Urine specific gravity

How concentrated your urine is, compared with plain water.

It is the simplest check on hydration, and it is what tells you whether the rest of a urine result is diluted or concentrated.

It says more about how much you drank than about your kidneys, and a first-morning sample is naturally the most concentrated of the day. Read every other urine value in light of it -- a dilute sample makes everything else look low.

Worth mentioning a value that stays very low even when you are not drinking much.