SODIUM concentrations have become a part of every patient's hospital record. Interpreting them correctly depends on two fundamental facts. First, a concentration is a ratio—the amount of sodium compared with the amount of plasma (plasma water) in which it is dissolved. Thus, an abnormal value may result from a change in the amount of sodium, a change in the amount of water, or a change in the amount of both. This leads to situations that all have a low serum sodium concentration but arise from different mechanisms and call for different treatments. The second key fact is that the serum concentration of sodium gives no information about the concentration inside the cells and other special compartments such as bone. It is therefore possible for serum sodium concentration and total body sodium to vary in opposite directions.
The two most common types of hyponatremia are illustrated as follows:
Berman LB. Serum Sodium: A Bedside Problem. JAMA. 1974;227(9):1063. doi:10.1001/jama.1974.03230220053021
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