Interstitial Cystitis/Bladder Pain Syndrome (IC/BPS) is a complex chronic condition characterized by bladder pain, hypersensitivity, and urinary symptoms. Accurate assessment of bladder capacity is crucial for understanding disease pathophysiology. Functional Bladder Capacity (FBC) derived from a voiding diary, reflects real-life bladder function and may serve as a surrogate for sensory bladder capacity. This study evaluates the clinical significance of FBC compared to maximum bladder capacity (MBC) and cystometric bladder capacity (CBC) in IC/BPS patients.

A retrospective cohort study was conducted on 198 non-Hunner lesion IC/BPS patients treated with intravesical platelet-rich plasma injections. Bladder capacity was assessed using a three-day voiding diary (FBC), video urodynamic study (CBC), and cystoscopic hydrodistention under anesthesia (MBC). Pearson correlation and multiple linear regression analyses examined the associations between bladder capacities, symptom severity, and treatment outcomes.

FBC significantly correlated with MBC (r =0.301, p <0.01), symptom severity (ICSI, r =-0.360, p <0.01), and treatment outcome (GRA_6M, r =0.251, p <0.01), whereas CBC showed a weaker correlation with clinical parameters. Multiple regression analysis identified full sensation (β =0.357, p =0.044), MBC (β =0.292, p =0.004), and treatment outcome of GRA (β =0.189, p =0.044) as significantly associated with FBC.

FBC is a clinically relevant measure of bladder function and symptom burden in IC/BPS, demonstrating strong associations with MBC and treatment outcomes. Given its non-invasive and well-founded, FBC may serve as a primary evaluation tool, reducing the need for invasive procedures while guiding personalized treatment strategies. Further prospective studies are needed to validate these findings.