Chloride

Table salt is sodium chloride, and in your body those two stay together. Sodium is the positive particle that attracts water and determines your fluid balance; chloride is the negative particle that keeps it company. If sodium rises because you are dehydrating, chloride rises along; if sodium falls through dilution, chloride falls along. On its own chloride is therefore often an echo of sodium. But chloride has a second job. Your blood has to stay electrically neutral, so opposite the positive particles there is always a fixed amount of negative ones. Chloride and bicarbonate are the two largest of those, and they complement each other like communicating vessels: if bicarbonate disappears, chloride shifts up to fill the gap, and the other way round. Bicarbonate is the buffer that protects your blood against acidification, and so chloride takes part in acidity. If you vomit stomach acid, which is full of chloride, the kidney holds on to bicarbonate and chloride falls; if you have acid to spare through diarrhoea or kidney damage, chloride rises to replace the lost bicarbonate. Your report shows chloride in mmol/L. Normal is about 101 to 108 mmol/L, where the labs overlap. The value usually sits in a row with sodium, potassium and bicarbonate, and that row is where you read it.

Chloride is rarely the value you wait for, and that is the honest message of this page. It is almost always measured together with sodium and potassium because it comes from the same test, and in most cases it confirms what sodium already says: both up is dehydration, both down is dilution. Anyone who is healthy and sees a chloride within the band need do nothing with it. Where chloride does add something is when it moves independently of sodium, because then the story is not about water but about acid. A low chloride with a normal sodium fits persistent vomiting, diuretics or a body holding on to too much carbon dioxide through a lung disease, and goes with a high bicarbonate. A high chloride with a normal sodium fits long-lasting diarrhoea, kidneys that cannot get rid of acid well or a drip with a lot of salt, and goes with a low bicarbonate. That pattern, chloride against bicarbonate, is what a doctor uses to see which way the acid-base balance has tipped. For anyone who does a lot of sport or works in the heat, chloride is like sodium a measure of what has gone with the sweat. Sweat is salt water, and anyone who sweats for hours and drinks back only water sees sodium and chloride fall together. That is not a chloride problem but a water problem, and the solution is the same as with sodium: drinking to thirst, with salt alongside if it lasts long.

A low chloride means loss or dilution. In order of how often it occurs: loss of stomach acid through persistent vomiting, or through a stomach tube; diuretics, which make the kidneys release chloride and hold on to bicarbonate; and dilution through too much water, in which sodium falls along. If sodium moves along, it is about water; if sodium stays normal and bicarbonate is high, it is about acid and vomiting or the diuretic is the explanation. A high chloride usually means concentrating. With dehydration from fever, diarrhoea, heat or drinking too little, sodium and chloride rise together, and that recovers with water. If chloride rises while sodium stays normal and bicarbonate is low, the body is losing bicarbonate or cannot release acid: with long-lasting diarrhoea, with advanced kidney damage or after a drip with a lot of salt. That is a pattern to look at together with creatinine and bicarbonate. Always read chloride beside sodium, because moving together means water, beside bicarbonate, because moving against each other means acid, and beside potassium and creatinine, which say whether the kidneys are involved.