Acid-Base Physiology

9.2 Systematic Evaluation of Acid-Base Status

The next stage of assessment is to systematically evaluate the arterial blood gas results and other results to make a complete diagnosis of the acid-base disturbance. An overview of the six sequential steps involved are outlined below and then again in detail on the opposite page.

CAUTION: An occasional problem occurs due to incorrect transcription of blood-gas results. If you are working from a hand-written copy of results then you should always consider whether there has been an error in writing the results down (eg mis-heard over the phone for example). A check of pH, pCO2 & HCO3 against the Henderson-Hasselbalch equation is usually difficult without a calculator. However, a quick check of the logical consistency of the results is often possible. For example, pH must be less then 7.4 if PCO2 is high & HCO3 is low. It is preferable to review the result print-out from the machine.

The Six Steps of Systematic Acid-Base Evaluation

1. pH: Assess the net deviation of pH from normal

2. Pattern: Check the pattern of bicarbonate & pCO2 results

3. Clues: Check for additional clues in other investigations

4. Compensation: Assess the appropriateness of the compensatory response

5. Formulation: Bring the information together and make the acid base diagnosis

6. Confimation: Consider if any additional tests to check or support the diagnosis are necessary or available & revise the diagnosis if necessary

The first step is to look at the arterial pH. A net acidaemia means that an acidosis must be present. A net alkalaemia means that an alkalosis must be present. A normal pH gives 2 possibilities: no acid-base disorder or a mixed disorder with an alkalosis compensating for an acidosis

The next step is to determine whether any disorder is of the respiratory or metabolic type by reviewing the pattern and magnitude of the bicarbonate and pCO2 results. If the disorder is a simple one (ie only one primary disorder present) then the acid-base disorder is diagnosed at this step. But the real problem is that this is not known so the evidence must always be checked for evidence of a mixed disorder. This is an important part of steps 2, 3 and 4


Systematic Approach to Blood Gas Analysis

1. pH: Check arterial pH

Principle: The net deviation in pH will indicate whether an acidosis or an alkalosis is present (but will not indicate mixed disorders)


  • IF an acidaemia is present THEN an acidosis must be present
  • IF an alkalaemia is present THEN an alkalosis must be present
  • IF pH is normal pH THEN Either (no acid-base disorder is present)  or (Compensating disorders are present ie a mixed disorder with an acidosis and an alkalosis)

2. PATTERN: Look for suggestive pattern in pCO2 & [HCO3]

Principle: Each of the simple disorders produces predictable changes in [HCO3] & pCO2.


  • IF Both [HCO3] & pCO2 are low THEN Suggests presence of either a Metabolic Acidosis or a Respiratory Alkalosis (but a mixed disorder cannot be excluded)
  • IF Both [HCO3] & pCO2 are high THEN Suggests presence of either a Metabolic Alkalosis or a Respiratory Acidosis (but a mixed disorder cannot be excluded)
  • IF [HCO3] & pCO2 move in opposite directions THEN a mixed disorder MUST be present

Which disorder is present is dependent on which change is primary and which is compensatory, and this requires an assessment based on the history, examination & other results.

3. CLUES: Check for clues in the other biochemistry results

Principle: Certain disorders are associated with predictable changes in other biochemistry results

Examples: See separate list of 'Aids to Interpretation' below

4. COMPENSATION: Assess the Compensatory Response

Principle: The 6 Bedside Rules are used to assess the appropriateness of the compensatory response.


  • If the expected & actual values match => no evidence of mixed disorder
  • If the expected & actual values differ => a mixed disorder is present

5. FORMULATION: Formulate the Acid-Base Diagnosis

  • Consider all the evidence from the history, examination & investigations and try to formulate a complete acid-base diagnosis

6. CONFIRMATION: Check for specific biochemical evidence of particular disorders for confirmation

Principle: In some cases, further biochemical evidence can confirm the presence of particular disorders. Changes in these results may be useful in assessing the magnitude of the disorder or the response to therapy.

Examples: Lactate, urinary ketones, salicylate level, aldosterone level, various tests for renal tubular acidosis

Step 3 involves reviewing other results looking for specific evidence of particular disorders. Some of these 'clues' are outlined in the table below. In most circumstances, these clues are confirmatory of the expected diagnosis but on occasion can alert to the presence of an unanticipated second disorder. An elevated anion gap can be particularly useful. Most of these 'clues' are obtained from the biochemistry profile. An alert clinician can often correctly pick the diagnosis before the gas results are back.

Some Aids to Interpretation of Acid-Base Disorders



High anion gap

Always strongly suggests a metabolic acidosis.


If ketones also present in urine -> diabetic ketoacidosis

Hypokalaemia and/or hypochloraemia

Suggests metabolic alkalosis


Common with normal anion gap acidosis

Elevated creatinine and urea

Suggests uraemic acidosis or hypovolaemia (prerenal renal failure)

Elevated creatinine

Consider ketoacidosis: ketones interfere in the laboratory method (Jaffe reaction) used for creatinine measurement & give a falsely elevated result; typically urea will be normal.

Elevated glucose

Consider ketoacidosis or hyperosmolar non-ketotic syndrome

Urine dipstick tests for glucose and ketones

Glucose detected if hyperglycaemia; ketones detected if ketoacidosis

The 4th step is to assess acid-base compensation. The approach discussed here involves the use of a set of six rules. These are discussed in Section 9.3. Much of the emphasis here is to pick the presence of a second acid-base disorder.

Step 5: The stage should now be reached in that a definitive overall acid-base assessment can be made.

Step 6: Sometimes the diagnosis suggests additional tests that can be used to confirm the diagnosis or at least allow more precise diagnosis. An example would be a measurement of blood salicylate level in a child which if high can confirm a clinical suspicion of a salicylate overingestion. If a diagnosis of renal tubular acidosis is suspected then further specific tests can be done to further specify the diagnosis.