What is the most appropriate screening test for a 27-year-old man with a sister who has autosomal dominant polycystic kidney disease?

Ultrasound Abdomen is the Most Appropriate Screening Test

For a 27-year-old man with a sister who has ADPKD, abdominal ultrasound is the recommended first-line screening test. 1

Rationale Based on 2025 KDIGO Guidelines

The 2025 KDIGO Clinical Practice Guideline explicitly recommends abdominal imaging by ultrasound to screen adults at risk of ADPKD who have a positive family history 1. This patient fits this exact scenario—he is an at-risk adult (27 years old) with a first-degree relative (sister) affected by the disease.

Age-Specific Diagnostic Criteria

For this 27-year-old patient, the ultrasound diagnostic criteria are straightforward:

• ≥3 total kidney cysts confirms ADPKD (ages 15-39 years)
• ≤1 cyst rules out ADPKD (ages 15-39 years)
• Equivocal findings (2 cysts total) would warrant follow-up with MRI or genetic testing 1

Why Not the Other Options?

CT abdomen is not appropriate for initial screening because:

• It exposes the patient to unnecessary ionizing radiation
• It is reserved for detecting complications (hemorrhage, infection, rupture) rather than diagnosis 2
• While CT can be used as an alternative to MRI for equivocal cases, it is not the first-line screening modality 1

Urine microscopy has no role in screening or diagnosing ADPKD—it does not detect cysts and is not mentioned in any diagnostic algorithm 1

Anti-polycystin 1 antibodies do not exist as a clinical diagnostic test. ADPKD is diagnosed through imaging or genetic testing for PKD1/PKD2 gene mutations, not through antibody testing 1

Clinical Algorithm for This Patient

1. First step: Renal ultrasound (non-invasive, no radiation, cost-effective)
2. If ≥3 cysts detected: ADPKD diagnosis confirmed—proceed to clinical management and prognostication
3. If 0-1 cyst: ADPKD effectively ruled out at this age
4. If 2 cysts (equivocal): Consider MRI (>10 cysts confirms, <5 cysts rules out for ages 16-40) or genetic testing for PKD1/PKD2 mutations 1

Important Caveats

Pre-test counseling is essential before performing any screening test 1. The patient should understand:

• The implications of a positive diagnosis (insurance, employment, psychological impact)
• The benefits of early diagnosis (blood pressure monitoring, lifestyle modifications, potential for disease-modifying therapy)
• The option to defer testing if he prefers not to know his status

Genetic testing is reserved for specific situations in this context 1:

• Equivocal imaging findings
• If he were being considered as a living kidney donor
• Family planning decisions
• If the family's genetic variant is already known (targeted testing is simpler)

Diagnostic Performance

Ultrasound has excellent diagnostic accuracy in this age group. High-resolution ultrasound achieves approximately 97% sensitivity and 98% specificity for detecting ADPKD in at-risk individuals aged 16-40 years 3. MRI offers 100% sensitivity and specificity with >10 cysts as the threshold, but is more expensive and not necessary as first-line screening 3.

The bottom line: Proceed with renal ultrasound as the initial screening test, ensuring appropriate pre-test counseling about the implications of testing.