As part of routine electrolyte measurement in blood or when symptoms or medical conditions could indicate problems with acid-base (acid-alkali) balance in the body. It is most useful when measured alongside pH, partial pressure of carbon dioxide (PCO2) and other electrolytes (e.g. sodium, potassium and chloride). It can help detect or monitor disorders as well as the response to treatment.
A blood sample taken from a vein in the arm is needed for bicarbonate. A blood sample from an artery may be taken at the bedside if blood gases (including calculated bicarbonate) are also being tested.
No test preparation is needed before blood is collected. For accurate results, blood should be spun and tested promptly in full and capped tubes. For accurate blood gas measurement (pH, PCO2 and calculated bicarbonate), arterial blood should be collected in a heparin tube that is capped, well mixed, free from air bubbles and analysed immediately at the bedside.
A significant amount of acid is produced within the normal human body. However, hydrogen ion concentration ([H+], described by pH) must be kept within tight limits to prevent major health problems. The ratio between blood bicarbonate (HCO3-) concentration and partial pressure of carbon dioxide (PCO2) determines blood pH. The kidneys and lungs are important in maintaining acid-base (acid-alkali) balance for normal pH. The kidneys remove acid and reclaim bicarbonate, which impacts on blood bicarbonate concentration. Bicarbonate is a base (alkali) and is the metabolic (kidney) part of acid-base balance. Bicarbonate is also important for minimising (buffering) changes in blood pH. Breathing brings oxygen (O2) into and removes carbon dioxide (CO2) from the body in the lungs and this helps to control PCO2. PCO2 in water is proportional to the amount of carbonic acid and is the respiratory (lung) part of acid-base balance. In the blood, carbon dioxide is present in different forms including bicarbonate (HCO3-), carbonic acid (H2CO3), dissolved CO2, carbonate (CO32-), and carbamino compounds (of carbon dioxide and proteins). Total carbon dioxide is measured routinely in the blood. However, since the majority of total carbon dioxide is bicarbonate it is often just called ‘bicarbonate’. Bicarbonate is the second most common negatively charged ion. As well as helping to maintain acid-base balance (pH) it works with sodium, potassium and chloride to balance positive and negative charges to keep the overall charge in the body neutral.
How is it used?
Measuring bicarbonate alongside pH and gases dissolved in the blood gives an estimation of acid-base (acid-alkali) status. Acid-base balance is linked to fluid and electrolyte (charged ion) balance so bicarbonate may be measured with sodium, potassium, and chloride in an electrolyte profile to give the most information.
Bicarbonate is not used in isolation. The blood pH may show acid-base disturbance (acidaemia with low pH and alkalaemia with high pH). The blood bicarbonate and partial pressure of carbon dioxide (PCO2) will then help find the main cause of the disturbance, as well as any attempt by the body to correct it. Blood electrolytes (including sodium, potassium, chloride, calcium and magnesium) are often measured since these are frequently abnormal in acid-base disturbance. Blood anion gap (measured concentration of sodium + concentration of potassium – concentration of chloride – concentration of bicarbonate) will help to determine the cause of metabolic acidosis. If the anion gap is raised the osmolar gap (difference between measured and calculated osmolality) may help determine if ethanol or toxic alcohols are present. If the anion gap is normal, the urine anion gap (concentration of sodium + concentration of potassium – concentration of chloride in urine) may be helpful. Other tests like lactate, glucose, ketones and toxicology will be requested as needed.
When is it requested?
As part of routine electrolyte (charged ion) measurement in blood or when symptoms or medical conditions could indicate problems with acid-base (acid-alkali) balance in the body e.g. diabetes, chronic kidney disease, diarrhoea and drug overdose. In some laboratories bicarbonate is part of a renal profile (a collection of tests which help investigate kidney function); In other laboratories it must be requested separately. It is most useful when measured alongside pH, partial pressure of carbon dioxide (PCO2) and other electrolytes (e.g. potassium, sodium and chloride). As well as detecting problems, it may be used to monitor known conditions or treatments that could alter acid-base or electrolyte status
What does the test result mean?
