Background: A previous analysis of 161 patients (pts) tested for nuclear-localized AR-V7(+) CTCs showed a therapy interaction between AR-V7 positivity and improved overall survival (OS) on taxane chemotherapy vs. androgen receptor signaling inhibitors (ARSI). To validate the use of the biomarker result for physician decision making, we prospectively analyzed an independent, multicenter, blinded, cross-sectional cohort (n = 225) to confirm a therapy interaction with AR-V7. We corrected for possible pt selection bias by the treating physician by analyzing the association of therapy to OS in low and high risk groups defined by the test cohort. Methods: Two analyses were performed: (1) the validation of a therapy interaction between AR-V7 positivity and superior OS benefit for pts treated with taxanes in the context of use for 2^nd+ line pts; and (2) as the choice between ARSI or taxanes was at the discretion of the attending physician, pt risk was incorporated into the predictive biomarker assessment. A pt-specific risk score was developed from line of therapy and other covariates to stratify pts as low and high risk, and the association of AR-V7 status and OS was performed within each risk group to correct for physician decision making and address possible confounding with treatment assignment. Results: (1) A therapy interaction between AR-V7(+) pts and lower risk of death on taxanes vs. ARSI (HR: 0.23, p = 0.003, 95% CI: 0.09 – 0.61) was validated. (2) In the validation cohort, high risk AR-V7(+) pts had an OS benefit when treated with taxanes (p = 0.02) and the AR-V7(-) pts had an improved OS with ARSI therapy (p = 0.02). AR-V7 incidence was low in the low risk pts, precluding the analysis for this sub-cohort. Conclusions: The results validate that the nuclear-localized AR-V7(+) biomarker has an interaction with therapy and improved survival on taxanes. Further, when adjusting for pt risk, the biomarker is predictive of OS in the high risk group. Nuclear-localized AR-V7 protein in CTCs can aid in the decision between ARSI and taxane chemotherapy in the 2^nd or greater line of therapy for mCRPC.