What is the next step in evaluating a patient with a non-visualized left kidney on ultrasound, considering potential risks and consequences, and taking into account their medical history, including any previous kidney issues, surgeries, or medications, and symptoms such as abdominal pain or hematuria?

Non-Visualized Left Kidney on Ultrasound: Next Steps

When a kidney is not visualized on ultrasound, the immediate next step is to obtain cross-sectional imaging with CT urography (if no contraindications exist) or MR urography (if contrast-enhanced CT is contraindicated) to definitively determine whether the kidney is absent (congenital agenesis), atrophic/nonfunctioning, or present but obscured by technical factors. 1, 2

Initial Diagnostic Algorithm

Confirm Technical Adequacy

• Repeat ultrasound with optimized technique using intercostal approach in multiple patient positions (supine, lateral decubitus, prone) to ensure the kidney was not simply missed due to bowel gas, body habitus, or suboptimal scanning windows 1, 3
• The left kidney lacks a hepatic acoustic window and requires careful intercostal scanning between the anterior axillary line and posterior midline 1

Obtain Cross-Sectional Imaging

CT urography is the gold standard for evaluating a non-visualized kidney, as it can:

• Differentiate between renal agenesis, severe atrophy, ectopic kidney, or a small nonfunctioning kidney 1, 2
• Detect hydronephrosis, masses, or anatomic abnormalities that ultrasound may have missed 1, 4
• Provide detailed assessment of the collecting system and surrounding structures 1

If CT is contraindicated (chronic kidney disease, contrast allergy):

• MR urography is the recommended alternative 1
• For patients with contraindications to both CT and MRI, consider non-contrast CT combined with retrograde pyelography 1

Clinical Context Considerations

If Hematuria is Present

Proceed with complete hematuria workup regardless of the non-visualized kidney 1, 5:

• CT urography evaluates both the missing kidney AND screens for urothelial malignancy in the visualized kidney and collecting system 1
• White light cystoscopy is mandatory to evaluate the bladder 1
• The contralateral kidney requires thorough evaluation as it bears the entire renal function burden 1, 5

If Renal Insufficiency is Present

• Assess whether the visualized right kidney shows compensatory hypertrophy (suggests chronic absence of left kidney) or is normal-sized (suggests acute process) 6, 7
• Obtain serum creatinine and estimated GFR to establish baseline renal function 1
• Avoid iodinated contrast if acute kidney injury is present unless the diagnostic benefit outweighs risk; consider MR urography with Group II gadolinium agents at lowest diagnostic dose 1

If Flank Pain or Suspected Obstruction

• CT urography remains first-line to identify ectopic kidney, obstructed kidney, or alternative pathology causing symptoms 1, 4
• Color Doppler ultrasound with resistive index (RI) measurement can help differentiate obstructive from non-obstructive processes if kidney is eventually located 4

Differential Diagnosis to Consider

The non-visualized kidney may represent 2, 6:

• Congenital renal agenesis (most common unilateral cause)
• Severe chronic kidney disease with atrophic kidney (small, echogenic, difficult to visualize)
• Ectopic kidney (pelvic, crossed fused, thoracic locations)
• Prior nephrectomy (verify surgical history)
• Complete hydronephrosis with non-functioning kidney
• Technical factors (obesity, bowel gas, inadequate scanning technique)

Critical Pitfalls to Avoid

• Do not assume congenital absence without definitive imaging – an ectopic or severely diseased kidney requires different management 2
• Do not delay evaluation if hematuria is present – the visualized kidney and collecting system must be thoroughly evaluated for malignancy regardless of the missing kidney 1, 5
• Do not perform renal venography as first-line imaging – this outdated approach has been replaced by CT/MRI 2
• In patients with risk factors for urothelial cancer (smoking >30 pack-years, age ≥60, occupational exposures), ultrasound alone is insufficient and must be combined with cystoscopy and cross-sectional imaging 1

Special Populations

Neurogenic Bladder Patients

• Obtain upper tract imaging of the visualized kidney annually if high-risk neurogenic lower urinary tract dysfunction is present 1, 8
• Assess for hydronephrosis, stones, and renal parenchymal changes in the solitary functioning kidney 1

Pediatric or Pregnant Patients

• Optimize ultrasound technique first before proceeding to CT, as radiation exposure is particularly concerning 1
• MR urography without gadolinium may be appropriate if ultrasound remains non-diagnostic 1