At a glance

Also known as

HCO3-; Total CO2; TCO2

Why get tested?

Usually as part of a renal profile (collection of tests which help investigate the kidney) to screen for an electrolyte or acid-base imbalance in conditions known to cause such disturbances, or to monitor a known imbalance

When to get tested?

May be part of a routine blood test that includes electrolyte measurements or may be requested by your doctor if you have a medical condition or are experiencing symptoms that could indicate problems with the acid-base balance of your body

Sample required?

A blood sample drawn from a vein in the arm

Test preparation needed?


What is being tested?

When you breathe, you bring oxygen (O2) into your lungs and release carbon dioxide (CO2). The bicarbonate test measures the total carbon dioxide in your blood [present in three forms: carbonic acid (H2CO3, CO2 dissolved in blood, and HCO3- (bicarbonate, the dominant form)]. HCO3- is an electrolyte that is excreted and reabsorbed by your kidneys. Its main job is to help maintain the acid-base balance (pH) in your body and secondarily to work with sodium, potassium and chloride to maintain electrical neutrality at the cellular level.

The bicarbonate test, since it measures all three forms at once, gives your doctor a rough estimation of acid-base balance. This is usually sufficient, but measurements of gases dissolved in the blood may be done if more information is needed. Bicarbonate may be measured along with sodium, potassium, and possibly chloride in an electrolyte profile as it is the balance of the three of four that gives your doctor the most information.

How is the sample collected for testing?

A blood sample is drawn by needle from a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

A bicarbonate level is almost always done along with other electrolytes to tell your doctor whether your sodium, potassium, chloride and bicarbonate levels are in balance. They may be measured as part of routine blood testing or when your doctor suspects an imbalance. Bicarbonate may also be measured when your doctor is evaluating your acid-base balance, to screen for an imbalance, and to monitor a known problem during treatment.

When is it requested?

Bicarbonate testing may be requested, usually as part of an electrolyte profile when:

  • you are having a routine blood test
  • your doctor suspects that you may be retaining water or are dehydrated, upsetting your electrolyte balance
  • to help evaluate your body's acid-base balance (pH)
  • to monitor a condition or treatment that might cause an electrolyte imbalance.

What does the test result mean?

When bicarbonate levels are higher than normal, it suggests that your body is having trouble maintaining its pH balance either by failing to remove carbon dioxide or perhaps because of an electrolyte imbalance, particularly a deficiency of potassium. Both of these imbalances may be due to a wide range of dysfunctions. Bicarbonate elevations may be seen with severe vomiting, chronic lung-related problems, such as emphysema, and some hormonal disorders such as Cushings disease.

Low bicarbonate levels may be seen with chronic diarrhoea, diabetic ketoacidosis and kidney failure.

Reference Intervals

22-32 mmol/L
0 to <1 week             15-28mmol/L
1 week - 2 years       16-29 mmol/L
2 years - 10 years     17-30 mmol/L
10 years - 18 years   20-32 mmol/L

The reference intervals shown above are known as a harmonised reference interval. This means that eventually all laboratories in Australia will eventually use this same interval so wherever your sample is tested, the reference interval should be the one shown above. Laboratories are in the process of adopting these harmonised intervals so it is possible that the intervals shown on the report of your results for this test may be slightly different until this change is fully adopted. More information can be found under Reference Intervals – An Overview.

Is there anything else I should know?

Some drugs may increase bicarbonate levels especially diuretics such as frusemide (usually as a consequence of potassium deficiency). Other drugs may cause slightly low levels. Your doctor can advise if this appears to be a problem.

Common Questions

If bicarbonate levels are too high or low, what treatments can help?

If your bicarbonate is high or low, your doctor will identify and treat the underlying cause. For example, high bicarbonate may be caused by emphysema, which may be treated with oxygen therapy and medications, or by severe diarrhoea or vomiting (which would be treated by treating the reason for the diarrhoea or vomiting). Low bicarbonate may be caused by diabetic ketoacidosis, which can be treated in part by treating the blood sugar problem that led to the ketoacidosis or kidney diseases.

What other gases circulate in blood?

Oxygen, both dissolved and bound to haemoglobin, is vital to the functioning of all cells in the body. Smokers and people who have been exposed to carbon monoxide may have measurable amounts of it in their blood. Carbon monoxide also binds to haemoglobin, diminishing your body's ability to carry oxygen. Nitrogen, from air, is dissolved in blood at low concentrations but is usually not of any significance.

If my doctor has measured bicarbonate, why do they want to do blood gases?

Blood gas tests, in which blood is drawn from an artery instead of a vein, can give your doctor a more accurate assessment of the body's pH (acid/base status) and indicate whether your body is taking in enough oxygen and getting rid of enough CO2.

Last Review Date: February 15, 2020

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