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14. Disturbances of non-immunoglobulin plasma proteins

Albumin

Albumin, more specifically human serum albumin, is a plasma protein whose main function is to transport bioactive molecules and to maintain oncotic pressure. It's a negative acute phase protein. Albumin is the most abundant plasma protein, accounting for 60% of plasma proteins.

Approximately 12 - 24 g of albumin is synthesised in the liver daily, but hepatocytes can more than double their synthetic capacity if needed. The normal serum level is approx 35 - 45 g/L.

Hypoalbuminaemia

Hypoalbuminaemia is characterised by serum albumin < 35 g/L. It's a common finding in hospitalised patients. It may be due to:

Hyperalbuminaemia

Hyperalbuminaemia is usually a sign of dehydration.

Bisalbuminaemia

Bisalbuminaemia is a harmless disorder where a person has two "peaks" in the albumin fraction on serum electrophoresis, showing that the person produces both normal and a mutated abnormal albumin.

Prealbumin

Prealbumin is a negative acute phase protein. It binds thyroxin (T3) and vitamin A in the serum. It's sometimes used as a marker of nutritional status but it performs very poorly as such a marker.

Alpha-1 antitrypsin

Alpha-1 antitrypsin (A1AT) is a plasma protein and a protease inhibitor. It protects tissues from proteases like neutrophil elastase.

Alpha-1 antitrypsin deficiency predisposes to pulmonary emphysema (a form of COPD) and cirrhosis.

Alpha-1 microglobulin

Alpha-1 microglobulin (A1M) is a small globular plasma protein synthesised by the liver. Its function is to bind and degrade haeme. It's not a very important molecule clinically. It may be measured in urine as an indicator of proteinuria.

Beta-2 microglobulin

Beta-2 microglobulin is a globular plasma protein and a component of MHC class I molecules, meaning that it exists on all nucleated cells. It can be measured in the urine as a marker of proteinuria. Elevated levels is a poor prognostic factor in lymphoma, chronic lymphocytic leukaemia, multiple myeloma, and AIDS.

Caeruloplasmin

Caeruloplasmin is a copper-binding plasma protein synthesised in the liver. It's a positive acute phase protein.

Low levels of caeruloplasmin is suggestive (but not diagnostic of) Wilson disease.

Thyroid-binding globulin

Thyroid-binding globulin (TBG) is a globular plasma protein whose function is to bind and transport thyroxine and triiodothyronine. Congenital TBG deficiency is asymptomatic, making TGB's physiological importance uncertain.

The level of TBG can be measured if congenital TBG deficiency is suspected, as this will interfere with regular thyroid hormone measurement.

Sex hormone binding globulin

Sex hormone binding globulin (SHBG) is a globular plasma protein whose function is to bind and transport testosterone, dihydrotestosterone, and oestradiol.

Measurement of SHBG is important because only the unbound fraction of sex hormones in the plasma is biologically active, and so when evaluating sex hormonal disorders one can only determine the biologically active fraction of sex hormones is one also knows the SHBG level.

The hepatic synthesis of SHBG increases when oestrogens and thyroid hormones increase, and the synthesis decreases when androgens and glucocorticoids increase.