Carbon dioxide is a gas that dissolves in the blood. Dissolved carbon dioxide helps to neutralize acid. Measuring carbon dioxide in a blood test can provide information about the blood’s acid level, or pH.

A blood carbon dioxide level that’s much higher than normal can be seen with a number of medical conditions. These include:

• Severe breathing problems, such as emphysema
• Vomiting for a long period of time
• Taking high doses of diuretics or antacids
• Certain hormone conditions that cause acids to leak into the urine
• Low blood levels of potassium

There’s probably nothing wrong when the carbon dioxide level is just a point or two outside of the normal range. That’s particularly true if your test turned up abnormal on otherwise routine blood work.

This raises an interesting question: What defines an abnormal result on a lab test? Some tests are abnormal because there is a clear link between that result and a known health condition. For example, high levels of lead in the bloodstream are known to produce lead poisoning.

But for other tests the normal range is determined statistically by performing the test in a large population. The average value, plus a certain buffer zone, defines what is normal. For those trained in statistics, this buffer is typically two standard deviations on either side of the mean.

For example, the normal range for carbon dioxide is calculated by looking at average levels in a large group of otherwise healthy people. The bottom line: “Abnormal” means you are not in the average range. But it doesn’t always mean there is problem. Some healthy people will simply have carbon dioxide levels that are slightly higher or lower than average.

Another quirky statistical fact is that a typical person is likely to have one or more “abnormal” values when they have a large number of blood tests. Simply put, it is hard to be average on every test. It may be alarming to see an “abnormal” label on one of your test results. But you can rest assured that this may simply be — well, normal.