Acute Kidney Injury: Anuria and Oliguria

Introduction

Acute Kidney Injury (AKI) is a frequent clinical problem encountered in Canadian hospitals and ambulatory care. For the MCCQE1, understanding the approach to a patient presenting with decreased urine output (oliguria or anuria) is a high-yield objective.

The Medical Council of Canada expects a candidate to demonstrate the ability to define, classify, investigate, and manage AKI, specifically focusing on distinguishing between prerenal, intrinsic, and postrenal causes. This guide aligns with the CanMEDS framework, emphasizing the Medical Expert role in diagnosis and management, and the Health Advocate role in preventing iatrogenic kidney injury.

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Definitions for MCCQE1:

Acute Kidney Injury (AKI): A sudden decrease in kidney function defined by an increase in serum creatinine by $\ge$ 26.5 µmol/L within 48 hours, OR increase in serum creatinine to $\ge$ 1.5 times baseline, OR urine volume < 0.5 mL/kg/h for 6 hours.
Oliguria: Urine output < 400 mL/day (or roughly < 0.5 mL/kg/hour).
Anuria: Urine output < 100 mL/day.

MCCQE1 Objectives

When preparing for the MCCQE1, focus on the following key competencies regarding AKI:

Identify patients at high risk for AKI (e.g., elderly, diabetics, existing CKD).
Differentiate between prerenal, renal (intrinsic), and postrenal causes.
Manage life-threatening complications (Hyperkalemia, Pulmonary Edema).
Recognize indications for urgent dialysis.

Classification and Etiology

The classic approach to AKI is anatomical. This is the most effective framework for the MCCQE1 clinical reasoning process.

Prerenal (Perfusion)

Prerenal AKI is caused by decreased renal perfusion without damage to the renal parenchyma (initially). It is the most common cause of AKI in hospitalized patients.

True Volume Depletion: Hemorrhage, GI losses (vomiting/diarrhea), burns, diuretics.
Decreased Effective Arterial Blood Volume (EABV): Heart failure (Cardiorenal syndrome), Cirrhosis (Hepatorenal syndrome), Sepsis (vasodilation).
Afferent Arteriolar Vasoconstriction: NSAIDs, Cyclosporine, Contrast dye.
Efferent Arteriolar Vasodilation: ACE inhibitors (ACEi), Angiotensin Receptor Blockers (ARBs).

Clinical Assessment

History Taking (Canadian Context)

In the Canadian context, always ask about:

Medications: Recent use of NSAIDs (common OTC), ACEi/ARBs, antibiotics (Gentamicin, Penicillins).
Procedures: Recent iodinated contrast (CT scans), cardiac catheterization.
Volume Status: Oral intake, vomiting, diarrhea, blood loss.
Urinary Symptoms: Hesitancy, frequency, urgency (suggests BPH/Obstruction).
Systemic Symptoms: Rash/fever (AIN), hemoptysis (Vasculitis/Goodpasture’s), bloody diarrhea (HUS - E. coli O157:H7 is relevant in Canadian outbreaks).

Physical Examination

Focus on volume status assessment and ruling out obstruction.

System	Signs of Hypovolemia (Prerenal)	Signs of Hypervolemia (Intrinsic/Postrenal)
Vitals	Tachycardia, Hypotension, Orthostasis	Hypertension (sometimes)
JVP	Flat	Elevated
Cardio	Normal	S3 gallop
Resp	Clear	Crackles (Pulmonary Edema)
Abdomen	Normal	Distended bladder (palpable), flank pain
Extremities	Cool, decreased turgor	Edema (Pedal/Sacral)

🚨 Clinical Pearl: The Blocked Catheter

In a hospitalized patient with a urinary catheter who suddenly develops anuria, the most likely cause is a blocked catheter. The immediate management is to flush the catheter or replace it, not to administer fluids or diuretics.

Diagnostic Investigations

1. Urinalysis and Microscopy

This is the “liquid biopsy” of the kidney.

Hyaline casts: Concentrated urine (Prerenal).
Muddy brown (granular) casts: Acute Tubular Necrosis (ATN).
WBC casts/Eosinophils: Acute Interstitial Nephritis (AIN).
RBC casts: Glomerulonephritis (GN).

2. Urine Electrolytes

Useful to distinguish Prerenal from ATN when the patient is oliguric.

Fractional Excretion of Sodium (FeNa) Formula:

FeNa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

Parameter	Prerenal (Kidney holding onto salt/water)	ATN (Kidney cannot reabsorb)
FeNa	< 1%	> 2%
Urine Na	< 20 mmol/L	> 40 mmol/L
Urine Osmolality	> 500 mOsm/kg	< 350 mOsm/kg (Isosthenuric)
BUN:Cr Ratio	> 20:1 (urea reabsorbed)	< 10-15:1

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Note on Diuretics: If the patient is on diuretics, FeNa is invalid (diuretics force Na excretion). Use Fractional Excretion of Urea (FeUrea) instead. FeUrea < 35% suggests prerenal state.

3. Imaging

Point of Care Ultrasound (POCUS) / Bladder Scan: Essential to rule out urinary retention (Postrenal).
Renal Ultrasound: Look for hydronephrosis (obstruction) and kidney size (small kidneys suggest Chronic Kidney Disease).

Step-by-Step Management Approach

Follow this algorithm for a patient presenting with Oliguria/Anuria:

Step 1: Rule Out Obstruction (Postrenal)

Perform a physical exam (palpate bladder) and a bladder scan.

