Tests
The goals of testing are to identify whether a patient 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 disorder. Testing is also done to monitor critically ill patients, as well as patients 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 and electrolytes.
Blood gases are a group of tests performed together 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 dissolved in the blood), and PCO2 (the amount of carbon dioxide dissolved in the blood). From these results HCO3- (the amount of bicarbonate) can be calculated.
Results seen:
| Acid-base disorder |
H+ |
pH |
HCO3- |
PCO2 |
Body compensation |
| Metabolic acidosis |
↑ |
↓ |
↓↓ |
↓ |
Increased breathing rate (hyperventilation) to increase CO2 elimination |
| Metabolic alkalosis |
↓ |
↑ |
↑↑ |
↑ |
Slowed breathing (hypoventilation) to decrease CO2 elimination |
| Respiratory acidosis |
↑ |
↓ |
↑ |
↑↑ |
Kidney increases production of HCO3- and excretion of H+ (acid) |
| Respiratory alkalosis |
↓ |
↑ |
↓ |
↓↓ |
Decreased production of HCO3- and excretion of H+ |
Electrolytes refer to a group of four tests: Na+ (sodium), K+ (potassium), Cl- (chloride) and bicarbonate (total CO2 content).
An anion gap can be calculated from the electrolytes and provides a clue to the cause of the acid/base imbalance.
Depending on the suspected cause, a number of other tests may be ordered: E/LFT, ketones in blood and urine, lactate, salicylates, ethylene glycol, and methanol, to name a few.
Treatment
Most acid-base disorders do not require treatment of the abnormal pH. The goal is to identify what has caused the acid-base disorder and, when necessary, treat this underlying cause. However, doctors may give intravenous HCO3- to patients with dangerously low blood pH levels.
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