Traditional methods and equations

The equations, reference intervals, interpretation rules and limitations behind the Traditional approach.

A structured conventional interpretation: pH, bicarbonate and PaCO₂, expected compensation, albumin-corrected anion gap, then mixed-disorder checks.

The Traditional reading sequence

Establish the measured state. pH below 7.35 is acidaemia; above 7.45 is alkalaemia. A value inside this interval can coexist with an acid–base disorder.
Assess the primary pattern. Relate PaCO₂ and bicarbonate to pH. Consider alternative respiratory and metabolic explanations when both could fit.
Check the expected response. Use the compensation equation for the proposed process and the history-selected respiratory course.
Read the corrected gap. Compare a no-potassium AG with the appropriate reference interval, correcting for albumin when available.
Look for additional processes. Use the compensation departure, corrected gap and delta screen together with prior measurements and clinical context.

American Thoracic Society — Interpretation of arterial blood gases · SRLF/SFMU expert panel — Metabolic acidosis guideline (2019)

Measurements, units and gas calculation

The calculation uses adult samples at 37°C. Pressure is normalised to mmHg and albumin to g/L; 1 kPa = 7.50061683 mmHg and 1 g/dL = 10 g/L. Sodium, chloride and bicarbonate are expressed in mmol/L.

HCO₃⁻ = 0.0307 × PaCO₂ × 10^{(pH − 6.105)}

These are the Henderson–Hasselbalch constants already used by the Stewart calculator, retained for comparison between approaches. Gas bicarbonate is calculated from pH and PCO₂. If a reported actual bicarbonate is supplied, agreement checks arithmetic and entry consistency; it is not an independent validation of the sample.

Chemistry total CO₂ is a separate measurement. When entered and contemporaneous, it is used for the chemistry-based AG and delta calculations. Otherwise an AG calculated with gas-derived bicarbonate is labelled as an estimate. Do not enter standard bicarbonate as actual bicarbonate.

MSD Manual Professional — Acid–base disorders · AARC Clinical Practice Guideline — Blood gas analysis and haemoximetry (2013)

Expected compensation

Empirical compensation conventions used in this app
Primary process	Expected secondary response	Units
Metabolic acidosis	PaCO₂ = 1.5 × HCO₃⁻ + 8 ±2	mmHg
Metabolic alkalosis	PaCO₂ = 40 + (0.6–0.75) × (HCO₃⁻ − 24)	mmHg
Respiratory acidosis · acute	HCO₃⁻ = 24 + (1–2) × [(PaCO₂ − 40) / 10]	mmol/L
Respiratory acidosis · chronic	HCO₃⁻ = 24 + (3–4) × [(PaCO₂ − 40) / 10]	mmol/L
Respiratory alkalosis · acute	HCO₃⁻ = 24 − (1–2) × [(40 − PaCO₂) / 10]	mmol/L
Respiratory alkalosis · chronic	HCO₃⁻ = 24 − (4–5) × [(40 − PaCO₂) / 10]	mmol/L

The coefficients above are the source conventions chosen for this app. They use reference anchors of PaCO₂ 40 mmHg and HCO₃⁻ 24 mmol/L; they are empirical rules rather than universal physiological boundaries.

A further 2 mmol/L screening margin is applied outside respiratory bicarbonate bands, and 2 mmHg outside the metabolic-alkalosis PaCO₂ band. Results outside the formula band but within the added margin are labelled borderline. The margins are implementation conventions, not validated prediction intervals. Winter’s ±2 is part of its published equation.

Acute and chronic estimates are shown separately when the course is unknown. Acute-on-chronic and rapidly changing states need prior measurements. Severe values and very high compensatory PaCO₂ require particular caution with a linear rule.

For stable chronic hypercapnia up to 70 mmHg, the González study also supports a larger response: approximately 24 + 0.5 × (PaCO₂ − 40) ±2 mmol/L. When this published variation could explain a bicarbonate above the conventional chronic band, the app preserves uncertainty rather than automatically diagnosing metabolic alkalosis.

