How do I interpret an ABG?

Follow a systematic approach: (1) Look at pH — acidemia (<7.35) or alkalemia (>7.45)? (2) Identify the primary disorder — is the pCO₂ or HCO₃⁻ driving the pH change? (3) Assess compensation — is the other value moving in the expected direction? (4) Calculate anion gap if metabolic acidosis is present. This structured method ensures you don't miss mixed disorders.

What are normal ABG values?

Normal ranges: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg, SaO₂ 95–100%. Values outside these ranges indicate an acid-base or oxygenation disorder. Remember that 'normal' varies by altitude (PaO₂ is lower at high altitude) and age (PaO₂ declines with age).

What is Winter's formula?

Winter's formula predicts the expected PaCO₂ in a primary metabolic acidosis: Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (±2). If the actual PaCO₂ matches, compensation is appropriate. If lower, there is a concurrent respiratory alkalosis. If higher, there is a concurrent respiratory acidosis. Winter's formula only applies to metabolic acidosis, not other acid-base disorders.

Can there be mixed acid-base disorders?

Yes. Patients can have two or even three simultaneous acid-base disorders. Clues include: pH that is more extreme than expected for either disorder alone, PaCO₂ that does not match expected compensation formulas, or an anion gap that does not correlate with the bicarbonate change (delta-delta analysis). Mixed disorders are common in critically ill patients.

How long does compensation take?

Respiratory compensation (lungs adjusting for metabolic disorders) begins within minutes and maximizes in 12-24 hours. Renal compensation (kidneys adjusting for respiratory disorders) takes 3-5 days to reach full effect. In acute disorders, you may see the primary abnormality without full compensation yet. In chronic disorders, compensation is typically complete, which is why pH may be near normal despite significant PaCO₂ or HCO₃⁻ abnormalities.

What is the anion gap and why does it matter?

The [anion gap](/tools/anion-gap) (AG) = Na⁺ - (Cl⁻ + HCO₃⁻), normal 8-12 mEq/L. It represents unmeasured anions. In metabolic acidosis, an elevated anion gap indicates accumulation of unmeasured acids (lactate, ketones, uremic acids, toxins). A normal anion gap indicates loss of bicarbonate (diarrhea) or failure to excrete acid (renal tubular acidosis). The differential diagnosis and treatment differ completely, so always calculate the anion gap in metabolic acidosis.

What is the A-a gradient and when should I calculate it?

The [A-a (alveolar-arterial) gradient](/tools/aa-gradient) assesses the efficiency of oxygen transfer from alveoli to blood. Calculate it when PaO₂ is low to determine the cause. A normal A-a gradient (<15 mmHg on room air) with hypoxemia indicates hypoventilation (high PaCO₂ explains low PaO₂). An elevated A-a gradient indicates a lung parenchymal problem (pneumonia, pulmonary edema, pulmonary embolism, ARDS) or shunt.

How do I distinguish lactic acidosis from ketoacidosis?

Both cause high anion gap metabolic acidosis. Lactic acidosis is diagnosed by elevated serum lactate (>2 mmol/L, often >4 in severe cases), typically from tissue hypoperfusion (shock, sepsis) or impaired clearance (liver failure). Ketoacidosis is diagnosed by elevated serum or urine ketones (beta-hydroxybutyrate >3 mmol/L), typically from uncontrolled diabetes (DKA), alcohol use (alcoholic ketoacidosis), or starvation. Check lactate and ketones to distinguish.

What is the delta-delta ratio?

The delta-delta ratio compares the change in anion gap to the change in bicarbonate: Δ AG / Δ HCO₃⁻. In pure high anion gap metabolic acidosis, this ratio should be ~1 (for every increase in AG, HCO₃⁻ drops by the same amount). If ratio <1, there is concurrent normal anion gap metabolic acidosis (extra bicarbonate loss). If ratio >2, there is concurrent metabolic alkalosis (bicarbonate preserved). This detects mixed metabolic disorders.

When should I get an ABG vs. a VBG (venous blood gas)?

