Blood pressure

Watched a few episodes of ER, and started wondering: how do doctors interpret the blood pressure levels if they do not know a particular patient's baseline? Isn't, for example, 90/60 normal for some people but low for most? How do they know whether 80/50 is somebody who is normally 90/60 and had a little drop in blood pressure or somebody who is normally 140/100 and had a big drop? Are they interested in the actual levels much at all (unless they are extreme), or are they measuring it to see whether it's stable or going up or down?