What is the significance of total kidney volume on CT in adults with known or suspected kidney disease, such as polycystic kidney disease (PKD)?

Total Kidney Volume on CT in Adults with Kidney Disease

In adults with autosomal dominant polycystic kidney disease (ADPKD), total kidney volume (TKV) measured on CT is a critical prognostic biomarker that predicts future kidney function decline and timing of kidney failure, and should be routinely measured using height-adjusted TKV with the Mayo Imaging Classification system. 1

Primary Clinical Significance in ADPKD

The KDIGO 2025 guidelines recommend employing the Mayo Imaging Classification (MIC) to predict future decline in kidney function and timing of kidney failure in ADPKD patients. 1 The MIC uses height-adjusted total kidney volume (htTKV), adjusted for age, to stratify patients with typical imaging findings into 5 prognostic groups (1A–1E), indicating accelerating decline in kidney function. 1

Measurement Methodology on CT

• Height-adjusted TKV is measured most accurately by CT or MRI using an automated or semiautomated tool, or alternatively can be estimated using the ellipsoid equation. 1
• CT-based TKV measurements using ultra-low-dose protocols (CT dose index 1.2 mGy) with ellipsoid equations demonstrate excellent correlation with the reference standard of MRI planimetry (r² > 0.98), with root mean square error of only 5.9-11.5% of mean TKV. 2
• The ellipsoid method requires only 4.6-5.2 minutes compared to 27.7 minutes for manual planimetry, making it clinically practical. 2
• Manual planimetry and stereology methods provide the highest accuracy and reproducibility, particularly on CT images, and are preferred when assessing drug treatment effects in clinical trials. 3

Prognostic Value and Clinical Application

• The MIC should only be used in patients with typical ADPKD (class 1) who have pathogenic variants in PKD1 or PKD2 genes—it should not be used in patients with pathogenic variants in other genes. 1
• Kidney volume has been accepted by the US Food and Drug Administration as an enrichment biomarker and surrogate outcome for trials in ADPKD. 1
• In patients with early ADPKD (eGFR >60 mL/min/1.73m²), htTKV >1,000 mL/m is associated with increased pain and quality of life impairment. 4
• TKV measurement is integral to clinical management decisions, including eligibility for disease-modifying therapy with tolvaptan in patients at risk of rapid progression. 5

Significance in Non-ADPKD Kidney Disease

Limited Diagnostic Value in General CKD

• In chronic kidney disease without polycystic disease, CT is generally not considered a first-line imaging modality for initial evaluation. 1
• Unenhanced CT is useful for determining kidney size/volume and evaluating hydronephrosis, but ultrasound is typically preferred as the initial modality unless US is not feasible or nondiagnostic due to body habitus. 1
• CT can determine if there is hydronephrosis and assess renal size/volume, but provides limited information about the etiology of CKD. 1

Specific Clinical Scenarios Where CT TKV Assessment Adds Value

• Unenhanced CT is the most sensitive modality for detecting urinary tract calculi and determining the level and cause of obstruction in patients with hydronephrosis. 1
• In patients with renal colic and moderate to severe hydronephrosis on ultrasound, CT can identify stone characteristics to guide surgical management planning. 1
• CT may be considered when ultrasound is nondiagnostic or when evaluating for renal artery stenosis (found in 4.3% of CKD patients), though MRA or Doppler ultrasound are alternative options. 1

Practical Implementation Algorithm

For Suspected or Known ADPKD:

1. Confirm diagnosis through genetic testing (PKD1/PKD2 variants) or imaging criteria (>10 cysts on MRI for positive family history). 1
2. Obtain baseline CT or MRI to measure htTKV using automated/semiautomated tools or ellipsoid equation. 1
3. Apply Mayo Imaging Classification only if patient has typical ADPKD (class 1) with PKD1/PKD2 variants. 1
4. Stratify into MIC classes 1A-1E based on htTKV/age to predict kidney function decline trajectory. 1
5. Use TKV data to guide decisions about intensive blood pressure control, lifestyle modifications, and eligibility for tolvaptan therapy. 5

For General CKD Evaluation:

1. Start with ultrasound to assess kidney size, echogenicity, and presence of hydronephrosis. 1
2. Reserve CT for situations where ultrasound is inadequate (body habitus), or when specific information is needed (stone disease, level of obstruction). 1
3. Use unenhanced CT when possible in patients with renal impairment to avoid contrast-induced nephropathy risk. 1, 6
4. Consider contrast-enhanced CT only when diagnostic information is essential and cannot be obtained otherwise (mesenteric ischemia, vascular thrombosis), using minimum necessary contrast dose with adequate hydration. 6

Critical Pitfalls to Avoid

• Do not apply the Mayo Imaging Classification to patients with atypical ADPKD (class 2) or those with pathogenic variants in genes other than PKD1 or PKD2—the prognostic information is not valid in these populations. 1
• Do not use ultrasound-determined TKV as a substitute for CT/MRI-based measurements when precise prognostication is needed—ultrasound measurements are less precise, though they retain some prognostic value. 1
• Do not order contrast-enhanced CT in CKD patients without first calculating eGFR and considering non-contrast alternatives—the risk-benefit ratio must be carefully assessed. 1, 6
• Do not assume kidney volume alone determines symptoms in early ADPKD—pain is not related to kidney size except in patients with very large kidneys (htTKV >1,000 mL/m). 4
• Do not use simplified measurement methods (mid-slice, kidney length alone) when monitoring disease progression or assessing treatment effects—these provide only rough estimates and lack the precision needed for clinical decision-making. 3