If retention is present: Insert Foley catheter.
If catheter already in place: Flush the catheter.
If hydronephrosis on US without bladder distension: Consult Urology for nephrostomy (ureteral obstruction).

Step 2: Assess Volume Status (Prerenal)

Determine if the patient is “wet” (overloaded) or “dry” (depleted).

Hypovolemic: Administer IV fluid challenge (Balanced crystalloids like Ringer’s Lactate are preferred over Normal Saline in Canada to prevent hyperchloremic metabolic acidosis).
Hypervolemic: Consider diuretics (Furosemide) only if the patient is volume overloaded. Do not “wring out” a dry kidney.

Step 3: Review Medications (Iatrogenic)

Stop nephrotoxins immediately:

NSAIDs
ACEi / ARBs (hold during acute illness)
Aminoglycosides
Metformin (hold to prevent lactic acidosis, though it doesn’t cause AKI itself).

Step 4: Manage Complications

Monitor for the “Killer Ks”:

Potassium: Treat Hyperkalemia aggressively (Calcium gluconate, Insulin+Glucose, Ventolin, Kayexalate/Lokelma).
Acidosis: Metabolic acidosis.
Pulmonary Edema: Oxygen, Nitrates, Furosemide (if kidneys responsive).

Step 5: Consider Dialysis

Consult Nephrology if indications for Urgent Dialysis are met.

Indications for Urgent Dialysis (AEIOU Mnemonic)

A - Acidosis: Severe metabolic acidosis (pH < 7.1) refractory to medical therapy.
E - Electrolytes: Severe hyperkalemia (K > 6.5 mmol/L) or symptomatic, refractory to medical therapy.
I - Intoxication: Toxic alcohols (Methanol, Ethylene glycol), Lithium, Salicylates.
O - Overload: Volume overload causing respiratory failure, refractory to diuretics.
U - Uremia: Uremic pericarditis, uremic encephalopathy, or uremic bleeding.

Canadian Guidelines & Choosing Wisely

Kidney Disease: Improving Global Outcomes (KDIGO): These are the globally accepted guidelines adopted by the Canadian Society of Nephrology (CSN). They define AKI staging based on creatinine and urine output.
Choosing Wisely Canada:
- Don’t start dialysis for AKI without a specific indication (AEIOU).
- Don’t use NSAIDs in individuals with hypertension or heart failure or CKD of all causes, including diabetes.
- Don’t administer contrast media to patients with AKI without weighing risks/benefits.

Key Points to Remember for MCCQE1

Anuria usually implies obstruction (postrenal) or complete vascular occlusion (rare). Always rule out obstruction first with a catheter/scan.
Prerenal AKI is characterized by avid sodium and water retention (FeNa < 1%).
Muddy brown casts are pathognomonic for ATN.
Contrast-induced nephropathy is prevented by IV hydration (Isotonic saline or Sodium Bicarbonate) before the procedure. N-acetylcysteine is no longer recommended in Canadian guidelines due to lack of efficacy.
In elderly males with AKI, BPH is the most common cause of postrenal failure.

Sample Question

Clinical Scenario

A 78-year-old male presents to the emergency department with a 2-day history of inability to pass urine and lower abdominal pain. He has a history of benign prostatic hyperplasia (BPH) and hypertension. On examination, he is uncomfortable and diaphoretic. His blood pressure is 165/95 mmHg, and heart rate is 100 bpm. Abdominal examination reveals a firm, palpable mass in the suprapubic region that is dull to percussion. His serum creatinine is 320 µmol/L (baseline 90 µmol/L) and Potassium is 5.1 mmol/L.

Question

Which one of the following is the most appropriate immediate management step?

A. Administer IV Furosemide 40 mg
B. Initiate urgent hemodialysis
C. Insert a urethral catheter
D. Order a CT scan of the abdomen and pelvis
E. Administer IV bolus of 1L Normal Saline

Explanation

The correct answer is:

C. Insert a urethral catheter

Detailed Explanation:

Diagnosis: This patient presents with classic signs of Postrenal AKI secondary to acute urinary retention, likely due to BPH. The “firm, palpable mass” in the suprapubic region is a distended bladder.
Reasoning: The immediate priority is to relieve the obstruction. This is both diagnostic (confirming retention) and therapeutic (relieving pressure).
- Option A (Furosemide): Contraindicated. The patient is obstructed; forcing more urine production will increase bladder pressure and potentially cause rupture or worsening hydronephrosis.
- Option B (Dialysis): There are no urgent indications for dialysis (AEIOU). The potassium is 5.1 (mild), and there is no evidence of severe acidosis, overload, or uremia. The AKI is likely reversible with catheterization.
- Option C (Catheter): Correct. This relieves the obstruction immediately.
- Option D (CT Scan): Unnecessary delay. The diagnosis is clinical. A bladder scan or ultrasound could be used, but with a palpable bladder, catheterization is the next step.
- Option E (Fluids): The patient is hypertensive and obstructed. Adding volume is not the primary issue and could worsen the situation if the obstruction isn’t cleared first.

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.
Choosing Wisely Canada. Nephrology: Five Things Physicians and Patients Should Question. Available at: https://choosingwiselycanada.org/nephrology/
Medical Council of Canada. Objectives for the Qualifying Examination Part I. Ottawa: MCC.
Canadian Society of Nephrology. Clinical Practice Guidelines.
Toronto Notes 2023. Nephrology Chapter. Toronto: Toronto Notes for Medical Students, Inc.