Written by Samuel Rouleau
Spoon Feed
Drop the delta-delta and learn a simpler approach to acid-base calculations! Once you learn the basics, check your math with the JournalFeed Acid-Base Calculator.
Do hydrogen ions actually exist in isolation?
The traditional methods for acid-base calculations are confusing and time consuming. This approach is quick and simple!
Step 1: What’s the pH?
- pH < 7.35 → acidemia
- pH > 7.45 → alkalemia
- Note: A normal pH does not mean the patient’s physiology is “normal;” they could have multiple processes leading to a normal/compensated pH.
Step 2: What’s the pCO2?
- 35 – 45 → eucarbia
- > 45 → hypercarbic acidosis. This applies if the pH is low; if normal and the pCO2 is > 45, this could be the patient’s baseline, or they have a concurrent metabolic alkalosis.
- < 35 → hypocarbic alkalosis
Step 3: What’s the strong ion difference (SID)?
SID = Na – Cl.
The SID takes into account how the chloride level is impacting acid-base status and replaces the delta.
- > 40 → elevated SID, so there are more positive charges (sodium) than negative charges (chloride), meaning there is a metabolic alkalosis.
- Commonly caused by: vomiting, dehydration, administration of sodium bicarbonate
- < 36 → decreased SID, there is excess chloride relative to sodium, meaning there is a metabolic acidosis (non-anion gap). All non-anion gap metabolic acidoses (NAGMA) are hyperchloremic.
- Common causes: diarrhea, renal tubular acidosis, excessive chloride containing solution (i.e. normal saline)
Step 4: What’s the anion gap (AG)?
AG = Na – (Cl + bicarb)
- < 8 → AG alkalosis, typically due to loss of albumin or the presence of other unmeasured cations (i.e. lithium).
- > 15 → AG acidosis, time to break out MUDPILES/GOLDMARK.
How does this change my practice?
Overall, I like this approach!
- The SID calculation takes the place of the delta/delta by identifying if there is an additional metabolic process in addition to AG metabolic acidosis.
- The authors are critical of the Winter’s formula (Expected PaCO₂ = (1.5 × [HCO₃⁻]) + 8 ± 2), but in a patient with a severe metabolic acidosis, understanding an estimated compensated pCO2 can be helpful with regard to respiratory compensation (though to the author’s point, the pH matters infinitely more than the pCO2).
Editor’s note: It feels wrong, training under Corey Slovis, to have 4 steps. There should always be 5, right? You could also try our handy JournalFeed Acid-Base Calculator. ~Clay Smith
Source
Quantitative Acid-Base: A Simplified Approach for the Emergency Physician. J Emerg Med. 2025 Oct;77:50-60. doi: 10.1016/j.jemermed.2025.07.021. Epub 2025 Jul 15. PMID: 40850064.
