The purpose of this study is to develop and characterize a surface plasmon resonance SPR-based assay that can specifically detect binding of APF to its cellular receptor, cytoskeleton associated protein 4 CKAP4, immobilized on a sensor chip surface and to test the ability of this SPR-based assay to discriminate and measure the concentration of APF in urine from well-defined IC patients vs. age-matched, asymptomatic controls. Our results demonstrate that we have successfully developed an SPR assay using a CKAP4127-360 biosensor with sufficient binding efficiency to detect as-APF in urine with detection limits in the high nM to uM range. Urine specimens from 14 47 of 30 women diagnosed with ICPBS demonstrated as-APF binding activity to the CKAP4127-360 biosensor compared with 22 73 of 30 asymptomatic control women. Thus, urine specimens from women with IC were less likely to demonstrate significant APFCKAP4 binding activity 47 than specimens from asymptomatic control women 73 . When compared to cellular proliferation assay results, agreement with clinical diagnosis is decreased by the SPR method vs. cellular proliferation method 47 vs. 77 .Importantly, the correlativity between both assays was 58 35 of 60 samples, suggesting that the SPR method has potential utility as a non-invasive means for diagnosing IC in women. The observation that APF activity is detectable in urine specimens from 47 of asymptomatic control women by the cellular proliferation assay and 73 of asymptomatic control women by the SPR binding assay, suggests that other urinary constituents may be contributing to the measured response. Using a targeted mass spectroscopy method, we determined that APFas-APF were present in the pM to nM range in all urine samples regardless of patient diagnosis control vs. IC suggesting that both peptides may exist at basal levels in normal bladder physiology.