What Blood Chemistry Tests Tell Us

With the growing automation of biochemical blood testing over the last forty years, the number of blood tests carried out on patients has increased enormously. Blood chemistry testing is generally carried out on the watery part of blood (serum or plasma) as opposed to the full blood count which looks at the number and condition of the cells in whole blood.

Because of the large number of chemical constituents that can be analysed these tests are often used as a screen when the patient has symptoms that the doctor can't make sense of. Occasionally the result of a particular test will clinch a diagnosis. However this is unusual, history and clinical examination providing much of the useful information.

Common Biochemical Tests

There are very many substances that are tested for but those listed below are probably the most commonly requested.

• Electrolytes: sodium, potassium and chloride. These elements are important in maintaining fluid balance across cell membranes. They can give an indication of level of dehydration and kidney function. Heart muscle is particularly sensitive to potassium level.
• Calcium: this is also important in cell membrane function and correct levels are vital for proper muscle activity. Tetany – marked muscle spasm – can occur if calcium becomes too low. Calcium is also important for proper bone formation. Testing for calcium can be helpful in diagnosing kidney disease, parathyroid gland dysfunction and malabsorption problems in the gastrointestinal tract.
• Urea: sometimes referred to as blood urea nitrogen (BUN). This is a waste product from the metabolism of protein. The test is used as an indication of kidney efficiency.
• Glucose: this is the sugar that provides much of the body's energy. Its main use is as a test to diagnose and monitor diabetes. Low blood glucose levels are sometimes seen in some cancers, liver disease, alcoholism, starvation and certain hereditary disorders.
• Liver enzymes: liver cells contain very many enzymes to facilitate the large number of chemical processes that the liver is responsible for. If sufficient numbers of liver cells are damaged by infection or toxins for example, these enzymes will spill out into the blood and give a useful indication of the extent of liver damage. Conditions that can cause such damage are hepatitis, poisoning (paracetamol for example), excessive alcohol consumption and liver tumours. Liver enzymes are known by acronyms such as γGT, AST and ALT
• Cholesterol: there can't be many adults in the west who have not had a cholesterol test. High levels of cholesterol have been linked to heart disease and the advent of statins - cholesterol lowering drugs – has encouraged doctors to get their patients' cholesterol levels as low as possible.

Blood Test Reports

Pathology test reports usually come with the normal range printed next to the result so it's possible to see at a glance whether or not the test is normal. However these reports need to be interpreted with caution.

Some people who are perfectly healthy will have the occasional abnormal test result. This is because a published normal range is good for only 95% of the population. Five percent of the population will always fall outside of the normal range. Another reason is that other factors can interfere with the test result - diet or medicines for example.

For these reasons it's a good idea to go through your path results with your doctor rather than just request them from the surgery; you may save yourself some worry.

Resource

Medicine. Eds Souhami and Moxham. Pub. Churchill Livingstone 2002

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