1. Basic Concepts - Ammonia

Ammonia (NH3) is a volatile, colorless, gas with a characteristic unpleasant odor. If present in increased concentrations it is toxic (as is discussed in detail in Module 2). Humans are unable to directly excrete ammonia in the urine or stools; it is metabolized to urea in the urea cycle and thereby detoxified. Ammonia is a constituent of all human body fluids and at neutral pH exists mostly (> 98%) in its ionized form as ammonium (NH+).

The concentration of ammonia in plasma is micromolar. It is important to note that the ammonia concentration depends on the type of sample (venous, arterial or capillary blood), on the mode of sampling (free flowing blood versus use of tourniquet) and on a number of further pre-analytical conditions which will be discussed later. (For more information, see sections 2.1 and 3 below).