I am helping out a doctor with some research and have on one sheet a list of
data (laid out in columns such as patient number, patient name, date of
birth, date of test, test 1 results, etc) and then in the second sheet I have
a table and a graph to display one patient's results.

I want it so that I can just enter the patient's number in and all the
results for that patient come up. I have tried with the DGET and the VLOOKUP
formulas and am having trouble with both; it is returning seemingly 'random'
results no related to what I am looking up.

What is the best method to do this, and what might I be doing wrong?

I have used formula's like this:
=LOOKUP(B3,TESTS!A:A,TESTS!B:B)
and like this
=DGET('Patient Results'!A2:M158,11,A1:A2)

also there is going to be results added continually to these files so there
isn't a definite range; how do you get around that?

thanks heaps!