MSD Manual Professional — Primary changes and compensation · González et al. — Chronic respiratory-acidosis response (2018)

The gap, albumin and the selected reference

AG = Na⁺ − Cl⁻ − HCO₃⁻
Corrected AG = AG + 0.25 × [40 − albumin (g/L)]

Potassium is excluded from this equation and its reference. Albumin 40 g/L is the correction anchor. At albumin 20 g/L, for example, the correction adds 5 mmol/L. Missing albumin prevents a corrected result; it is not replaced with a normal value.

The default RCPA interval is 4–13 mmol/L without potassium. Use a method-matched local interval when available. The interval determines whether the gap is raised; its midpoint or an explicitly selected baseline anchors the delta calculation. Those are different functions.

The default midpoint is 8.5 mmol/L. It is a numerical midpoint, not an established personal baseline or a published population mean. An unexplained change in gap should be checked against prior results and the laboratory method.

RCPA Manual — Anion gap · Figge et al. — Anion gap and hypoalbuminaemia (1998)

Delta calculations and mixed metabolic screening

ΔAG = corrected AG − selected AG anchor
ΔHCO₃⁻ = 24 − HCO₃⁻
Delta ratio = ΔAG / ΔHCO₃⁻
Delta gap = ΔAG − ΔHCO₃⁻
Delta-adjusted HCO₃⁻ = HCO₃⁻ + ΔAG

For an established high-gap metabolic acidosis, a ratio below 1 suggests an additional normal-gap component; 1–2 is compatible with a predominantly high-gap pattern; above 2 suggests metabolic alkalosis or a previously raised bicarbonate. These labels prompt further assessment and do not prove another diagnosis.

The ratio is withheld when its assumptions are unsupported, including a near-zero or negative denominator, bicarbonate not below 22 mmol/L, uncorrected or non-elevated gap, incompatible samples, or an uncertain primary process. Delta-adjusted bicarbonate can still support review of a raised gap with near-normal bicarbonate.

The ratio varies with the disorder, baseline and treatment. A study of diabetic ketoacidosis found that narrow ratio cutoffs substantially overcalled mixed disorders. The more cautious labels here are intentional; use the numeric result and clinical history together.

American Thoracic Society — Interpretation of arterial blood gases · Paulson & Gadallah — Mixed disorders in diabetic ketoacidosis (1993) · Wrenn — The delta gap and mixed disorders (1990)

How the visual explanations should be read

Compensation: the observed value is compared with the same expected band used in the written assessment. Any added screening margin is disclosed separately.
Anion gap: the raw and albumin-corrected values share a declared reference. The correction is a numerical adjustment, not a measured concentration of a particular acid.
Delta analysis: the gap rise and bicarbonate fall use the displayed anchors and the same bicarbonate source. An offset suggests a question to investigate.
Physiology simulation: changing model inputs explores equations. Holding other values fixed does not predict the response of a living patient to treatment.

Traditional and Stewart views describe the same measurements with different organising frameworks. The gas calculation and compensation conventions are shared. A change in view is not new evidence about the patient.

Sample and interpretation limits

The routine pH 7.35–7.45, PaCO₂ 35–45 mmHg and bicarbonate 22–26 mmol/L bands are adult screening references; local and situation-specific interpretation takes priority.
Non-arterial samples retain descriptive calculations. Arterial primary-disorder and compensation conclusions are withheld, and no fixed venous-to-arterial conversion is applied.
A chemistry panel from a different time can change the gap conclusion. Resolve timing or measurement discrepancies before interpreting a combined pattern.
A known respiratory course comes from the history. The app does not infer chronicity merely because a bicarbonate value fits a chronic band.
A normal pH, compatible compensation, or gap within range does not exclude all illness or every mixed acid–base process.
Synthetic examples illustrate the equations and limitations. They are not independent clinical validation. The software remains an educational aid, not a treatment recommendation.

AARC Clinical Practice Guideline — Blood gas analysis and haemoximetry (2013) · MSD Manual Professional — Acid–base disorders

Source methods and further reading

Equations and clinical rules are paraphrased and applied to original synthetic examples. Reference publications are linked so the conventions and their limits can be checked directly.

Adult educational interpretation at 37°C. Apply arterial rules to arterial samples, use the laboratory’s reference intervals, and review the result alongside the clinical history.