⁶⁸Ga-FAPI renal cortical uptake: a diagnostic and predictive biomarker for kidney disease in PET/CT imaging

Previous studies indicate that renal fibroblast activation protein expression inversely correlates with kidney function and that gallium-68 fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) positron emission tomography/computed tomography (PET/CT) can visualize renal fibrosis. However, its diagnostic and prognostic utility in kidney disease has not been investigated. This study aimed to further explore the correlation between increased ⁶⁸Ga-FAPI uptake in the renal cortex and kidney disease, focusing on its diagnostic and predictive capabilities to aid clinicians in image interpretation.

Patients who underwent whole-body ⁶⁸Ga-FAPI PET/CT examinations at our center from February to December 2022 were retrospectively enrolled. Spearman correlation analysis assessed the relationship between estimated glomerular filtration rate (eGFR) and maximum standardized uptake values (SUVmax) of renal cortex. The diagnostic performance of ⁶⁸Ga-FAPI PET/CT for kidney disease was evaluated using the receiver operating characteristic (ROC) curve and its area under the curve (AUC), alongside sensitivity, specificity, accuracy, positive predictive value (PPV), and negative predictive value (NPV). Predictive analysis was conducted using Cox proportional hazards model and Kaplan-Meier survival analysis. A P value <0.05 was considered statistically significant.

A total of 145 patients (82 men; mean age: 59.3±11.27 years; range, 23-83 years) were included. The SUVmax of renal cortex and eGFR was negatively correlated (R²=0.276). The AUC of SUVmax of renal cortex in diagnosing kidney diseases was 0.946 [95% confidence interval (CI): 0.908-0.983]. A renal cortex SUVmax threshold of 2.00 demonstrated a diagnostic sensitivity of 94.4% (17/18) and an NPV of 96.4% (27/28) for kidney disease, whereas the accuracy was 68.8% (44/64). The increased renal uptake of ⁶⁸Ga-FAPI is an independent risk factor for kidney disease. Non-kidney disease patients with an increased renal uptake of ⁶⁸Ga-FAPI have a higher incidence of kidney disease in the immediate future compared to non-kidney disease patients without increased renal uptake, which was 46.2% versus 0.0% (P<0.001).

The concentration of ⁶⁸Ga-FAPI in renal cortex is negatively correlated with eGFR. FAPI PET/CT is highly effective in the diagnosis and prediction of kidney diseases.