We present the development and performance evaluation of wearable passive UHF RFID tags based on slotted patch antennas that are electromagnetically optimized for operation in the close proximity of the human body. The antennas comprise electro-textile conductor on a light-weight and conformal textile substrate. The required patch-to-ground interconnections are formed by means of sewing with conductive yarn and by wrapping the electro-textile over the edges of the substrate. Our results show that the slotted patch tags on 2 mm and 4 mm substrates achieve high read ranges of 3.2 and 8.2 m, respectively, when affixed to the upper back.