I am sorry you are going through this.
Technically, the diagnostic criteria for preeclampsia is two readings of 140/90 or higher, and 300 mg of protein or more in a 24 hour urine. So you meet that if you have had readings of 140/90 twice, regardless of when/where. Being dehydrated and/or sick shouldn't necessarily cause you to spill protein, and someone can have 24 hour urines of something like 300, then 100, then 500, then 10000, then 200. (Not really literally, just throwing out numbers as an example.) Anyway, going from 300 to 100 doesn't *necessarily* mean it's a lot better, next week it could be 300 again or even higher.
Has your blood pressure always been high at the doctor but now it's even higher? Have you ever calibrated your blood pressure machine with the machine at the doctor's office? How often do you take your blood pressure at home? It may be that your machine is not as accurate. It may also be true that your blood pressure is higher at the doctor. But that may also mean it gets higher when you are at home and slightly stressed, and if it is regularly or frequently or easily getting up to 140/90 or higher throughout the course of a normal day (which would not really be good for your baby) and you just aren't catching it.
How high is your blood pressure at the doctor's office? Do they retake it there to see if it gets lower/normal after a bit? Have you had swelling/headaches/other symptoms? Have you had an ultrasound lately? I am just wondering if there aren't more symptoms/issues that the doctor may be worried about...they don't typically recommend hospitalization lightly. Also, if you really think it is nothing, it may be easier to just go in the hospital for a day so they can see your blood pressure over a period of a day or so, if it's really just high from white coat syndrome then it should be ok if you are in a hospital bed for awhile and then they can see that it's fine and relax.
Anyway, good luck, I hope you really don't have pre-e but please take any symptoms seriously (headaches, swelling, visual disturbances, right upper quadrant pain, etc.) as it can be very serious and get bad quickly.
Me (29) DH (30)
#1-Olivia Caetlyn-9-28-09-9-28-09, 23+2 wks, emergency classic c-section, class I HELLP, IUGR
#2- Lucas Oliver (rainbow baby)- April 2011, 36+2 wks, HELLP and pre-e free! (lovenox and LDA pregnancy)
#3-Matthew, late October 2012...mostly normal, 37 wks, (lovenox and LDA again)
My blog: http://www.butterflies-and-rainbows.blogspot.com/