Serum CO2 (total carbon dioxide, bicarbonate): what the result means

Carbon dioxide is made by the body as it burns fuel and is carried in blood in several forms: bicarbonate, carbonic acid, dissolved CO2, carbonate and compounds attached to proteins. Lab Tests Online UK explains that total carbon dioxide is measured routinely and that, since most of it is bicarbonate, it is often just called bicarbonate. Bicarbonate is the second most common negatively charged ion in blood.

Bicarbonate is a base. It mops up acid and helps keep blood pH within tight limits. The kidneys remove acid and reclaim bicarbonate, making bicarbonate the metabolic, or kidney, half of acid-base balance, whereas breathing controls the other half through the partial pressure of carbon dioxide in arterial blood.

It is usually part of an electrolyte profile with sodium, potassium and chloride, and in some laboratories part of a renal profile; in others it must be requested separately. Lab Tests Online UK lists situations in which acid-base balance may be disturbed: diabetes, chronic kidney disease, diarrhoea and drug overdose. It also helps balance positive and negative charges so the body stays electrically neutral.

Doctors rarely use it in isolation. The same source says bicarbonate is most useful alongside pH, PCO2 and other electrolytes, and that the anion gap, calculated from sodium, potassium, chloride and bicarbonate, helps find the cause of metabolic acidosis.

A vein sample is used for the routine test; an arterial sample may be taken at the bedside when blood gases are measured. No preparation is needed, but accuracy depends on the sample: blood should be spun and tested promptly in full, capped tubes, because exposure to air lets carbon dioxide escape and can falsely lower the result. Arterial gas samples go in a capped heparin tube, well mixed, free from air bubbles, and analysed immediately.

Metabolic acidosis occurs when low blood pH is caused by a low bicarbonate. If possible the body tries to breathe faster to blow off carbon dioxide and raise the pH. Lab Tests Online UK splits causes into raised anion gap and normal anion gap types. Raised anion gap causes include diabetic ketoacidosis, kidney disease, lactic acidosis, alcoholic ketoacidosis, toxic alcohol poisoning and drug overdoses such as paracetamol and salicylate. Normal anion gap (high chloride) causes include diarrhoea, renal tubular acidosis, Addison disease, gastrointestinal drainage and some medicines including potassium-sparing diuretics.

Low bicarbonate is also associated with deterioration in kidney function, with or without known kidney disease, so a persistently low result prompts a kidney review.

A high bicarbonate causes a high pH. The body tries to slow breathing, limited by the fall in oxygen this would cause. Lab Tests Online UK lists severe vomiting or nasogastric suction, hormone disorders such as Cushing syndrome or Conn syndrome, liquorice, low potassium, inherited disorders such as Bartter, Gitelman and Liddle syndromes, loop or thiazide diuretics, laxative abuse, milk-alkali syndrome, bicarbonate treatment, blood transfusions and cystic fibrosis. Low potassium is common and helps sustain the disturbance, and low ionised calcium may cause tingling and muscle cramps.

Treatment focuses on the underlying cause. The source says giving bicarbonate to people with acidosis is controversial and carries a risk of harm, although it may be used in some situations. Tests that commonly follow include a blood gas, lactate, glucose, ketones, kidney function and sometimes toxicology. Seek emergency care for rapid breathing, drowsiness, confusion, persistent vomiting or signs of diabetic ketoacidosis; call 999 or attend A&E.