What is the coaxial method for kidney biopsy?

The Coaxial Method for Kidney Biopsy

The coaxial method for kidney biopsy involves using a two-needle system where a larger outer guide needle (coaxial introducer) is placed adjacent to or into the renal capsule, followed by insertion of a smaller inner biopsy needle through this guide to obtain multiple tissue samples with a single capsular puncture. 1

Technique Components

Equipment

• Outer coaxial introducer needle (typically 16-17 gauge)
• Inner biopsy needle (typically 18 gauge)
• Automated biopsy gun (e.g., Bard Max-Core, Mission, or Biopince Ultra)
• Imaging guidance (ultrasound, CT, or MRI)

Procedural Steps

1. Positioning and imaging guidance: Patient is positioned appropriately for the selected imaging modality
2. Coaxial introducer placement:
   • A 16-17 gauge coaxial needle is advanced to the renal capsule
   • Can be placed using either:
     • Intracapsular technique: advancing into the renal cortex
     • Extracapsular technique: placing adjacent to the edge of the cortex 2
3. Inner needle deployment:
   • The stylet of the coaxial needle is removed
   • An 18 gauge biopsy needle is inserted through the introducer
   • Multiple core samples (typically 2-4) are obtained using an automated biopsy gun 3
4. Sample collection: Cores are immediately evaluated for adequacy
5. Track plugging: Optional plugging of the biopsy track with absorbable gelatin sponge to minimize bleeding risk 4

Advantages of the Coaxial Method

• Reduced needle passes: Allows multiple samples with only a single capsular puncture
• Minimized risk of tumor seeding: The outer needle shields the biopsy track during multiple sampling 1
• Higher diagnostic yield: Studies show 94-100% adequacy rates for diagnosis 3, 5
• Reduced complications: Particularly beneficial for reducing bleeding risk
• Versatility: Can be used with different imaging modalities (CT, ultrasound, MRI)
• Particularly suitable for challenging cases: Effective for obese patients or those with renal atrophy 5

Imaging Guidance Options

CT Guidance

• Allows precise needle placement
• Can be performed with low-dose protocols (120 kV, 25-50 mAs) without compromising accuracy 5
• Patient positioned prone or lateral decubitus
• Recommended under general anesthesia for accurate probe placement 1

Ultrasound Guidance

• Real-time visualization
• No radiation exposure
• Particularly useful for transplant kidney biopsies 2

MRI Guidance

• Requires MRI-compatible equipment
• Better soft tissue resolution
• Less commonly used due to logistical challenges 1

Needle Types Used with Coaxial Technique

Two main types of biopsy needles can be used with the coaxial technique:

• Side-cutting needles (e.g., Bard Max-Core, Mission): Traditional design but may have slightly higher inadequate sampling rates (1.0%)
• End-cutting needles (e.g., Biopince Ultra): May provide better sample adequacy (0.1% inadequate sampling) 2

Safety Considerations

• Complication rates: Similar between coaxial and non-coaxial methods (minor: 2.6-3.4%, major: 0.9-1.0%) 4
• Bleeding risk: Can be minimized by:
  • Using cortical tangential technique to avoid central artery transgression
  • Optional track plugging with absorbable gelatin sponge
  • Avoiding forceps use and preventing tissue stretching during sample removal 1
• Sample handling: Gentle handling with wooden sticks or 18G needles rather than forceps to avoid crush artifact 1

Clinical Applications

• Medical renal disease diagnosis: High diagnostic yield for glomerular diseases
• Renal mass evaluation: Helps distinguish benign from malignant solid renal masses
• Transplant kidney assessment: Particularly useful for diagnosing rejection or other transplant-related pathologies 2

The coaxial method has become the preferred technique for renal mass biopsy due to its high diagnostic yield and low complication rate, allowing for more informed clinical decision-making while avoiding unnecessary interventions for benign lesions 6.