Originally Posted by evavar
When you increase the upper limit this will increase specificity and will automatically increase PPV.
When you decrease the upper limit this will increase sensitivity and will automatically increase NPV.
This does not sound completely accurate to me. From what you've said, if you increase the upper limit and then decrease it back down to baseline, you have just increased the specificity and increased the sensitivity of the test, despite the fact that there is no actual change.
Could the OP give more information as to what "upper limit" he is talking about? Most diagnostic tests have a lower limit cutoff for positive or negative (i.e. there is enough of a substance, therefore the test is positive). I am not familiar with many situations where a one will say "there is too much of a substance, therefore the test is negative"