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Background: Skin temperature measurements after peripheral nerve block can be used as an easy and objective method to help predict block success. Thermochromic nail polishes are popular cosmetic products especially among young women. The colour change of nail polish is based on a thermochromic reaction as the temperature changes., Objective: The aim of this study was to test the hypothesis that the success of infraclavicular brachial plexus blocks (IBPBs) can be predicted by the colour change in thermochromic nail polish, which depends on skin temperature changes., Design: A prospective cohort study., Setting: Training and research hospital from December 2018 to March 2019., Patients: A total of 50 patients who received IBPB for forearm, wrist or hand surgery were included., Main Outcome Measures: Thermochromic nail polish was applied to the nails of both hands of all patients before the block. Reaction of the nail polish in both hands was photographed immediately after application of nail polish and at 30 min after performing the block. The digital photographs of each patient were evaluated by observers. To evaluate the validity of the colour change in nail polish in predicting a successful IBPB, sensitivity, specificity, positive and negative predictive values and positive and negative likelihood ratios were estimated., Results: The positive predictive value for colour change in thermochromic nail polish predicting a successful IBPB was 96% [95% confidence interval (CI) 90 to 98] and sensitivity was 94% (95% CI 87 to 97). Fleiss kappa value showed substantial agreement (0.76; 95% CI 0.59 to 0.93) in the assessment of interobserver agreement., Conclusion: The current study demonstrates that the colour change in thermochromic nail polish is a valid and reliable indicator for the prediction of block success., Trial Registration: ClinicalTrials.gov identifier: NCT03767868.