When blood bicarbonate levels are higher or lower than normal, acid-base and electrolyte disturbances may be present.
Metabolic acidosis occurs when low blood pH (acidaemia) is caused by a low bicarbonate concentration. If possible, the body will try to hyperventilate to remove more carbon dioxide in the lungs and increase pH towards normal. Metabolic acidosis can be classified as either raised or normal (high chloride) anion gap.
Causes of raised anion gap metabolic acidosis include:
- Diabetes (diabetic ketoacidosis)
- Kidney disease
- Lactic acidosis
- Alcoholic excess (alcoholic ketoacidosis)
- Toxic alcohol poisoning (e.g. methanol and ethylene glycol)
- Drug overdose (e.g. paracetamol and salicylate)
Causes of normal (high chloride) anion gap metabolic acidosis include:
- Renal tubular acidosis. Can be inherited or acquired e.g. in people with diabetes with chronic kidney disease and Addison disease (adrenal insufficiency)
- Gastrointestinal drainage and fistulas
- Medications e.g. potassium-sparing diuretics and carbonic anhydrase inhibitors
Metabolic alkalosis occurs when a high blood pH (alkalaemia) is caused by a high bicarbonate concentration. If possible, the body will try to decrease breathing rate to remove less carbon dioxide in the lungs and decrease pH towards normal but this is limited by the resulting decrease in oxygen. Low potassium is common in metabolic alkalosis, and helps cause and sustain this disturbance.
Increased bicarbonate levels may be due to:
- Severe vomiting or nasogastric suction
- Hormone disorders such as Cushing syndrome or Conn syndrome (primary aldosteronism)
- Low potassium
- Genetic disorders e.g. Bartter, Gitelman and Liddle syndromes
- Medications e.g. loop or thiazide diuretics, laxative abuse
- Milk (or calcium)-alkali syndrome
- Bicarbonate treatment
- Blood transfusions
- Cystic fibrosis
The causes can be classified according to whether or not they respond to saline/chloride and whether or not high blood pressure is present. Arterial blood gases and urinary chloride can help with this classification. In metabolic alkalosis, ionised calcium may be low causing symptoms like tingling, muscle cramps and involuntary muscle contractions.
Is there anything else I should know?
Normal blood bicarbonate levels are slightly different in different laboratories. Blood bicarbonate is slightly higher in adults than in children. Some medications can increase or decrease bicarbonate concentration. Lower blood bicarbonate is associated with deterioration in kidney function both with and without existing kidney disease.
If bicarbonate levels are too high or low, what treatments can help?
If blood bicarbonate is high or low, the most important thing is to identify and treat the underlying cause.
Giving bicarbonate (e.g sodium bicarbonate) or other buffers to people with acidosis is controversial and there is a risk of causing harm. However, it may be used in certain situations e.g. severe and symptomatic acute metabolic acidosis, severe metabolic acidosis with acute kidney injury, metabolic acidosis in severe chronic kidney disease, renal tubular acidosis and chronic acidosis (particularly in children). Blood pH, bicarbonate and electrolytes (including potassium and ionised calcium) should be monitored regularly during and after treatment. Low potassium should be at least partly corrected before giving bicarbonate, and bicarbonate should not be given with some other medications e.g. calcium salts.
In metabolic alkalosis, low potassium should be treated and saline may be used in people with saline/chloride-responsive causes.
What other gases circulate in blood?
If my doctor has measured bicarbonate, why do they want to do blood gases?
Blood gas tests, in which blood is taken from an artery instead of a vein, can give a better assessment of acid-base (acid-alkali) and oxygen status. pH, partial pressure of carbon dioxide, partial pressure of oxygen, calculated bicarbonate and electrolyte levels are available in blood gas panels. Blood gas measurement is advantageous in that it can be done quickly by the bedside.