Interpret arterial blood gas results including pH, PaCO2, HCO3, base excess, anion gap, and delta-delta ratio for acid-base disorders.
Arterial blood gas (ABG) analysis is one of the most important diagnostic tests in critical care and emergency medicine. By measuring arterial pH, partial pressure of carbon dioxide (PaCO₂), bicarbonate (HCO₃⁻), and partial pressure of oxygen (PaO₂), clinicians can rapidly identify life-threatening acid-base disturbances and guide treatment.
The interpretation of ABG results follows a systematic approach: first identifying whether the patient is acidemic or alkalemic, then determining whether the primary disorder is respiratory (driven by PaCO₂) or metabolic (driven by HCO₃⁻), and finally assessing whether appropriate compensation is present. Complex mixed disorders require additional calculations including the anion gap and delta-delta ratio.
This comprehensive ABG interpreter automates the entire analysis pipeline. It identifies the primary acid-base disorder, calculates base excess, evaluates compensation using Winter's formula, computes the anion gap with albumin correction, and performs delta-delta analysis to detect concurrent mixed disorders. The tool is designed around the stepwise approach taught in medical education and used daily in ICUs and emergency departments worldwide. It handles all four primary acid-base disorders plus mixed presentations.
This ABG interpreter provides a systematic, complete analysis of arterial blood gas results in seconds. It identifies the primary disorder, checks compensation, calculates anion gap with albumin correction, and detects mixed acid-base disorders—all steps that are easy to overlook or miscalculate under clinical pressure. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain.
Base Excess ≈ 0.9287 × HCO₃⁻ + 13.77 × pH − 124.58. Anion Gap = Na⁺ − Cl⁻ − HCO₃⁻ (normal 8–12). Corrected AG = AG + 2.5 × (4 − Albumin). Winter's Formula (expected PaCO₂) = 1.5 × HCO₃⁻ + 8 ± 2. Delta Ratio = (AG − 12) / (24 − HCO₃⁻).
Result: Primary: Metabolic Acidosis. Anion gap = 21 mEq/L (elevated). Appropriate respiratory compensation.
pH 7.25 (acidemia) with low HCO₃⁻ of 14 indicates metabolic acidosis. PaCO₂ of 30 shows respiratory compensation. Expected PaCO₂ by Winter's formula = 1.5 × 14 + 8 = 29 ± 2, so compensation is appropriate. AG = 140 − 105 − 14 = 21 (elevated, suggesting AG metabolic acidosis such as DKA, lactic acidosis, or toxins).
The systematic approach to ABG interpretation taught in medical schools follows a clear algorithm. First, determine the pH: is the patient acidemic (pH < 7.35), alkalemic (pH > 7.45), or normal? Next, examine PaCO₂ and HCO₃⁻ to determine whether the primary process is respiratory or metabolic. The direction of the abnormality that "matches" the pH direction identifies the primary disorder.
The body compensates for acid-base disturbances to bring pH back toward normal, but compensation never fully corrects pH. In acute respiratory acidosis, HCO₃⁻ rises approximately 1 mEq/L per 10 mmHg rise in PaCO₂. In chronic respiratory acidosis (>3–5 days), HCO₃⁻ rises approximately 3.5 mEq/L per 10 mmHg. For metabolic acidosis, Winter's formula predicts the expected PaCO₂.
When metabolic acidosis is identified, calculate the anion gap. An elevated AG indicates accumulation of unmeasured anions (lactic acid, ketoacids, toxic alcohols, uremia). A normal AG acidosis suggests bicarbonate loss (diarrhea, RTA). The delta-delta ratio then checks for additional hidden disorders superimposed on the AG acidosis.
Normal ABG values: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg, Base excess −2 to +2.
The anion gap (AG = Na⁺ − Cl⁻ − HCO₃⁻) represents unmeasured anions in serum. An elevated AG (>12) suggests accumulation of acids like lactate, ketoacids, or toxins.
Winter's formula predicts expected PaCO₂ in metabolic acidosis: Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. If actual PaCO₂ differs, a concurrent respiratory disorder is present.
The delta-delta ratio compares the change in AG to the change in HCO₃⁻. A ratio <1 suggests concurrent non-AG metabolic acidosis; >2 suggests concurrent metabolic alkalosis; 1–2 indicates pure AG metabolic acidosis.
Albumin carries negative charges that contribute to the anion gap. Hypoalbuminemia (common in hospitalized patients) lowers the AG, potentially masking an elevated AG acidosis.
Yes. Air bubbles falsely elevate PaO₂ and lower PaCO₂. Delayed analysis allows continued cellular metabolism, lowering pH and PaO₂. Samples should be analyzed within 10–15 minutes or kept on ice.