One exchange rate is BTC - USD. That does not make the currency itself USD denominated. Bitcoin is almost always denominated in BTC these days. There are other bitcoin denominations, like the Satoshi [1], but these are rarely used as BTC has not appreciated enough in value for them to be useful. You seem to be confused about what denomination means in the context of currency, see [2].
1. https://en.bitcoin.it/wiki/FAQ#What_do_I_call_the_various_de... 2. http://en.wikipedia.org/wiki/Denomination_(currency)