ABG is the gold standard and required when you need precise PaO₂ measurement, are managing mechanical ventilation, or need highly accurate acid-base assessment. VBG is acceptable for screening acid-base status in stable patients. Venous pH is ~0.03-0.05 lower and venous PCO₂ is ~3-8 mmHg higher than arterial values. VBG cannot assess oxygenation or calculate A-a gradient. Use ABG in critical illness, respiratory failure, or when precise data is needed.

What are common causes of metabolic alkalosis?

Metabolic alkalosis results from loss of acid or gain of bicarbonate. Common causes include: vomiting or nasogastric suction (loss of gastric HCl), diuretic use (loop or thiazide diuretics cause chloride depletion), volume depletion (contraction alkalosis), hypokalemia, mineralocorticoid excess (hyperaldosteronism, Cushing syndrome), and massive blood transfusion (citrate is metabolized to bicarbonate). Treatment involves correcting the underlying cause, replacing chloride and potassium, and restoring volume.

How do I interpret ABGs in COPD patients?

COPD patients often have chronic respiratory acidosis (elevated PaCO₂) with full renal compensation (elevated HCO₃⁻), resulting in near-normal pH. This is their baseline. During exacerbations, PaCO₂ may rise further (acute-on-chronic respiratory acidosis) or they may develop concurrent metabolic acidosis (from lactic acidosis if septic). Compare to prior ABGs when available. Avoid aggressive oxygen therapy—target SpO₂ 88-92% to prevent suppression of hypoxic drive and worsening hypercapnia.

What should I do if the ABG values don't make sense?

If the values seem internally inconsistent or don't match the clinical picture, first verify the sample: was it arterial (not venous)? Was it processed promptly (delays cause pH to fall and PaCO₂ to rise from ongoing cell metabolism)? Check for air bubbles in the sample (lowers PaCO₂). Verify the patient's clinical status—sometimes surprising values are correct and reflect an unexpected diagnosis. If in doubt, repeat the ABG. Also check that HCO₃⁻ from the ABG matches the serum HCO₃⁻ from the chemistry panel—large discrepancies suggest lab or sampling error.

Online Medical Tools — ABG Interpreter

Printed on 3/17/2026

For informational purposes only. This is not medical advice.

Clinical

ABG Interpreter

Arterial blood gas (ABG) analysis is fundamental to assessing a patient's acid-base status and respiratory function. This interpreter analyzes pH, PaCO₂, and HCO₃⁻ to determine the primary acid-base disorder (respiratory vs. metabolic acidosis or alkalosis), assess the degree of compensation, and identify mixed disorders. It applies Winter's formula and other compensation rules.

Compare with Anion Gap

Formula: Systematic analysis using pH, PaCO₂, HCO₃⁻, Winter's formula, and compensation rules

How It Works

Assess pH (Acidemia or Alkalemia?)

Begin by looking at the pH. Normal arterial pH is 7.35–7.45. A pH below 7.35 indicates acidemia (too much acid), while a pH above 7.45 indicates alkalemia (too much base). If pH is exactly 7.40, the patient may still have an acid-base disorder if compensation has normalized the pH (a compensated disorder). The pH tells you the overall acid-base status but not the underlying cause—that requires the next steps.

Identify the Primary Disorder (Respiratory vs. Metabolic)

Determine which system is driving the pH change. If PaCO₂ is abnormal in the direction that explains the pH (high PaCO₂ with low pH, or low PaCO₂ with high pH), the primary disorder is respiratory. If HCO₃⁻ is abnormal in the direction that explains the pH (low HCO₃⁻ with low pH, or high HCO₃⁻ with high pH), the primary disorder is metabolic. The system that moved first and caused the pH change is the primary disorder; the other system's response is compensation.

Check Compensation and Mixed Disorders

The body compensates for the primary disorder by adjusting the other system. In metabolic acidosis, the lungs hyperventilate to lower PaCO₂. In respiratory acidosis, the kidneys retain bicarbonate. Use compensation formulas (such as Winter's formula for metabolic acidosis) to predict the expected compensatory response. If the actual value matches the predicted value, compensation is appropriate. If not, a mixed acid-base disorder is present—meaning two or more simultaneous processes. Mixed disorders are common in critically ill patients and require careful attention.

