Tests
The goals of testing are to identify whether somone has an acid-base disorder, to determine how severe the disorder is, and to help diagnose underlying diseases or conditions (such as diabetic ketoacidosis or the ingestion of a toxin) that have caused the acid-base disturbance. Testing is also done to monitor critically ill people, as well as those with conditions known to affect acid-base balance, such as chronic lung disease and kidney disease.
The primary tests used to identify, evaluate, and monitor acid-base disorders are:
Blood gases are a group of tests performed together, usually on an arterial blood sample (blood obtained from an artery instead of a vein). They are a snapshot of the blood's pH, PO2 (the amount of oxygen in the blood), and PCO2 (the amount of carbon dioxide the blood). From these results, bicarbonate (HCO3-) can be calculated.
In most acid-base disorders, both HCO3- and PCO2 are outside the reference range. That is because the body tries to keep the pH close to normal. Therefore, if one of these substances is abnormally high or low, the other one will change in the same direction to bring the pH back toward its healthy range. This process is called compensation. The table below shows the expected values for each of the four main acid-base disorders.
Results seen in the four primary acid-base disorders:
| Acid-Base Disorder | pH | HCO3- | PCO2 | Body Compensation |
|---|---|---|---|---|
| Metabolic acidosis | Less than 7.4 | Low | Low | Increased breathing rate (hyperventilation) to increase CO2 elimination |
| Metabolic alkalosis | Greater than 7.4 | High | High | Slowed breathing (hypoventilation) to decrease CO2 elimination |
| Respiratory acidosis | Less than 7.4 | High | High | Kidney increases retention of HCO3- and excretion of acid |
| Respiratory alkalosis | Greater than 7.4 | Low | Low | Kidney decreases retention of HCO3- and excretion of acid |
Electrolytes refers to a group of four tests: Na+ (sodium), K+ (potassium), Cl- (chloride), and bicarbonate (usually measured as total CO2 content). Body fluid levels, electrolyte concentrations, and acid-base balance are interconnected, and one or more of the electrolytes is usually increased or decreased in metabolic acid-base disorders. In patients with a metabolic acidosis, the anion gap is calculated using the results of an electrolyte panel to help determine which disorders might be responsible for the acid-base abnormality. For example, an increase in the anion gap can indicate diabetic acidosis. (For more on anion gap, see FAQ #1 here.) Patients with a metabolic alkalosis usually have low chloride (Cl-) and potassium (K+) values, which again provides clues as to the cause of the acid-base disturbance. Based on these results, other tests may be ordered to diagnose the disease or condition that is producing the acidosis or alkalosis (e.g., glucose, lactate, ketones, osmolality, methanol, ethylene glycol).
It is possible for a person to have more than one acid-base disturbance at the same time. Examples include ingestion of aspirin (which can produce both a respiratory alkalosis and metabolic acidosis) and those with lung disease who are taking diuretics (respiratory acidosis plus metabolic alkalosis). By using formulas to calculate the expected degree of compensation, it is possible to determine if a mixed acid-base disorder is present.
