HCO3 (bicarbonate) on a blood gas: the kidney side of acid-base balance

Bicarbonate is the main buffer in blood: it neutralises acid and so limits swings in pH. Lab Tests Online UK explains that the ratio between blood bicarbonate and the partial pressure of carbon dioxide determines blood pH, that the kidneys remove acid and reclaim bicarbonate, and that bicarbonate is the metabolic, or kidney, part of acid-base balance.

In its blood gas guide the same source notes that HCO3 is excreted and reabsorbed by the kidneys in response to pH imbalances, is directly related to the pH, and may be a calculation rather than a direct measurement. In its bicarbonate guide it adds that calculated bicarbonate comes from the gas analysis, whereas a vein sample on a routine panel measures total carbon dioxide, most of which is bicarbonate.

HCO3 is read together with pH and PCO2. If pH is low and HCO3 is low, the problem is metabolic. If pH is low and PCO2 high, the problem is respiratory. When both change, the body is usually compensating. Bicarbonate is not used in isolation, and electrolytes including sodium, potassium, chloride, calcium and magnesium are often checked because they are frequently abnormal in acid-base disturbance.

Conditions that disturb acid-base balance include diabetes, chronic kidney disease, diarrhoea and drug overdose, as the source lists.

For accurate blood gas measurement, arterial blood should be collected in a heparin tube that is capped, well mixed, free from air bubbles and analysed immediately at the bedside. Air in the syringe or delay can alter carbon dioxide and therefore the calculated bicarbonate. The sample is usually from the radial artery at the wrist and is most often handled in hospital.

Low blood pH caused by low bicarbonate is metabolic acidosis, and the body tries to breathe faster to blow off carbon dioxide. Raised anion gap causes include diabetic ketoacidosis, kidney disease, lactic acidosis, alcoholic ketoacidosis, toxic alcohol poisoning and drug overdose such as paracetamol and salicylate. Normal anion gap causes include diarrhoea, renal tubular acidosis, Addison disease, gastrointestinal drainage and certain medicines.

The anion gap, calculated from sodium, potassium, chloride and bicarbonate, helps sort the possibilities, and lactate, glucose, ketones and toxicology tests are requested as needed.

High pH with high bicarbonate is metabolic alkalosis. Lab Tests Online UK lists severe vomiting or nasogastric suction, Cushing syndrome, Conn syndrome, liquorice, low potassium, loop or thiazide diuretics, laxative abuse, bicarbonate treatment, blood transfusions and cystic fibrosis. Low potassium is common and sustains the disturbance, and low ionised calcium may cause tingling, cramps and involuntary muscle contractions. Treatment targets the cause and often includes replacing potassium and fluids.

Treatment focuses on identifying and treating the underlying cause. The source says giving bicarbonate to people with acidosis is controversial and carries a risk of harm, so it is used in selected situations. Call 999 for rapid breathing with drowsiness, confusion or persistent vomiting in someone with diabetes.