Who Uses the ABG Interpreter

Emergency Department Dyspnea

Emergency physicians, hospitalists

A patient presents with acute shortness of breath. ABG shows pH 7.28, PaCO₂ 58, HCO₃⁻ 26. This is acute respiratory acidosis (elevated CO₂ with acidemia and minimal renal compensation yet). Differential includes COPD exacerbation, severe asthma, pneumonia, pulmonary edema, or neuromuscular weakness. The ABG guides immediate management (oxygen, bronchodilators, BiPAP, or intubation).

Diabetic Ketoacidosis (DKA)

Emergency physicians, endocrinologists, intensivists

A patient with diabetes presents with altered mental status and Kussmaul respirations. ABG shows pH 7.12, PaCO₂ 22, HCO₃⁻ 8, anion gap 28. This is high anion gap metabolic acidosis with appropriate respiratory compensation (Winter's formula predicts PaCO₂ ≈ 20). The low bicarbonate and high anion gap confirm DKA. Use the [Anion Gap](/tools/anion-gap) calculator to track acidosis severity and the [Sodium Correction](/tools/sodium-correction) for hyperglycemia-induced hyponatremia. Serial ABGs track treatment response (normalization of pH and anion gap closure).

Septic Shock in ICU

Intensivists, critical care nurses

A ventilated septic patient has worsening hypotension. ABG shows pH 7.18, PaCO₂ 28, HCO₃⁻ 10, lactate 6.2. This is metabolic acidosis (from lactic acidosis) with respiratory compensation. Expected PaCO₂ by Winter's formula is 23 (±2), but actual is 28—suggesting concurrent respiratory acidosis or inadequate ventilatory support. Calculate [SOFA Score](/tools/sofa-score) or [APACHE II](/tools/apache-ii-score) to assess organ dysfunction severity. Adjusting ventilator settings and continuing resuscitation is critical.

Post-Operative Metabolic Alkalosis

Surgeons, hospitalists, anesthesiologists

A post-surgical patient on nasogastric suction has pH 7.52, PaCO₂ 48, HCO₃⁻ 38. This is metabolic alkalosis (from loss of gastric acid) with respiratory compensation (hypoventilation to raise CO₂). Treatment includes stopping NG suction, replacing chloride and potassium, and correcting volume depletion. The ABG guides electrolyte replacement and monitors improvement.

Chronic COPD Patient

Pulmonologists, primary care physicians

A patient with severe COPD has pH 7.38, PaCO₂ 62, HCO₃⁻ 36. This is chronic respiratory acidosis with full renal compensation (pH is normal despite high CO₂ because kidneys have retained bicarbonate over days to weeks). Aggressive oxygen therapy can suppress hypoxic drive and worsen hypercapnia. Target SpO₂ 88–92% in these patients, and avoid over-oxygenation.

Salicylate Overdose

Emergency physicians, medical toxicologists

A patient with altered mental status has pH 7.48, PaCO₂ 18, HCO₃⁻ 13. This is a mixed disorder: respiratory alkalosis (from direct stimulation of the respiratory center by salicylates) plus high anion gap metabolic acidosis (from salicylic acid and lactic acid). The mixed picture is characteristic of salicylate toxicity. Calculate the [Anion Gap](/tools/anion-gap) to confirm high AG acidosis and assess level of consciousness with the [Glasgow Coma Scale](/tools/glasgow-coma-scale). Check serum salicylate level and initiate alkalinization therapy.

Pro Tips

Always use a systematic approach

Never skip steps. Always assess pH first, then identify the primary disorder, then check compensation. This structured method prevents missing mixed disorders and ensures accurate interpretation even in complex cases.

Check the anion gap in metabolic acidosis

If you identify metabolic acidosis, immediately [calculate the anion gap](/tools/anion-gap). This distinguishes high anion gap acidosis (lactic acidosis, ketoacidosis, renal failure, toxins) from normal anion gap acidosis (diarrhea, RTA, ureteral diversions). The differential diagnosis changes completely based on the anion gap.

