Patients with chronic kidney disease (CKD) are at higher cardiovascular risk than the general population. Cardiovascular diseases, vascular calcification among them, are the leading cause of death in these patients. Factors influencing vascular calcification are oxidative stress, inflammation, and accumulation of uremic toxins during CKD. Uric acid is a cardiorenal toxin that accumulates in the case of kidney malfunction. The primary therapy for replacing kidney function and removing toxins from end-stage renal disease patients is hemodialysis. Effective removal of toxins can be estimated by blood or dialysate lab analysis or optical monitoring. In this study, the authors tested a miniaturized optical sensor for monitoring uric acid levels and removal for the first time in a more extensive clinical study, including Hemodialysis (HD) and Post-dilutional online hemodiafiltration (HDF) procedures with different settings in Tallinn, Estonia. The results (Mean±SD, Lab vs. Sensor) of the uric acid concentration 57.20±34.05 vs. 57.22±33.09 µmol/L, reduction ratio 68.72±10.91 vs. 67.89±12.48 %, and total removed amount 7.00±2.10 vs. 7.33±2.29 mmol did not differ significantly from the values obtained from the clinical laboratory (p<0.05).Clinical Relevance-During this study, a miniaturized optical sensor was tested for the first time in the clinic in different dialysis settings. The results confirm that the sensor is reliable for regularly monitoring cardiorenal toxin uric acid removal during hemodialysis.