Tests for the Acid-base Balance Status

Clinical assesment

 * The most clinical useful information comes from the clinical description of the patient by the history and physical examination.
 * The H&P usually gives an idea of what acid base disorder might be present even before collecting the ABG sample.


 * The major causes of simple and mixed disturbances shoud be kept in mind so that such disorders may be predicted from the patient’s clinical picture. For example,
 * patients with severe vomiting and those on long-term thiazide therapy can be expected to have metabolic alkalosis,
 * those with renal failure may have metabolic acidosis
 * and patients with pneumonia or congestive cardiac failure may have respiratory alkalosis…


 * The possibility of an acid-base balance disturbance suggested by clinical presentation should then be evaluated by appropriate laboratory tests.

Blood gases

 * In order to interpret acid-base disturbances, the following five factors are considered:
 * pH
 * HCO3-
 * PCO2
 * Anion gap
 * and assessment for compensation


 * The first step is to determine if the patient is acidemic or alkalemic, based on pH.
 * Second, the primary disorder is determined by evaluating HCO3– and PCO2.
 * If HCO3– is elevated and pH is elevated, there is metabolic alkalosis.
 * If both are decreased, there is metabolic acidosis.


 * Next, one must look at the PCO2 in the context of the HCO3–.
 * If HCO3– is within the normal reference range and PCO2 is elevated but the patient is acidotic,
 * the condition is respiratory acidosis.
 * If bicarbonate is within the normal reference range and PCO2 is decreased but the patient is alkalotic,
 * the condition is respiratory alkalosis.


 * Next determine the anion gap, using standard formulas below, to determine the etiology of metabolic acidosis.
 * Anion gap = [Na] – ([Cl-] + [HCO3-])


 * Finally the pH, HCO3–, and PCO2 are considered to determine if compensation is as expected based on the typical ratio of 20:1 for bicarbonate to carbonic acid.
 * For example, both decreased HCO3– and PCO2 should produce a slightly decreased or nearly normal pH if they are in metabolic acidosis compensation.
 * To determine the actual ratio of bicarbonate to carbonic acid, PCO2 is converted to H2CO3 using the relationship
 * PCO2 * 0.03 = H2CO3


 * Compensation for metabolic acidosis or alkalosis is achieved initially by the respiratory system.
 * Respiratory compensation for acidosis means that the lungs increase the level of alveolar ventilation, which raises the pH toward normal.
 * The increased ventilation eliminates or blos off CO2, which eliminates carbonic acid.
 * Also, the presence of acidosis normally increases respiratory drive.
 * The respiratory system compensates for a metabolic defect.


 * In metabolic alkalosis, some decrease in ventilation occurs but the PCO2 generally remains normal since respiratory compensation doesn’t occur until alkalosis has been severe and prolonged.
 * Compensation for metabolic alkalosis is less complete since hypoventilation is not a naturally sustainable condition.