; tip at atriocaval junction
• Femoral: ≥20-25 cm length; tip must reach inferior vena cava
• Ultrasound guidance for insertion - reduces mechanical complications
• Noncuffed temporary catheters preferred in acute/critically ill
• Replace with cuffed tunneled catheter if prolonged RRT anticipated (lower infection risk)
• Brenner & Rector's The Kidney, p. 2820

Anticoagulation for the Extracorporeal Circuit

Dialysis Dose & Adequacy

• IHD: Measure Kt/V per session; target at minimum Kt/V of 1.2-1.4 per session (3×/week)
• CRRT: Effluent volume (mL/kg/h) is the surrogate for dose. Target ≥20 mL/kg/h delivered (prescribe 25-30 mL/kg/h to account for downtime)
• Higher dose does NOT improve outcomes: The ATN trial (1124 patients) showed no difference in 60-day mortality between 35 mL/kg/h vs. 20 mL/kg/h CRRT (53.6% vs. 51.5%, P = 0.47)
• Membrane choice: Modern synthetic membranes are all relatively biocompatible; no single membrane has shown superiority in outcomes
• Brenner & Rector's The Kidney, p. 1245-3200

Drug Dosing on RRT

1. Nephrotoxins - avoid/minimize (aminoglycosides, amphotericin B, contrast, NSAIDs, hydroxyethyl starches)
2. Drug accumulation - renally-cleared drugs accumulate rapidly; dose-reduce or extend intervals
3. Drug clearance by RRT - vital drugs (e.g., antibiotics, antivirals) may be removed by the filter; supplemental dosing required. CRRT clears drugs more continuously than IHD - consult pharmacokinetic references for each drug

Modality Selection Summary

Complications of RRT

⚠️ Important Disclaimer

Clinical Analysis: 23-Year-Old Female with Bilateral Ankle Pitting Edema + Severe Leg Pain × 25 Days

Lab Results Summary

Differential Diagnosis

1. Poststreptococcal Reactive Arthritis (PSRA) - Most Likely

• History of Group A Streptococcal (GAS) throat/skin infection 10-14 days prior (ask specifically)
• ASO 130 IU/mL - evidence of recent streptococcal exposure (streptozyme panel including anti-DNAse B, ASKase, anti-NAD is positive in 80-95% of PSGN cases; ASO alone may be less sensitive)
• Bilateral lower limb arthritis/arthralgia is the hallmark - non-migratory, severe, prolonged, resistant to salicylates (unlike ARF)
• Edema can occur secondary to joint inflammation and dependent positioning
• Proteinuria (UPCR 0.7) may represent a subclinical poststreptococcal glomerulonephritis component
• Tintinalli's Emergency Medicine

2. Acute Rheumatic Fever (ARF) - Must Rule Out (High priority in India = high-risk population)

• Major criteria: Carditis (clinical or subclinical), arthritis (mono or polyarthritis OR polyarthralgia), chorea, subcutaneous nodules, erythema marginatum
• Minor criteria: Fever ≥38.5°C, elevated ESR ≥60 mm/h or CRP ≥3 mg/dL, prolonged PR interval

• Red Book 2021, Jones Criteria Table 3.58

3. Poststreptococcal Glomerulonephritis (PSGN) with nephrotic features

• Mild proteinuria (UPCR 0.7) with edema is possible
• Classic PSGN presents with hematuria, casts, hypertension - none seen here
• Subclinical PSGN (no hematuria, low-level proteinuria, low C3) is 4-5× more common than clinical disease
• Check serum C3, C4, and creatinine urgently

4. Other causes to consider

Immediate Investigations Required

Blood Tests

Imaging & Other

Management

Immediate General Measures

1. Rest with limb elevation - reduces edema
2. Avoid NSAIDs initially until ARF is ruled out (in PSRA, NSAIDs are treatment; in ARF, aspirin is used with specific dosing)
3. Do NOT start empiric steroids without diagnosis
4. Salt restriction and fluid monitoring

Once Diagnosis Confirmed

If Poststreptococcal Reactive Arthritis (PSRA):

• Eradication of streptococcal infection: Oral penicillin V or amoxicillin × 10 days (benzathine penicillin 1.2 MU IM single dose is equally effective)
• NSAIDs for arthritis pain: Naproxen or diclofenac (PSRA arthritis is often salicylate-resistant)
• Short-term antibiotic prophylaxis (monthly benzathine penicillin) for 1 year while monitoring for late carditis
• Monitor for development of ARF features over follow-up

If Acute Rheumatic Fever (ARF):

• Penicillin to eradicate GAS (as above)
• Aspirin 50-100 mg/kg/day (max 4-6 g/day) for arthritis/carditis
• If carditis with heart failure: Corticosteroids (prednisone 1-2 mg/kg/day × 2-4 weeks, then taper)
• Long-term secondary prophylaxis (benzathine penicillin 1.2 MU IM every 4 weeks for years, per risk stratification) - mandatory to prevent recurrence and rheumatic heart disease
• Echocardiographic follow-up

