Hypoalbuminemia

Excerpt

In adult humans, albumin is the most abundant plasma protein with a concentration ranging from 35 to 50 g/L. Albumin represents 50% of the total protein content of plasma, with globulins making up most of the rest. It is a single peptide chain of 585 amino acids in a globular structure. The molecular weight of albumin is approximately 66 kDa, and it has a half-life of 21 days. Albumin is exclusively synthesized by the liver, initially a pre-proalbumin and then proalbumin, which in the Golgi apparatus is converted to albumin, which is the final form secreted by the hepatocyte. The synthetic rate is about 10 to 15 grams per day and then secreted into the circulation of which around 40% remains in circulation with a fraction moving from the intravascular to the interstitial space. Factors that stimulate albumin synthesis include the action of hormones such as insulin and growth hormone. Albumin production may be inhibited by pro-inflammatory mediators such as interleukin-6 (IL-6), interleukin-1 (IL-1) and tumor necrosis factor. In fetal life, alpha-fetoprotein (AFP) produced by the liver and yolk sac is the most abundant plasma protein. AFP is thought to be the fetal counterpart of albumin, and both are transcribed by genes located close together on chromosome 4. Approximately 100 variant forms of albumin have been described.

Albumin has several physiological roles. One of the most important is to maintain the oncotic pressure within the vascular compartments preventing leaking of fluids into the extravascular spaces. It accounts for around 80% of the colloid osmotic pressure. Additionally, albumin functions as a low-affinity, high-capacity carrier of several different endogenous and exogenous compounds acting as a depot and a carrier for these compounds. Binding of compounds to albumin may reduce their toxicity such as in the case of unconjugated bilirubin in the neonate and drugs. Also, albumin binds at least 40% of the circulating calcium and is a transporter of hormones such as thyroxine, cortisol, testosterone, among others. Albumin also is the main carrier for fatty acids and has significant anti-oxidant properties. Albumin is also involved with maintaining acid-base balance as it acts as a plasma buffer. Albumin is used as a marker of nutritional status and disease severity in particular in chronic and critically ill patients.,,. Renal and gut loss of albumin may account for around 6% and 10% respectively of albumin loss in healthy individuals. A decrease in serum albumin levels below the reference interval hypoalbuminemia. This article reviews the causes and diagnosis of hypoalbuminemia.