‘Rules of 5’ for acid-base problem-solving

1 Determine the arterial pH status
pH < 7.40 is acidemic, pH > 7.44 is alkalemic
But a normal pH does not rule out an acid-base disorder
2 If the arterial pH is abnormal, determine whether the primary process is respiratory, metabolic, or both
pHEmbedded Image Bicarbonate
Respiratory acidosisLowHigh
Metabolic acidosisLowLow
Mixed respiratory and metabolic acidosisLowHighLow
Respiratory alkalosisHighLow
Metabolic alkalosisHighHigh
Mixed respiratory and metabolic alkalosisHighLowHigh
3 Calculate the anion gap
Anion gap = sodium – (chloride + bicarbonate)
If serum albumin is low, add 2.5 mmol/L to the anion gap for every 1 g the serum albumin is below normal
An anion gap > 10 mmol/L is elevated
4 Check the degree of compensation (respiratory or metabolic)
Embedded Image and bicarbonate should move in the same direction
Nominal normal levels: bicarbonate 25 mmol/L and Embedded Image mm Hg
In respiratory acidosis, for every 10-mm Hg increase in Embedded Image , bicarbonate should increase by 1 mmol/L in the first 48 hours and 4 mmol/L afterward
In metabolic acidosis, for every 1-mmol/L decrease in bicarbonate, Embedded Image should decrease by 1.3 mm Hg
In respiratory alkalosis, for every 10-mm Hg decrease in Embedded Image bicarbonate should decrease by 2 mmol/L in the first 48 hours and 5 mmol/L afterward
In metabolic alkalosis, for every 1-mmol/L increase in bicarbonate, Embedded Image may increase by 0.6 mm Hg
5 If the patient has metabolic acidosis with an elevated anion gap, check whether the bicarbonate level has decreased as much as the anion gap has increased
In metabolic acidosis, the anion gap should increase by the same amount that bicarbonate decreases; a difference in these two changes is called a delta gap
  • Embedded Image = partial pressure of carbon dioxide

  • Based on information in reference 1