The point, I believe, is that one's blood volume status (i.e., presence or absence of hypovolemia) cannot be measured based on urine output in such patients. If the patient were hypovolemic, the condition would have to be measured in another way.

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Note added at 6 mins (2009-06-02 14:50:25 GMT)
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NB: "Volume status" does not refer to level of hydration, as one's level of hydration definitely can be measured by urine output.

Here is a useful paper on the topic: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=12...