@MissDaphne - I can't think why it would be a different test. The urine test for protein is the best way to diagnose preeclampsia; I think high protein in the urine plus high blood pressure is the definitive diagnosis.
What it basically does is narrow your blood vessels, which is why your blood pressure goes up. I'm not sure why that causes protein in the urine, but I'm guess that it's because your kidneys and/or liver don't work as well without enough blood getting to them? The article mentions protein in the urine being a sign of kidney problems.