How is renal injury graded?

Grading of Renal Injury

Renal injuries are graded using the American Association for the Surgery of Trauma (AAST) Organ Injury Scale, which classifies injuries from Grade I through Grade V based on anatomic severity, with hemodynamic status determining the clinical management approach through the World Society of Emergency Surgery (WSES) classification system. 1

AAST Organ Injury Scale (Anatomic Grading)

The AAST scale provides the anatomic foundation for injury classification:

Grade I (Minor)

• Contusion: Microscopic or gross hematuria with normal urological studies 1
• Hematoma: Subcapsular, non-expanding without parenchymal laceration 1

Grade II (Minor)

• Hematoma: Non-expanding perirenal hematoma confined to renal retroperitoneum 1
• Laceration: <1.0 cm parenchymal depth of renal cortex with no urinary extravasation 1

Grade III (Moderate)

• Laceration: >1.0 cm parenchymal depth of renal cortex without collecting system rupture or urinary extravasation 1

Grade IV (Severe)

• Laceration: Parenchymal laceration extending through renal cortex, medulla, and collecting system 1
• Vascular: Main renal artery or vein injury with contained hemorrhage 1

Grade V (Critical)

• Laceration: Completely shattered kidney 1
• Vascular: Avulsion of renal hilum that devascularizes kidney 1

WSES Clinical Classification (Management-Based)

The WSES system integrates hemodynamic status with anatomic grading to guide treatment decisions:

WSES Class I (Minor)

• Hemodynamically stable AAST Grade I-II injuries (blunt or penetrating) 1

WSES Class II (Moderate)

• Hemodynamically stable AAST Grade III injuries (blunt or penetrating) 1

WSES Class III (Severe)

• Hemodynamically stable AAST Grade IV-V injuries (blunt or penetrating) 1
• Any grade parenchymal lesion with arterial dissection/occlusion 1

WSES Class IV (Critical)

• Hemodynamically unstable AAST Grade I-V injuries (any grade, blunt or penetrating) 1

Key Clinical Considerations

Hematuria Does Not Predict Injury Grade

• Macro or micro-hematuria is present in 88-94% of renal trauma cases but does not correlate with injury severity 1
• Critical pitfall: 10-25% of high-grade kidney injuries present without hematuria 1, 2
• 24-50% of ureteropelvic junction and renal hilum injuries occur without hematuria 1
• In 0.1-0.5% of patients, hemodynamic stability and micro-hematuria coexist with significant urinary tract injury 1

Hemodynamic Stability Definitions

Adults:

• Unstable: SBP <90 mmHg with evidence of vasoconstriction (cool, clammy skin, decreased capillary refill), altered consciousness, or requiring bolus infusions/transfusions/vasopressors 1
• Unstable: Base excess >-5 mmol/L, shock index >1, or transfusion requirement of 4-6 units packed red blood cells within 24 hours 1

Pediatric patients:

• Stable: SBP of 90 mmHg plus twice the child's age in years 1
• Lower limit: <70 mmHg plus twice the child's age in years (or <50 mmHg in some studies) 1
• Acceptable response to resuscitation: 3 boluses of 20 mL/kg crystalloid before blood replacement, leading to heart rate reduction, cleared sensorium, return of peripheral pulses, normal skin color, increased blood pressure and urinary output, and increased warmth of extremities 1

Imaging Requirements

• Contrast-enhanced CT with immediate and delayed phases (10-15 minutes post-contrast) is mandatory for all stable patients with gross hematuria, microscopic hematuria with SBP <90 mmHg, or concerning mechanism/physical findings 2, 3, 4
• Delayed phase imaging is essential to identify collecting system injuries and contrast extravasation, which may be missed on initial imaging in up to 1% of high-grade injuries 3, 4

Pediatric Anatomic Vulnerabilities

• Children have less perirenal fat, thinner abdominal muscles, lack of rib cage ossification, larger kidney size, and fetal kidney lobulations making them more vulnerable to injury 1
• In children, micro-hematuria (<50 RBC/HPF) is frequent due to kidney anatomy and undiagnosed kidney diseases (1-36%), while macro-hematuria is more related to major renal injuries 1
• Perform imaging in children with blunt trauma and >50 RBCs/HPF, considering mechanism of injury, hypotension, flank hematoma/ecchymosis, rib fractures, and hematocrit drop 1

Management Implications

• Non-operative management is the standard of care for all hemodynamically stable patients regardless of injury grade, with 93% success rate 2, 3
• Immediate surgical exploration is mandatory for persistent hemodynamic instability despite resuscitation 2, 3
• Angioembolization should be considered for ongoing bleeding in stable or transiently responsive patients, with 63-100% success rates 2, 3, 4

Common Pitfalls to Avoid

• Do not rely solely on hematuria presence to determine imaging need, as significant injuries can occur without hematuria 1, 2
• Do not fail to perform delayed phase CT imaging, as collecting system injuries may be missed on initial imaging 3, 4
• Do not rush to surgery for devascularized kidney in stable patients without other laparotomy indications 3
• Recognize that the AAST grading system has fair inter-observer agreement (Kappa 0.36), improving to moderate (Kappa 0.59) when simplified to low-grade versus high-grade classification 5