Proteins and enzymes
Plasma proteins
The biochemistry laboratory routinely measures ‘total protein’ and ‘albumin’ concentrations, usually in a serum specimen, and reports the ‘globulin’ fraction as the difference between the first two results. Some proteins (e.g. immunoglobulins) are measured as classes, and immunochemical methods are available for measuring specific proteins and hormones. Enzymes are measured both by determining their activity and by immunochemical methods to assess their mass.
Total protein
Changes in total protein concentration are common. An elevated total protein concentration may mean the presence of a paraprotein. A decreased total protein usually means that the albumin concentration is low.
Albumin
Albumin is the major plasma protein and is synthesized and secreted by the liver. It has a biological half-life in plasma of about 20 days and it accounts for about 50% of the total hepatic protein production. Albumin makes the biggest contribution to the plasma oncotic pressure. If the albumin concentration falls very low, oedema is the result (Fig 25.1). There are four main reasons for the occurrence of a low plasma albumin concentration:
Abnormal distribution. Albumin can move into the interstitial space as a result of increased capillary permeability in the acute phase response.
Decreased synthesis. Due to malnutrition, malabsorption or advanced chronic liver disease.
Dilution. Hypoalbuminaemia can be induced by overhydration.
Abnormal excretion or degradation. The causes include the nephrotic syndrome, protein-losing enteropathies, burns, haemorrhage and catabolic states.
Although serum albumin measurements have previously been used to monitor a patient’s response to long-term nutritional support, they are unreliable and insensitive.
Specific proteins
Measurement of a number of specific proteins gives useful information in the diagnosis and management of disease (Table 25.1). Characteristic changes in the concentration of certain plasma proteins are seen following surgery or trauma, or during infection or tumour growth. The proteins involved are called acute phase reactants (pp. 110–111). These acute phase proteins may be used to monitor progress of the condition or its treatment.
Table 25.1
Specific proteins that are measured in serum
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
Get Clinical Tree app for offline access
| Protein name | Function | Reason for assay |
| α1-antitrypsin | Protease inhibitor | Reduced in α1-antitrypsin deficiency |
| β2-microglobulin | A subunit of the HLA antigen |