A urinary albumin (microalbumin) test in a random urine sample measures the precise amount of albumin protein excreted in the urine. Normally, healthy kidney glomeruli prevent albumin (a large molecule) from passing into the urine. The presence of small amounts (30–300 mg/g creatinine) indicates microalbuminuria, the first and most sensitive sign of damage to the lining of the small blood vessels in the kidneys (nephropathy) and blood vessels in general, particularly in diabetic patients.