If PSGN (mild/subclinical):

• Supportive - no specific treatment; no immunosuppressants
• Control hypertension (if present) with antihypertensives
• Salt/fluid restriction if edematous
• Treat active streptococcal infection with antibiotics
• Monitor renal function - most children/young adults recover fully in 3-6 weeks

Clinical Bottom Line

• Bilateral ankle pitting edema + severe leg pain × 25 days
• Borderline ASO 130 IU/mL (evidence of recent streptococcal exposure)
• Mild proteinuria (UPCR 0.7) with no hematuria or casts

Updated Clinical Analysis - New Reports Received

Echo Report Summary (16 July 2026 - CH. VINEELA, 23F)

ECG Summary

✅ Key Conclusions from New Data

1. Acute Rheumatic Fever is NOW DEFINITIVELY EXCLUDED

• No mitral regurgitation (most sensitive marker of rheumatic carditis)
• No aortic regurgitation
• All valves morphologically normal
• No pericardial effusion
• PR interval 140 ms - normal (no Jones minor criterion met)

2. Cardiac Cause of Edema Excluded

• EF 64%, normal LV function, no pulmonary hypertension (RVSP 25 mmHg normal)
• Congestive heart failure and cor pulmonale are excluded as causes of the bilateral leg edema

Revised Diagnosis

Most Likely: Poststreptococcal Reactive Arthritis (PSRA)

Co-diagnosis to Consider: Subclinical Poststreptococcal Glomerulonephritis (PSGN)

• No hematuria, no RBC casts (against active nephritic PSGN)
• No nephrotic-range proteinuria (UPCR <3.5)
• Normal dipstick (suggests non-albumin proteinuria - possibly tubular)
• Prior streptococcal infection

Management Plan (Definitive)

Step 1: Eradicate Residual Streptococcal Infection

Benzathine Penicillin G 1.2 MU IM - single dose (preferred in India for compliance)

OR oral Amoxicillin 500 mg TID × 10 days / Penicillin V 500 mg BD × 10 days

If penicillin allergic: Azithromycin 500 mg Day 1, then 250 mg × 4 days

Step 2: Treat Arthritis and Pain

Naproxen 500 mg BD (preferred NSAID for poststreptococcal arthritis - note PSRA is often salicylate-resistant and partially NSAID-resistant)

Add Pantoprazole 40 mg OD for gastroprotection while on NSAIDs

Short courses only - review in 2-4 weeks

Step 3: Secondary Prophylaxis with Penicillin

Benzathine Penicillin G 1.2 MU IM every 4 weeks

Duration: Minimum 1 year, with repeat echocardiogram at the end. If echo remains normal and no recurrence, prophylaxis may be stopped. If any valvular changes develop - continue for 5-10 years.

Step 4: Monitor Renal Function (Subclinical PSGN)

Step 5: Supportive Measures

• Leg elevation (above heart level) for 20-30 min 3× daily - reduces dependent edema
• Compression stockings (Class I, 15-20 mmHg) for ankle edema
• Moderate salt restriction (no added salt diet, <5g NaCl/day)
• Adequate hydration - do not restrict fluids unless hypertensive
• Rest during acute phase, then gradual activity resumption

Step 6: Rule Out Filariasis (Still Pending)

• Andhra Pradesh coastal belt is endemic for lymphatic filariasis
• Order nocturnal peripheral blood smear (taken between 10 PM - 2 AM for microfilariae)
• If positive → Diethylcarbamazine (DEC) 6 mg/kg/day × 12 days ± albendazole

Follow-Up Plan

Summary

Does This Patient Need a Renal Biopsy?

The Short Answer: No - not at this stage

"The diagnosis of poststreptococcal glomerulonephritis rarely requires a kidney biopsy."

• Harrison's Principles of Internal Medicine 22e

"Renal biopsy is not indicated for diagnosis, unless atypical clinical features are present."

• Tintinalli's Emergency Medicine

"Kidney biopsy is not routinely indicated in PSGN but may be required for confirmation when clinical features are atypical."

• Comprehensive Clinical Nephrology, 7th Edition

Why This Patient Specifically Does NOT Need Biopsy Right Now

When Would Biopsy Become Necessary? (Watchful Waiting Triggers)

What She Needs Instead of Biopsy

Important Note Specific to This Patient's Age

• She does not need biopsy right now given her mild, non-nephrotic presentation
• But she needs closer follow-up than a child would
• If her proteinuria does not fully resolve by 3-6 months, biopsy should be reconsidered to exclude an underlying GN (IgA nephropathy or lupus nephritis are important differentials in a young Indian woman)

Summary