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Student Education

Investigations: Essential clinical chemistry

BMJ 2003; 327 doi: https://doi.org/10.1136/sbmj.0309314 (Published 01 September 2003) Cite this as: BMJ 2003;327:0309314
  1. Suneeta Kochhar, fourth year medical student1,
  2. William Marshall, reader and honorary consultant in clinical biochemistry1
  1. 1Guy's, King's, and St Thomas's School of Medicine, London

Suneeta Kochhar and William Marshall kick off a new series on basic clinical investigations. They explain what biochemistry results can tell you and how you should interpret them

We have all seen the bits of paper that come back from the laboratory full of endless numbers referring to many different biochemical ions. But what are they used for and what can they tell us?

Clinical biochemistry is important to:

  • help formulate a diagnosis or make treatment decisions

  • screen for a condition

  • help with a prognosis and to follow the course of an illness

  • monitor the response or side effects of treatment.

Biochemical tests can be done on blood plasma or serum, urine, or cerebrospinal fluid. Tests may be sent to a laboratory for analysis or be done at the bedside using reagent sticks--for example, monitoring blood glucose. The results are then compared with a reference range. If the results are wildly unexpected you may need to repeat the test or reconsider the diagnosis. If you need any advice about choosing the appropriate test, do not be afraid to ask someone more senior for help.

This article focuses on the abnormal biochemistry results that demand urgent attention.

Electrolyte imbalance

Most of the ATP your cells produce is used up in maintaining the correct balance of electrolytes in your body. Electrolyte imbalances have potentially dangerous consequences for all important organs, especially the heart, so recognising the signs and responding quickly is essential.

Remember that every hospital and every laboratory has a slightly different range for all the tests they carry out.

View this table:

Normal range of some important electrolytes

Potassium

When potassium falls too low

Mild hypokalaemia (low potassium) can cause lethargy and weakness. If the potassium is less than 2.5 mmol/l the patient needs urgent attention because of increased risk of cardiac arrhythmias. Furthermore, having low potassium means that patients taking …

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