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Tests

Tests are performed to determine whether a patient has an acid-base disorder and 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 those that measure blood gases and electrolytes.

"Blood gases" refers to a group of tests performed together on an arterial blood sample (blood obtained from an artery instead of a vein). They provide 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 pH HCO3 PCO2 Body Compensation
Metabolic acidosis Less than 7.35 Low Low Increased breathing rate (hyperventilation) to increase CO2 elimination
Metabolic alkalosis Greater than 7.45 High High Slowed breathing (hypoventilation) to decrease CO2 elimination
Respiratory acidosis Less than 7.35 High High Kidney increases production of HCO3- and excretion of H+ (acid)
Respiratory alkalosis Greater than 7.45 Low Low Decreased production of HCO3- and excretion of H+

"Electrolytes" refers to a group of four tests that measure Na+ (sodium), K+ (potassium), Cl- (chloride) and bicarbonate (total CO2 content).

An anion gap (see electrolytes /Common questions, for an explanation) can be calculated from the electrolytes; this provides a clue to the cause of the acid/base imbalance.

Depending on the suspected cause, a number of other tests may be ordered, including an E/LFT, a test for ketones in blood and ketones in urine, and tests for lactate, salicylates, ethylene glycol and methanol, to name but 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.


Last Review Date: April 5, 2017