What is the exact order for ultrasound to check for urinary obstruction and post-void residual (PVR) volume?

Ultrasound Order for Urinary Obstruction and Post-Void Residual Assessment

Scan the bladder first (pre-void), have the patient void, then immediately measure post-void residual (PVR), and finally scan both kidneys to assess for hydronephrosis. 1

Optimal Scanning Sequence

Step 1: Pre-Void Bladder Assessment

• Scan the bladder when full to assess baseline volume and identify any bladder wall abnormalities, stones, or masses 1
• Image the bladder from top to bottom and side to side in both transverse and sagittal planes 1
• A full bladder facilitates bladder scanning but must be avoided when subsequently scanning the kidneys, as distension causes artifactual hydronephrosis 1

Step 2: Patient Voids

• Have the patient void completely before proceeding to PVR measurement 1
• The interval between voiding and PVR measurement should be of short duration (ideally within 30 minutes) 2, 3

Step 3: Immediate Post-Void Residual Measurement

• Measure PVR immediately after voiding using ultrasound rather than catheterization when possible to minimize infection risk 2, 3
• Ultrasound bladder volume measurement is preferred to urethral catheterization 3
• Repeat PVR measurement at least 2-3 times due to marked intra-individual variability to ensure reliable results 2, 3, 4
• PVR >200-300 mL may indicate significant bladder dysfunction 2, 3

Step 4: Kidney Assessment for Obstruction

• Scan both kidneys after the patient has voided to avoid artifactual hydronephrosis from bladder distension 1
• Image both kidneys in longitudinal and transverse planes for comparison and to exclude absence of either kidney 1
• Assess for hydronephrosis grading: Mild (Grade I), Moderate (Grade II with "bear's paw" appearance), or Severe (Grade III with parenchymal effacement) 1

Technical Approach for Kidney Scanning

Right Kidney

• Use anterior subcostal approach with liver as acoustic window, or intercostal approach in right flank between anterior axillary line and midline posteriorly 1
• Patient positioning: left lateral decubitus or prone position may facilitate visualization 1
• Deep breath may extend the liver window to include the inferior pole 1

Left Kidney

• Requires intercostal approach (lacks hepatic window) 1
• Use same positioning techniques as right kidney 1

Critical Pitfalls to Avoid

• Never scan kidneys before the patient voids, as a distended bladder causes false-positive hydronephrosis 1
• Do not base clinical decisions on a single PVR measurement - always confirm with repeat testing due to high intra-individual variability 2, 3, 4
• Do not delay PVR measurement after voiding - measure within 30 minutes for accuracy 2, 3
• Recognize that ultrasound can miss obstruction in early cases or when the collecting system is non-compliant, and can show false-positive hydronephrosis in conditions like pregnancy, high urine flow states, or vesicoureteral reflux 5

Equipment Specifications

• Use a general-purpose curved array abdominal probe with frequency range of 2-5 MHz 1
• A small footprint or phased array probe may facilitate intercostal scanning 1