Electrolyte composition of intracellular and extracellular fluids, and why urine as well as serum testing is performed in a hospital setting

Firstly: the usual electrolyte levels of the ICF (intracellular fluid, the actual fluid within the cells), ISF (interstitual fluid, the fluid that bathes the cells) and Plasma (the fluid within the blood vessels) are as follows:
Plasma ISF ICF

Na+ 142 144 10
K+ 4 4 160
Ca2+ 2.5 2.5 1.5
Mg2+ 1.0 0.5 13
Cl- 102 114 2
HCO3- 26 30 8
PO42- 1.0 1.0 57
SO42- 0.5 0.5 10
Organic acid 3 4 3
Protein 16 0 55

In a 70kg male, the ICF is approximately 28L, or 35% of lean bodyweight
the ISF compirises of 12%of body weight, or 9.4L
the plasma is approximately 4.6L or 5% of total body weight.

The reason electrolyte testing of urine in addition to serum occurs in a hospital setting is to ascertain the endocrine and renal responses to the osmolar state of the patient ie. to see if the reason their sodium serum levels are low is because of renal failure, or because of excessive infusion of dextrose solution.