Chloride is a major anion in the body. It is involved in water and acid-base balance. It predominates in the extracellular space, generally occurs in binary compounds, where one of the elements is chlorine. The most common example of chloride is sodium chloride, which is commonly known as table salt. Chloride levels are usually measured in blood or more exactly in the serum, but can also be measured in the urine.

The test of blood chloride, along with other electrolytes, is a part of a more in depth blood testing, and is required to diagnose problems related to prolonged vomiting, diarrhea, weakness, and some respiratory problems.

In people having excess of base, urine chloride test helps doctors determine whether it is due to dehydration from lack of water intake or due to vomiting. The other factor can be use of diuretics, where urine chloride would be extremely low. Urine tests for chloride together with sodium, is useful in monitoring patients on a low-salt diet. Higher level of sodium and chloride indicates that the patients have failed in following a specific diet plan. Testing for chloride helps monitor problems, such as heart failure, liver and kidney disease.

When chloride level is high, the condition is called hyperchloremia. It normally indicates dehydration, but it can also be caused by other problems, which lead to high level of sodium in the blood. Low chloride level indicates hypochloremia, which occurs due to low serum sodium, prolonged vomiting or gastric suction. It is also caused by persistent diarrhea and chronic lung disease.

Drugs affecting level of sodium in blood affects chloride level as well. Additionally, intake of large amounts of baking soda and antacids also lead to a low level of chloride.