Acid–Base Assessment

Blood Gas Analyser

ABG and VBG Interpretation Tool for Medical Students and Junior Doctors

A step-by-step ABG and VBG interpretation tool for medical students and junior doctors. Enter pH, PCO₂, bicarbonate, electrolytes, albumin, lactate, and oxygenation values to identify the primary acid–base disorder, assess compensation, calculate anion gap, and detect possible mixed disorders.

ABG / VBG Acid–Base Anion Gap Compensation Medical Student Tool
Enter Blood Gas Values

Required

Enter pH between 6.80 and 7.80.
mmHg
Enter PCO₂ between 5 and 150 mmHg.
mmol/L
Enter HCO₃⁻ between 3 and 60 mmol/L.
mmol/L
Enter Na⁺ between 90 and 180 mmol/L.
mmol/L
Enter Cl⁻ between 50 and 140 mmol/L.

Optional

Enter albumin 1–7 g/dL or 10–70 g/L.
mmol/L
Enter lactate between 0 and 30 mmol/L.
%
mmHg
Enter PO₂ between 20 and 700 mmHg.

Enter values on the left and click Analyse to see the step-by-step interpretation.

How to Interpret a Blood Gas

A blood gas should be interpreted systematically. First decide whether the pH shows acidaemia, alkalaemia, or a near-normal value. Then identify whether the primary process is metabolic or respiratory by comparing PCO₂ and bicarbonate. After that, assess whether compensation is appropriate. Finally, calculate the anion gap, consider albumin correction, and look for evidence of mixed acid–base disorders.

Common Acid–Base Patterns

Pattern pH PCO₂ HCO₃⁻ Example Causes
Metabolic acidosis Low Low/compensatory Low DKA, lactic acidosis, renal failure, diarrhoea
Metabolic alkalosis High High/compensatory High Vomiting, diuretics, mineralocorticoid excess
Respiratory acidosis Low High Normal/high COPD, hypoventilation, CNS depression
Respiratory alkalosis High Low Normal/low Anxiety, pain, sepsis, pregnancy, liver disease

Formula Reference

-- Anion gap --
AG = Na+ − (Cl− + HCO3−)
-- Albumin-corrected anion gap --
Corrected AG = AG + 2.5 × (4.0 − albuming/dL)
-- Winter's formula (metabolic acidosis compensation) --
Expected PCO₂ = 1.5 × HCO3− + 8 ± 2
-- Metabolic alkalosis compensation --
Expected PCO₂ = 0.7 × (HCO3− − 24) + 40 ± 5
-- Delta ratio --
Delta ratio = (AG − 12) / (24 − HCO3−)
-- P/F ratio (oxygenation) --
P/F ratio = PaO2 / FiO2

Worked Examples

Example 1 — High Anion Gap Metabolic Acidosis
Given: pH 7.20, PCO₂ 25 mmHg, HCO₃⁻ 10 mmol/L, Na 140, Cl 100
AG: 140 − (100 + 10) = 30 mmol/L (High)
Winter's formula: Expected PCO₂ = 1.5 × 10 + 8 = 23 ± 2 mmHg. Actual PCO₂ 25 — within expected range.
Interpretation: High anion gap metabolic acidosis with appropriate respiratory compensation.
Example 2 — Respiratory Acidosis
Given: pH 7.25, PCO₂ 70 mmHg, HCO₃⁻ 30 mmol/L
Acute expected HCO₃⁻: 24 + (30/10) × 1 = 27 mmol/L
Chronic expected HCO₃⁻: 24 + (30/10) × 3.5 – 4 = 34.5–36 mmol/L
Interpretation: Primary respiratory acidosis. Raised bicarbonate suggests renal compensation; compare with acute and chronic expected bicarbonate ranges.
Example 3 — Metabolic Alkalosis
Given: pH 7.52, PCO₂ 48 mmHg, HCO₃⁻ 38 mmol/L
Expected PCO₂: 0.7 × (38 − 24) + 40 = 49.8 ± 5 mmHg. Actual PCO₂ 48 — within expected range.
Interpretation: Primary metabolic alkalosis with appropriate respiratory compensation.

Frequently Asked Questions

Yes. The tool can help interpret acid–base patterns in both ABG and VBG samples. However, oxygenation assessment should be based on ABG PaO₂ rather than VBG PO₂.

Albumin is a major unmeasured anion. Low albumin can reduce the measured anion gap and may hide a high anion gap metabolic acidosis. Albumin correction helps adjust for this effect.

Winter's formula estimates the expected respiratory compensation in metabolic acidosis. If the actual PCO₂ is outside the expected range, an additional respiratory disorder may be present.

No. A normal or near-normal pH can occur in compensated disorders or mixed acid–base disorders. Always interpret pH together with PCO₂, bicarbonate, anion gap, and clinical context.

Limitations

  • This tool is for educational interpretation. It does not replace clinical assessment, senior review, or urgent management.
  • Compensation formulas are approximations. Individual variation and mixed disorders can produce results outside expected ranges.
  • VBG PO₂ does not reflect arterial oxygenation — oxygenation should be assessed with ABG PaO₂ where possible.
  • Normal anion gap ranges vary between laboratories depending on the analyser and whether potassium is included.
  • Albumin correction uses a standard formula and may not account for all sources of variation.

References

  1. Standard acid–base interpretation formulas used in clinical medicine.
  2. Winter's formula for respiratory compensation in metabolic acidosis.
  3. Cockcroft-style educational medical calculator format already used on atmedstu tools.
  4. Local laboratory and hospital protocols should be followed for clinical decisions.
Reference Ranges
pH normal7.35–7.45
PCO₂ normal35–45 mmHg
HCO₃⁻ normal22–26 mmol/L
Anion gap normal8–12 mmol/L
Lactate normal<2 mmol/L
P/F ratio normal≥300

Reference ranges are approximate. Always check local laboratory ranges.

Urgent Thresholds
  • pH < 7.20 or > 7.60
  • PCO₂ > 70 mmHg
  • HCO₃⁻ < 10 mmol/L
  • Lactate ≥ 4 mmol/L
  • P/F ratio < 200
Delta Ratio
<0.4HAGMA + NAGMA
0.4–0.8Mixed AG acidosis
0.8–2.0Pure HAGMA
>2.0HAGMA + met. alk.

Calculated only when AG >12 and HCO₃⁻ <24.

Lactate Levels
Normal<2 mmol/L
Raised2–3.9 mmol/L
High≥4 mmol/L
Related Calculators
Anion Gap Calculator

Calculate anion gap and albumin-corrected anion gap for acid–base assessment.
Corrected Calcium Calculator

Correct serum calcium for abnormal albumin using mg/dL or mmol/L.
Creatinine Clearance Calculator

Estimate creatinine clearance using the Cockcroft–Gault equation.
Educational use only. This blood gas analyser is designed for medical students and junior doctors. It does not replace clinical judgement, senior review, local protocols, laboratory reference ranges, or urgent management of unstable patients.