Here are the calculations that I used in my presentation at ASA 2025.

pH versus  PaCO₂ 

An increase of 10 mmHg in PaCO₂ results in a pH drop of about 0.08

Respiratory Acidosis PaCO₂  vs HCO₃^–

In Acute Respiratory Acidosis (e.g. patient hypoventilating in the OR) the Bicarbonate Increases by 1mmol/L for every 10mmHg Increase in PaCO₂

In Chronic Hypercarbia (COPD) the Bicarbonate ↑ by 4mmol/L for every 10mmHg Increase in PaCO₂ and Cl^– falls by an equivalent amount

Modern Anion Gap = [Na⁺ + K⁺] – [Cl^– + HCO₃^– + La^–+ βOH^–] = Albumin^– + PO₄^2- + UMA^– mEq/L

[Albumin⁻]= [Albumin g/L] × (0.123 × pH−0.631)

Albumin Charge is simplified to 2.5 (albumin in g/dl) = Alb in mEq/L

Change in Albumin is (44 – Albumin) / 4

To determine the effectiveness of respiratory compensation use the Winters formula:

(Bicarbonate Version) Expected PaCO₂ in Acute Metabolic Acidosis is
1.5 x [HCO₃^–] + 8 (in mmHg)

(Base Deficit Version) Expected PaCO₂ in Acute Metabolic Acidosis is
Normal PaCO₂ – BD (in mmHg)

The Base Excess Gap (Fencl Story with my modification)

Identify the BE on the ABG

Calculate the SID for Na⁺-Cl^–+H₂O by [Na⁺ – Cl^– – 35] BD_NaCl

Calculate the SID for La^– and β-OH^– (1mmol = 1mEq) BD_LβOH

Calculate the Impact of Albumin (44 – Alb^– g/L)/4 BE_ALB

Add these together BE_NaCl BD_LβOH BE_ALB

Subtract from BE on the Blood Gas

The result is UMA in mEq/L

Finally, the Strong Ion Gap (arguably the gold standard)

The calculation for the strong ion gap (SIG) is:

Strong Ion Gap (SIG) = SIDa-SIDe

SIDa (apparent SID) = ([Na⁺] + [K⁺] + [Mg²⁺] + [Ca²⁺]) – ([Cl^–] + [Lactate^–] + [βOH^–])

SIDe (effective SID) = [HCO₃^–] + [charge on albumin] + [charge on Pi]

The degree of ionization for weak acids is pH dependent, so one must calculate for this:

[charge on albumin] = [albumin] (in g/L) x (0.123 x pH – 0.631)

[charge on Pi] = [Pi] (in mg/dL) /10 x pH – 0.47

The SIG quantifies UMA