Stewart Light

Acid-base calculator

Start with the familiar Boston question: does the respiratory compensation fit? Then use Stewart Light to ask what the metabolic SBE is made of: strong-ion/chloride effects, albumin effects, and residual unmeasured ions.

Original Stewart Light manuscript

Why this exists

Stewart Light decomposes multiple metabolic processes.

Traditional bicarbonate/SBE-plus-compensation reasoning is often clear when one process dominates. ICU acid-base problems are frequently less transparent: chloride shifts, hypoalbuminemia, lactate, unmeasured ions, fluids, diuresis, and chronic CO2 retention can move in opposite directions and make the total SBE look deceptively simple.

This tool keeps Boston-style compensation assessment visible, then partitions measured SBE into chloride/SID, albumin/weak-acid, and residual unmeasured-ion components using the Stewart Light method proposed by Duška et al. in the original manuscript.

Inputs

Blood gas and chemistry

Blood gas
Chemistry
Units
Optional clinical context
Result layers

Selecting a case fills the form with synthetic teaching values.

Results

Interpretation

Orientation

What base excess means — and when it is misleading

SBE near zero can mean true metabolic normality or equal-and-opposite metabolic processes, but there are several types of processes that can result in the same HCO3 / SBE. Stewart Light is useful because it differentiates these by separating chloride/SID, albumin, and residual unmeasured-ion effects.

Example Metabolic Processes

These are fixed teaching examples, not values calculated from the current inputs.

UI -8
Isolated lactate / UI acidosis
Cl +7
Isolated hypochloremic alkalosis
-6 +6
Offset case, net SBE ≈ 0

A normal total does not guarantee a simple process.

Step 1

Clinical context, pH severity, and Boston rules

Start with the clinical picture, pH direction and severity, and expected respiratory compensation. For metabolic acidosis, this includes the Boston/Winter check of expected PaCO2.

Enter values and calculate to review respiratory compensation, then inspect what the metabolic SBE is made of.

Steps 2-4

Partition measured SBE

Base excess partitioning separates the measured metabolic SBE into SID/chloride, albumin/weak-acid, and residual unmeasured-ion components. Segments can point in opposite directions, so a near-normal total may reflect offsetting processes rather than one normal metabolic state.

  • Step 2: Strong-ion effect. SBE_SID = Na - Cl - SID reference; outside pH 7.30-7.50, use SID reference = 35 + 15 * (7.40 - pH), equivalent to 1.5 mmol/L per 0.10 pH unit, upward in acidemia and downward in alkalemia. Negative values suggest strong-ion acidosis; positive values suggest strong-ion alkalosis.
  • Step 3: Weak-acid effect. SBE_Alb = 0.3 * (40 - albumin g/L); hypoalbuminemia contributes a positive alkalinizing component.
  • Step 4: Remainder. SBE_UI = SBE - SBE_SID - SBE_Alb; if lactate is available, it is split from the residual unmeasured-ion component.

Calculate a case to see component direction and magnitude.

    Synthesis

    Synthesize the Boston and Stewart Light views

    Put the Boston compensation assessment together with the SID, albumin, and unmeasured-ion components. Clinically decide whether a positive SBE_SID is consistent with a primary chloride-depletion process or an expected renal response to chronic hypercapnia.

    Stewart Light decomposition

    Context checks

    Reconcile the partition with lactate, anion-gap context, chronic hypercapnia suspicion, lab timing, and the overall clinical picture.

    Why can the total look normal even when components are not?

    This simplified calliper view shows the sodium-chloride gap as the SID anchor, then separates weak-acid and residual unmeasured-ion effects. This is an educational bedside simplification, not a full physicochemical reconstruction. Use it as a mental model for how chloride/SID, albumin, and unmeasured ions can push SBE in different directions.

    Calculate a case to see the conceptual SID, albumin, bicarbonate, and UI spaces.