How to read your medical test report: Arterial blood gas

All you need to know about reading your arterial blood gas (ABG) report.

Gases are exchanged across a specialized lung surface. The inhaled oxygen is diffused into the blood, and carbon dioxide from the blood is released into the lungs where it is exhaled. Gases are transported throughout the body in the blood by the action of the heart and blood vessels. They are responsible for the acid-base balance (pH) in the blood. Lungs and kidneys maintain the proper blood pH in the body. The lungs affect the blood pH by the release of carbon dioxide, whereas the kidneys affect blood pH by expelling excess acids or bases. Many organs are severely affected by even a slight deviation from the normal range of acid-base balance.

Arterial blood gas test what is it?

An arterial blood gas (ABG) test measures the amounts of certain gases like oxygen and carbon dioxide dissolved in your blood. For the test, a sample of blood is drawn from an artery, mostly in your wrist. Commonly, the blood acidity (pH), partial pressure of oxygen and carbon dioxide (PaO2 and PaCO2 respectively) and bicarbonate levels (HCO3) are determined in the test.

When is the test recommended?

ABG test is recommended when your body s acid-base levels, oxygenation, and ventilation sufficiency need to be assessed and monitored. It is mainly used in respiratory diseases like asthma and COPD. It is also valuable when caring for patients in intensive care units (ICUs). The test helps determine the patient s oxygen-carbon dioxide exchange, indicate the severity of illness, and assess and monitor treatment.

Reading your ABG test report

Your ABG test report may show blood pH, PaCO2, HCO3 and PaO2 level results. The reference ranges may vary from lab to lab. The test result helps analyze some of the more common acid-base disorders. While lungs primarily control the PaCO2 levels, kidneys mostly control the bicarbonate levels. Changes in either the PaCO2 or the bicarbonate level initiate compensatory changes to minimize the change in pH.

Acidosis is increased blood acidity. There are two types - metabolic acidosis and respiratory acidosis. Metabolic acidosis is caused by increased acid or decreased bicarbonate in the blood as seen in diarrhea, diabetic ketoacidosis, damaged pancreatic ducts (pancreatic fistula) or failure of the kidneys to appropriately acidify the urine (renal tubular acidosis). Respiratory acidosis is caused by increased carbon dioxide in the blood due to poor lung function or too little breathing. Clinically it usually occurs in association with chronic lung disease¹.

Alkalosis is an excessively alkaline condition of the blood. There are different types of alkalosis. In metabolic alkalosis, there is an increased bicarbonate concentration and pH. It is caused by increased loss of hydrogen ions or depletion of chloride due to steroid therapy, severe vomiting, excessively vigorous diuretic therapy, sodium bicarbonate overdose, etc. Respiratory alkalosis is due to over-breathing which results in too little carbon dioxide in the blood (decreased PCO2), with a consequent increase in pH¹.

In pulmonary heart disease complication, metabolic alkalosis is superimposed on respiratory acidosis¹.

1. Blood pH

Your body maintains the blood pH within the normal range to avoid cell death.

7.35 7.45: Normal range

< 7.35: Acidosis

> 7.45: Alkalosis

Carbon dioxide is present in the blood as carbonic acid. Too little breathing increases CO2 levels in the blood causing a decrease in pH (respiratory acidosis). Excessive breaths (hyperventilation) cause too little carbon dioxide in the blood and lead to increased pH (respiratory alkalosis).

2. Carbon dioxide partial pressure (PaCO2)

PaCO2 level indicates carbon dioxide production and elimination.

Normal range: 35-45 mmHg

Higher values indicate respiratory acidosis due to too little breathing (hypoventilation). They could rarely be due to an abnormal increase in the body s metabolism. Lower values indicate respiratory alkalosis due to excessive breathing (hyperventilation)

3. Bicarbonates (HCO3)

The bicarbonate levels indicate metabolic problems like ketoacidosis.

Normal range: 22 26 mEq/L

Lesser values indicate metabolic acidosis, whereas higher values indicate metabolic alkalosis.

4. Oxygen partial pressure (PaO2)

Normal range: 75-100 mmHg

Lower values indicate decreased oxygen in the blood (hypoxia). Very low values increase the risk of death and require oxygenation.

Reference

1. Atkins, E. L. (1969). Assessment of acid-base disorders. A practical approach and review. Canadian Medical Association Journal, 100(21), 992 998.

Image source: Shutterstock

---