Essential reading in acid-base disorders

This compilation of resources is meant to guide the CICM Second Part Exam candidate in what the hell are they supposed to do to prepare for the Second Part Exam acid-base questions. Of these, a historically large number have required trainees to interpret an arterial blood gas result. These were generally viewed as easy marks, as it was possible to answer them with moderate success after reading nothing more than Chapters 3-10 of (by Kerry Brandis). It helped that these were generously repeated through the years, which was probably some form of poor assessment design. The reliance on data interpretation questions has been diminishing in recent history, and trainees can expect them to be fewer in number and greater in complexity. 

But first, some ground rules:

  • Where the college asks you to calculate the anion gap, you do not use potassium in the calculation.
  • The "normal" value for the anion gap is assumed to be 12.
  • Where albumin is offered in the stem of the question, it is expected that you will correct the anion gap for albumin; but otherwise the albumin is assumed to be 40g/L.
  • The delta ratio interpretation is conventional, i.e.:
    • 0.4 = normal anion gap metabolic acidosis
    • 0.4-0.8 = mixed high and normal anion gap acidosis exists.
    • 0.8-1.0 = purely due to a high anion gap metabolic acidosis
    • 1.0-2.0 = still purely a high anion gap metabolic acidosis
    • Over 2.0 = high anion gap acidosis with pre-existing metabolic alkalosis

Useful guides for blood gas and acid-base data interpretation


Again, this unsexy topic does not attract a whole lot of attention from critical care societies, as they prefer to write multiple subtely different duplicates of the same ARDS guidelines


Morgan, Thomas J. "Acid–base balance and disorders." Oh's intensive care manual. Butterworth-Heinemann, 2014. 937-948.

Morgan, T. J. "What exactly is the strong ion gap, and does anybody care?" Critical Care and Resuscitation (2004) 6: 155-166.

Sirker, A. A., et al. "Acid− base physiology: the ‘traditional’and the ‘modern’approaches."  Anaesthesia 57.4 (2002): 348-356.

Kraut, Jeffrey A., and Nicolaos E. Madias. "Serum anion gap: its uses and limitations in clinical medicine." Clinical Journal of the American Society of Nephrology 2.1 (2007): 162-174.

Rastegar, Asghar. "Use of the ΔAG/ΔHCO3− Ratio in the Diagnosis of Mixed Acid-Base Disorders." Journal of the American Society of Nephrology 18.9 (2007): 2429-2431.

Figge, James, et al. "Anion gap and hypoalbuminemia." Critical care medicine 26.11 (1998): 1807-1810.

Reddi, Alluru S. "Interpretation of Urine Electrolytes and Osmolality." Fluid, Electrolyte and Acid-Base Disorders. Springer New York, 2014. 13-19.

Schlichtig, Robert, Alan W. Grogono, and John W. Severinghaus. "Human PaCO2 and standard base excess compensation for acid-base imbalance."Critical care medicine 26.7 (1998): 1173-1179.

Krahn, John, and Anna Khajuria. "Is Osmol Gap an Effective Screen in Accurate Prediction." Clin. Lab 57 (2011): 297-303.

Jung, Boris, et al. "Diagnosis and management of metabolic acidosis: guidelines from a French expert panel." Annals of intensive care 9 (2019): 1-17.

Davis, Michael D., et al. "AARC clinical practice guideline: blood gas analysis and hemoximetry: 2013." Respiratory care 58.10 (2013): 1694-1703.