Albumin is a type of protein found in large amounts in the blood. Because it is a small molecule in size, it is one of the first proteins able to pass through the kidneys into the urine when there are kidney problems.This presence of small amounts of albumin in the urine is the condition called microalbuminuria.lbumin is a type of protein found in large amounts in the blood. Because it is a small molecule in size, it is one of the first proteins able to pass through the kidneys into the urine when there are kidney problems.