Blood tests used to investigate liver, thyroid or kidney function and disease
The previous article in this series (PJ, 3 April p419–21), looked at general blood tests requested from haematology departments, such as full blood counts. Blood tests are used to investigate many aspects of health and disease. Some tests are done routinely, for example, tests for blood type and phenylketonuria in new-born babies.
Another example is when a patient has a suspected myocardial infarction. His or her levels of cardiac enzymes, such as creatine kinase (especially the MB isoenzyme) and troponin T, will be determined and this information will be used (in conjunction with electrocardiogram changes) to diagnose underlying disease and choose an appropriate treatment. Blood tests can even be used to assess a person’s risk of a heart attack (eg, tests for c-reactive protein and homocysteine).
Venous blood can be taken for microbiological tests (eg, blood cultures in cases of suspected sepsis) and immunological testing (eg, detecting rheumatoid factor when arthritis is suspected). Tests commonly requested from chemical pathology laboratories include those for renal, liver and kidney function as well as tests for cholesterol, triglycerides and glucose and drug assays.
Liver function tests
Liver function tests (LFTs) are a group of biochemical measurements that are used to identify patients who are suffering from liver or biliary tract disease. Substances measured include:
- Alanine aminotransferase (ALT)
- Aspartate aminotransferase (AST)
- Gamma-glutamyl transferase (GGT)
- Alkaline phosphatase (ALP)
None of the above tests is specific for liver or biliary disease and other diseases can cause abnormal levels of one of more of these substances. Even in cases of liver disease, blood levels of one or more of these substances might be within the reference range. However, it is unlikely that all the results would be within their respective reference ranges. Together, therefore, the combination of tests is more useful than a single test.
Liver enzymes (ALT, AST, etc) leak into the blood when liver cells are damaged so, strictly speaking, these tests indicate liver disease rather than liver function. ALT and AST levels provide an indication of the degree of inflammation as well as the possible causes or hepatocellular damage. Similarly, ALP and GGT increases can suggest the presence of obstructive liver disease.
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Citation: The Pharmaceutical Journal URI: 10997021
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