Blood urea nitrogen

The nitrogen part of urea, which is the same waste product reported on a different scale -- roughly half the urea number.

It carries exactly the meaning urea does; which of the two your report prints is a matter of local convention, not of what was measured.

Never chart both as separate lines: they are one measurement, and a series that switches between labs will step. Its ratio to creatinine is the part clinicians actually read, and dehydration is what usually raises it.

Worth discussing together with creatinine rather than on its own.