Dehydration is a complex and challenging syndrome to diagnose. It can be particularly debilitating even at mild levels if it is not caught in time. Some medication such as water pills can cause fluid and electrolyte imbalances that are dangerous to the patient. Reading lab work to determine if a patient is dehydrated takes some experience. Sodium, potassium and blood urea nitrogen (BUN) are all indicators of a patient's hydration status. They often tell the story of dehydration long before physical symptoms manifest.
Look at the sodium level. If it is higher than the laboratory's quoted range, then the patient may be suffering from hypertonic dehydration. This means that fluid is being pulled out of the cells due to the high amount of sodium in the bloodstream. It does this to replace the lost fluid. If the sodium level is lower than the laboratory's quoted range, then the patient has hypotonic dehydration. This is caused by the rapid loss of electrolytes from medicines such as diuretics or water pills.
Look at the potassium level. If it is lower than the laboratory's quoted range, then hypotonic dehydration may be at fault. This type of dehydration pulls fluid out of the vessels and into the cells. This is usually caused by diuretics.
Look at the blood urea nitrogen level (BUN) and creatinine level. In severe dehydration, the BUN level will be much higher than the creatinine level.