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 refers 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: CMP (comprehensive metabolic panel), ketones in blood and urine, lactate, salicylates, ethylene glycol, and methanol, to name a few.
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