Acute Kidney Injury, Anuria, and Oliguria

Introduction

This comprehensive guide is designed to help Canadian medical students prepare for the Medical Council of Canada Qualifying Examination Part I (MCCQE1) in the area of Acute Kidney Injury (AKI), Anuria, and Oliguria. Understanding these conditions is crucial for success in the MCCQE1 and for future practice in the Canadian healthcare system.

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This guide aligns with the CanMEDS framework, emphasizing the roles of Medical Expert, Communicator, and Health Advocate in managing patients with kidney dysfunction.

Definitions and Classification

Acute Kidney Injury (AKI)

AKI is a sudden decrease in kidney function occurring over hours to days, leading to the accumulation of waste products and fluid imbalance.

Anuria and Oliguria

Anuria: Urine output <100 mL/day
Oliguria: Urine output <400 mL/day or <0.5 mL/kg/h for 6 hours

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In Canada, the KDIGO (Kidney Disease: Improving Global Outcomes) criteria are widely used for AKI classification and staging.

KDIGO Classification of AKI

Stage	Serum Creatinine	Urine Output
1	1.5-1.9 times baseline OR ≥26.5 μmol/L increase	<0.5 mL/kg/h for 6-12 hours
2	2.0-2.9 times baseline	<0.5 mL/kg/h for ≥12 hours
3	3.0 times baseline OR Increase in serum creatinine to ≥353.6 μmol/L OR Initiation of renal replacement therapy	<0.3 mL/kg/h for ≥24 hours OR Anuria for ≥12 hours

Etiology of AKI

Understanding the causes of AKI is crucial for MCCQE1 success. The causes are typically categorized into three groups:

Pre-renal

Decreased renal perfusion
Common in Canadian emergency departments due to severe dehydration or shock

Intrinsic renal

Direct damage to kidney structures
Often seen in cases of nephrotoxic drug exposure or glomerulonephritis

Post-renal

Obstruction of urinary outflow
Frequently encountered in older Canadian patients with prostatic hypertrophy

Clinical Presentation

Recognizing the signs and symptoms of AKI is essential for MCCQE1 preparation:

Decreased urine output
Fluid overload (edema)
Shortness of breath
Fatigue
Confusion
Nausea and vomiting

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Remember: Not all patients with AKI present with oliguria or anuria. Some may have normal or even increased urine output, especially in certain causes of intrinsic AKI.

Diagnostic Approach

For the MCCQE1, focus on the following diagnostic steps:

History and Physical Examination
- Recent illnesses, medications, exposures
- Volume status assessment
Laboratory Tests
- Serum creatinine and blood urea nitrogen (BUN)
- Electrolytes, including potassium and bicarbonate
- Complete blood count (CBC)
- Urinalysis and urine microscopy
Imaging Studies
- Renal ultrasound to assess kidney size and rule out obstruction
Special Tests
- Fractional excretion of sodium (FENa) to differentiate pre-renal from intrinsic renal causes

FENa = [(UNa × PCr) / (PNa × UCr)] × 100

Management

The management of AKI in Canada follows these key principles:

Treat the underlying cause
Optimize hemodynamics and correct volume status
Avoid nephrotoxic agents
Manage electrolyte and acid-base disturbances
Adjust medication dosages
Consider renal replacement therapy when indicated

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In Canada, continuous renal replacement therapy (CRRT) is often preferred for critically ill patients with AKI, while intermittent hemodialysis is more common in stable patients.

Canadian Guidelines

The Canadian Society of Nephrology provides guidelines for AKI management:

Use balanced crystalloids rather than normal saline for initial volume expansion
Consider early nephrology consultation for severe AKI or high-risk patients
Implement electronic alert systems for early AKI detection in hospitals
Utilize KDIGO criteria for AKI diagnosis and staging

Key Points to Remember for MCCQE1

AKI is classified using KDIGO criteria in Canada
Understand the differences between pre-renal, intrinsic, and post-renal AKI
Know how to calculate and interpret FENa
Recognize that not all AKI presents with oliguria or anuria
Familiarity with Canadian guidelines for AKI management
Understand the indications for renal replacement therapy

Sample Question

# Sample Question

A 68-year-old man presents to the emergency department with a 3-day history of decreased urine output and fatigue. He has a history of hypertension and type 2 diabetes. His blood pressure is 100/60 mmHg, heart rate is 110 bpm, and he appears dehydrated. Laboratory results show:

Serum creatinine: 265 μmol/L (baseline 88 μmol/L)
BUN: 25 mmol/L
Sodium: 135 mmol/L
Potassium: 5.2 mmol/L

Which one of the following is the most appropriate next step in management?

- [ ] A. Start intravenous furosemide
- [ ] B. Initiate hemodialysis
- [ ] C. Administer intravenous balanced crystalloid solution
- [ ] D. Perform renal biopsy
- [ ] E. Start oral sodium bicarbonate

Explanation

The correct answer is:

C. Administer intravenous balanced crystalloid solution

This patient presents with signs of AKI (elevated creatinine) and volume depletion (low blood pressure, tachycardia, clinical dehydration). The most appropriate initial step is to restore intravascular volume with IV fluids. In Canada, balanced crystalloid solutions are preferred over normal saline for initial volume expansion in AKI.

Option A (furosemide) is incorrect as it may worsen the patient's volume depletion. Option B (hemodialysis) is premature without first attempting fluid resuscitation. Option D (renal biopsy) is not indicated as the initial management step. Option E (sodium bicarbonate) is not the priority in this case of likely pre-renal AKI.

This question tests the candidate's ability to recognize AKI, assess volume status, and initiate appropriate management according to Canadian guidelines.

References

Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012; 2: 1-138.
Rewa O, Bagshaw SM. Acute kidney injury-epidemiology, outcomes and economics. Nat Rev Nephrol. 2014;10(4):193-207.
Canadian Society of Nephrology. Choosing Wisely Canada: Nephrology. https://choosingwiselycanada.org/recommendation/nephrology/ (opens in a new tab)
Self WH, Semler MW, Wanderer JP, et al. Balanced Crystalloids versus Saline in Noncritically Ill Adults. N Engl J Med. 2018;378(9):819-828.
Mehta RL, Cerdá J, Burdmann EA, et al. International Society of Nephrology's 0by25 initiative for acute kidney injury (zero preventable deaths by 2025): a human rights case for nephrology. Lancet. 2015;385(9987):2616-2643.

Proteinuria Hyponatremia