Introduction: Emerging data suggest a beneficial ancillary effect of on-treatment follow-up urine testing on medication adherence among patients selectively prescribed preventative pharmacological therapy (PPT) for urinary stone disease (USD). While prior studies have investigated the association of on-treatment follow-up testing with medication adherence, none so far have examined causation. We aimed to estimate the ancillary causal effect of follow-up, on-treatment 24-hour urine testing on adherence to PPT. Methods: Using claims data from working-age, commercially insured adults with USD (2008 to 2019), we identified those prescribed PPT (a thiazide diuretic, alkali citrate therapy, or allopurinol) after receiving a baseline 24-hour urine test in the 180 days prior to the initial PPT fill. Within this group we identified patients that 1) received on-treatment follow-up 24-hour urine testing from 90 to 180 days after initiating PPT and 2) received no on-treatment follow-up testing. We defined PPT adherence based on percentage days covered using pharmacy claims and calculated it in four, non-overlapping 90-day intervals in the 0- to 360-day period following the initial prescription fill. We applied three established causal effect estimators to measure the impact of follow-up testing on medication adherence. Results: Among a cohort of 1,361 patients, 1,206 (88.6%) completed a baseline collection only, and 155 (11.4%) completed both baseline and on-treatment follow-up testing from 90 to 180 days since initiating PPT. At the end of the first 90 days of the prescription, patients that received no on-treatment testing had lower average medication adherence compared with patients that did (65.0% vs. 75.0%, P <0.001). After adjustment for demographic factors and baseline adherence in the 0- to 90-day period since initial prescription fill, we observed a pooled, statistically significant 12.1% increase in medication adherence caused by follow-up testing across three estimators (Fig 1). Conclusions: Obtaining a follow-up 24-hour urine test after initiating PPT had a statistically significant spillover causal effect to improve patient adherence to PPT. On-treatment 24-hour urine collections can not only indicate response to therapy; they can also motivate the patient to adhere to medication. SOURCE OF Funding: NIH 5R01DK121709-03