If you are interested in a particular country, you should use national poverty lines which are defined according to each country’s specific economic and social circumstances. The national poverty lines are typically lower in poorer countries and higher in richer countries.
If you are interested in comparing poverty measures across countries, you should use international poverty lines. The international poverty lines attempt to hold the real value of the poverty lines consistent across countries by accounting for differences in purchasing power across countries.
For additional details on international poverty lines, see the introduction and a list of publications from PovcalNet: the on-line tool for poverty measurement developed by the Development Research Group of the World Bank.