While purchasing power parity is certainly relevant, being poor in the US (by the official US definition) is still better than being poor almost anywhere else.

This paper goes so far as to argue that, in many ways, the average poor American lives at the same level or better (materially speaking) than an European middle class person:

http://www.timbro.se/bokhandel/pdf/9175665646.pdf

Also, note that the official definition's minimum yearly income doesn't include welfare benefits.