Albumin, the body's predominant serum-binding protein, has several important functions.
Albumin comprises 75-80% of normal plasma colloid oncotic pressure and 50% of protein content. When plasma proteins, especially albumin, no longer sustain sufficient colloid osmotic pressure to counterbalance hydrostatic pressure, edema develops.
Albumin transports various substances, including bilirubin, fatty acids, metals, ions, hormones, and exogenous drugs. One consequence of hypoalbuminemia is that drugs that are usually protein bound are free in the plasma, allowing for higher drug levels, more rapid hepatic metabolism, or both.
Alterations in albumin level affect platelet function.
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Reference serum values range from 3.5-4.5 g/dL, with a total body content of 300-500 g. Synthesis occurs only in hepatic cells at a rate of approximately 15 g/d in a healthy person, but the rate can vary significantly with various types of physiologic stress. The half-life of albumin is approximately 21 days, with a degradation rate of approximately 4% per day.
Hypoalbuminemia is a common problem among persons with acute and chronic medical conditions. At the time of hospital admission, 20% of patients have hypoalbuminemia. Hypoalbuminemia can be caused by various conditions, including nephrotic syndrome, hepatic cirrhosis, heart failure, and malnutrition; however, most cases of hypoalbuminemia are caused by acute and chronic inflammatory responses.
Serum albumin level is an important prognostic indicator. Among hospitalized patients, lower serum albumin levels correlate with an increased risk of morbidity and mortality.
The presentation, physical examination findings, and laboratory results associated with hypoalbuminemia depending on the underlying disease process.
Serum albumin levels are dependent on the rate of synthesis, the amount secreted from the liver cell, the distribution in body fluids, and the level of degradation. Hypoalbuminemia results from a derangement in one or more of these processes.
Albumin synthesis begins in the nucleus, where genes are transcribed into messenger ribonucleic acid (mRNA). The mRNA is secreted into the cytoplasm, where it is bound to ribosomes, forming polysomes that synthesize preproalbumin. Preproalbumin is an albumin molecule with a 24 amino acid extension at the N terminus. The amino acid extension signals insertion of preproalbumin into the membrane of the endoplasmic reticulum. Once inside the lumen of the endoplasmic reticulum, the leading 18 amino acids of this extension are cleaved, leaving proalbumin (albumin with the remaining extension of 6 amino acids). Proalbumin is the principal intracellular form of albumin. Proalbumin is exported to the Golgi apparatus, where the extension of 6 amino acids is removed prior to secretion of albumin by the hepatocyte. Once synthesized, albumin is secreted immediately; it is not stored in the liver.
Tracer studies with iodinated albumin show that intravascular albumin is distributed into the extravascular spaces of all tissues, with the majority being distributed in the skin. Approximately 30-40% (210 g) of albumin in the body is found within the vascular compartments of the muscle, skin, liver, gut, and other tissues.
Albumin enters the intravascular space via 2 pathways. First, albumin enters this space by entering the hepatic lymphatic system and moving into the thoracic duct. Second, albumin passes directly from hepatocytes into the sinusoids after traversing the Space of Disse.
After 2 hours, 90% of secreted albumin remains within the intravascular space. The half-life of intravascular albumin is 16 hours. Daily losses of albumin from the intravascular space are approximately 10%. Certain pathological conditions, such as nephrosis, ascites, lymphedema, intestinal lymphangiectasia, and edema, can increase the daily loss of albumin from the plasma.
Albumin distributes into the hepatic interstitial volume, and the concentration of colloids in this small volume is believed to be an osmotic regulator for albumin synthesis. This is the principal regulator of albumin synthesis during normal periods without stress.
Degradation of albumin is poorly understood. After secretion into the plasma, the albumin molecule passes into tissue spaces and returns to the plasma via the thoracic duct. Tagged albumin studies suggest that albumin may be degraded within the endothelium of the capillaries, bone marrow, and liver sinuses. Albumin molecules apparently degrade randomly, with no differentiation between old and new molecules.
Hypoalbuminemia is more frequent in older patients who are institutionalized, patients who are hospitalized with advanced stages of disease (eg, terminal cancer), and malnourished children.
No race predilection exists.
No sex predilection exists.
Hypoalbuminemia affects persons of all age groups, depending on the underlying cause.
Low serum albumin levels are an important predictor of morbidity and mortality. A meta-analysis of cohort studies found that, with every 10 g/L decrease in serum albumin, mortality was increased by 137% and morbidity increased by 89%. Patients with serum albumin levels of less than 35 at 3 months following discharge from the hospital have a 2.6 times greater 5-year mortality than those with a serum albumin levels greater than 40.
Hypoalbuminemia has also been studied as an important prognostic factor among subsets of patients, such as patients with severe sepsis, burns,  and regional enteritis (Crohn disease) and has recently been associated with an increased risk of reintubation. 
Whether or not hypoalbuminemia is merely a marker of severe protein malnutrition, which itself is a cause of increased morbidity and mortality or an independent risk factor for death, is unclear.
Specific dietary recommendations are based on the underlying disease.
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