Passive ultra-high frequency (UHF) RFID systems are conventionally assumed to be downlink limited due to the relatively high power consumption of a tag’s integrated circuit (IC). In this work, it is demonstrated that under certain conditions, tags’ uplink/downlink losses become significantly asymmetrical, causing tens of decibels more loss in the uplink, thus making the system uplink limited. Our study reveals the necessity of developing RFID readers with higher sensitivity. It is shown that in extreme cases, the uplink loss can be 40 dB greater than in the downlink.