# Acid–Base Analysis: Find Every Process

Calculate compensation and the gap, then use the mismatch to find an additional disorder; a plausible first diagnosis is not the stopping point.

![Infographic: Acid–Base Analysis: Find Every Process](https://urinenephrology.org/visual-reference/images/acid-base-approach.png?v=20261003c)

## Name the direction

Acidemia is pH <7.35; alkalemia >7.45. Low bicarbonate suggests metabolic acidosis, high bicarbonate metabolic alkalosis; PaCO₂ moves pH in the opposite direction. A normal pH can hide opposing disorders, so always inspect all three values.

## Test metabolic-acidosis compensation

Expected PaCO₂ ≈ 1.5 × HCO₃ + 8 ±2 mmHg. A measured value above this suggests added respiratory acidosis; below it suggests respiratory alkalosis. This is an approximation for metabolic acidosis, not a formula for every acid–base disorder.

## Calculate and contextualize the gap

AG = Na − (Cl + HCO₃), using the laboratory’s reference interval. Low albumin can conceal unmeasured acids; a common correction adds about 2.5 mEq/L for each 1 g/dL albumin below 4. Check lactate, ketones, kidney function, or toxins as indicated.

## Search for a second metabolic process

Compare the rise in AG with the fall in bicarbonate from their expected baselines. A disproportionately preserved/high bicarbonate suggests added alkalosis; a larger bicarbonate fall suggests added normal-gap acidosis. Baseline variation and treatment can distort delta comparisons.

## Worked calculation

Synthetic example: Na 140, Cl 100, HCO₃ 12 gives AG 28. Expected PaCO₂ is 26 ±2. If measured PaCO₂ is 40, respiratory compensation is inadequate and an additional respiratory acidosis is present; assess ventilation urgently alongside the metabolic cause.

## Recalculate as treatment changes physiology

Vomiting can add alkalosis to DKA; saline can add normal-gap acidosis during recovery. Use measurements obtained close together, treat the underlying causes and potassium abnormalities, and repeat assessment rather than assuming the first diagnostic label remains complete.

## Supporting evidence

- [PubMed 25295502](https://pubmed.ncbi.nlm.nih.gov/25295502/)
- [PubMed 39052901](https://pubmed.ncbi.nlm.nih.gov/39052901/)

## Source lessons

- [index](https://urinenephrology.org/2025_UDPA_Lectures_Live/electrolytes/acid-base-comprehensive/index.html)
- [case6 enhanced](https://urinenephrology.org/2025_UDPA_Lectures_Live/cases/case6_enhanced.html)

Read alongside the full lessons; the findings and decisions shown here require the stated clinical context.