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Module 2 : Clinical Assessment



Blood biochemistry


Serum biochemistry results can be used as an adjunct to other clinical observations in order to gain an idea of a patient’s fluid status.

Below are some of the urea and electrolyte (‘Us and Es’) changes which may be seen in the fluid depleted or overloaded patient:

Urea and creatinine: Levels increase in the dehydrated patient. Urea levels increase proportionately more than creatinine levels.


Hyponatraemia:  Most commonly caused by water overload either by excessive water drinking or IV fluids - notably glucose solutions. Therefore the ratio of sodium to water in the blood plasma decreases.


Hypernatraemia: This occurs in dehydration; simply referring to depletion of water, thus this increases the plasma sodium concentration.

Since ‘water follows salt’ and osmoreceptors detect a rise in serum osmolality and trigger thirst and ADH secretion as compensatory measures, it can generally only be seen in the following patients:

  • In the very old/young or neurologically disabled who have lost the perception of thirst
  • Too ill to drink
  • In those with an osmotic diuresis (e.g. caused by glycosuira) in which water is lost disproportionately to sodium.

Hypokalaemia: may result from losses of potassium rich GI tract fluid e.g. vomiting and diarrhoea. In such cases, extra potassium supplementation to fluid may be required. See Physiology : Electrolytes in disease states to revise this.

Generally, there are no typical serum marker thresholds to signify when a dehydrated patient should be given fluid, or when a fluid overloaded patient should be fluid restricted. This decision is subjective depending on these results and all the other clinical information you have about the patient.



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