Microalbuminuria

Excerpt

The main functions of albumin are to maintain plasma oncotic pressure via its negatively charged surface and colloidal nature, provide nutrition to renal tubular cells, and serve as an antioxidant. Hepatocytes produce approximately 10-15 g of albumin daily, which is regulated by interstitial colloidal pressure. Albumin exits the blood and is reabsorbed by the lymphatic system at a rate of 4.5% per hour.

There are many barriers to albumin within the glomerular filtration system of the nephron. At physiological pH, the glomerular capillary wall and endothelial cells repel albumin, as they are all negatively charged. The glomerular basement membrane (GBM) is a porous system, but normally these exits are too small to permit the passage of albumin. Additionally, the megalincubulin complex degrades albumin in the nephron, specifically the proximal convoluted tubule. The underlying function is to preserve amino acids for further use but is also another method of restricting the passage of albumin.

Through the dysfunction of the GBM filtration barrier, albumin can be secreted into the urine, and the amount that is present is important. The current definition of microalbuminuria (MA) is an amount of urinary albumin greater than the normal value but also lower than what is detected by a conventional dipstick. Thus, the rate of urine albumin excretion (UAE) in microalbuminuria is 30 to 300 mg/24 hours. In other units, it can also mean 30–300 mcg/mg creatinine or 20–200 mcg/min on two out of three urine collections. This value is derived from studies that evaluated adults but could also be applied to the pediatric population. Macroalbuminuria, on the other hand, is classified as greater than 100 mg/12 hours or 300 mg/24 hours. The diagnosis of diabetic kidney disease requires a person with type 1 or 2 diabetes to have persistently elevated albuminuria (more than 300 mg/24 hours), diabetic retinopathy, and the absence of other kidney diseases.

The current diagnosis of microalbuminuria also includes a urinary albumin/creatinine ratio (UACR) ranging between 2 to 20 mg/g. By including creatinine, it corrects the value for urine concentration and volume. However, other factors can affect the level of UACR, including gender, race, blood pressure, time of day, muscle mass, and amount of food, water, and salt intake. Thus, UACR can vary by up to 40% daily. In addition to the individual variability, one should be cautious that some cases have an elevated UACR at baselines, such as males, African Americans, Asians, smokers, people with higher muscle mass, patients with urinary tract infections, and genital leakage. Due to the considerable variation, one should obtain three UACR measurements that are each one month apart.