Urea / BUN

When your body uses protein, it produces ammonia, which is toxic. Your liver turns that ammonia into urea, a harmless molecule, and releases it into your blood.

Your kidneys then filter most of that urea out into your urine. So the level you see is a balance of how much you make and how well your kidneys clear it. BUN, or blood urea nitrogen, measures just the nitrogen part of the same molecule.

This number reads your kidney clearing and your hydration at the same time. When it runs high, it can be an early sign your kidneys aren't filtering as well as they could, which is useful to catch early. Dehydration, a high-protein diet, bleeding in your gut, or heart failure can push it up too, so it's usually read alongside creatinine, another kidney marker, rather than on its own. A low value is less common and usually harmless, often pointing to a low-protein diet or extra fluids.

The upside: much of what moves this number is modifiable, and an early read gives you and your clinician time to look closer. Treat any single result as a clue to investigate, not a diagnosis. It means the most read together with your other kidney markers.