Apply Winter's formula to every metabolic acidosis

Winter's formula (Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ±2) predicts the appropriate respiratory compensation. If actual PaCO₂ is higher than predicted, there is a concurrent respiratory acidosis. If lower, there is a concurrent respiratory alkalosis. This detects mixed disorders that change management.

Remember compensation never overcorrects

The body compensates to bring pH toward normal, but compensation alone never pushes pH past 7.40. If pH is on the opposite side of normal from the primary disorder, you have a mixed disorder. Example: if you think it's metabolic acidosis but pH is 7.48, there must be a concurrent metabolic alkalosis driving pH high.

Look for delta-delta in high anion gap acidosis

Delta-delta compares the change in anion gap to the change in bicarbonate. In pure high anion gap metabolic acidosis, these should change 1:1. If the bicarbonate drop is greater than the anion gap rise, there is concurrent normal anion gap acidosis. If the bicarbonate drop is less than the anion gap rise, there is concurrent metabolic alkalosis.

Consider the clinical context

ABG interpretation is not done in isolation. Integrate the results with the patient's history, exam, and other lab values. A pH of 7.35 might be normal for one patient but represent severe acidemia for another with chronic alkalosis. Context matters.

Respiratory compensation is fast, renal is slow

The lungs respond to metabolic disorders within minutes to hours, so respiratory compensation appears quickly. The kidneys take 3-5 days to fully compensate for respiratory disorders. If you see a respiratory acidosis with high bicarbonate, ask: is this chronic (full compensation) or acute (minimal compensation)? The timeline guides diagnosis and management.

Repeat ABGs to track trends

A single ABG is a snapshot. Serial ABGs show whether the patient is improving, worsening, or stable. In DKA, repeat ABGs every 2-4 hours to monitor anion gap closure. In respiratory failure, repeat after ventilator adjustments. Trends often matter more than single values.

Don't ignore the PaO₂ and A-a gradient

The ABG includes PaO₂ (partial pressure of oxygen), which is critical for assessing oxygenation. [Calculate the A-a gradient](/tools/aa-gradient) to determine if hypoxemia is from hypoventilation (normal gradient) or a parenchymal lung problem (elevated gradient). This completes the respiratory assessment.

Be cautious with chronic compensated disorders

Patients with chronic lung or kidney disease may have chronically abnormal pH, PaCO₂, or HCO₃⁻ that represents their baseline. Avoid aggressive correction—returning values to 'normal' can be harmful. Compare to prior ABGs when available, and target the patient's baseline, not textbook normal values.

Common Questions About Your Results

Compensation takes time. Respiratory compensation for metabolic disorders begins within hours but maximizes in 12-24 hours. Renal compensation for respiratory disorders takes 3-5 days. If you see an acute disorder, compensation may not be fully developed yet. Additionally, if the patient has concurrent illness affecting the compensating system (e.g., lung disease preventing respiratory compensation), compensation may be inadequate. Incomplete compensation is common in acute illness.

In acute respiratory acidosis, there is minimal renal compensation, so HCO₃⁻ is near normal (increases only ~1 mEq/L per 10 mmHg rise in PaCO₂). In chronic respiratory acidosis, the kidneys have had time to compensate, so HCO₃⁻ is significantly elevated (~4 mEq/L per 10 mmHg rise in PaCO₂). Example: PaCO₂ 60, HCO₃⁻ 26 suggests acute; PaCO₂ 60, HCO₃⁻ 34 suggests chronic. Clinical history (COPD patient vs. acute asthma) also helps.

This indicates a fully compensated acid-base disorder or a mixed disorder. If PaCO₂ and HCO₃⁻ are both elevated, it's likely compensated respiratory acidosis or compensated metabolic alkalosis. If both are low, it's likely compensated respiratory alkalosis or compensated metabolic acidosis. Look at which system moved first based on clinical context. Alternatively, there may be two opposing disorders (e.g., metabolic acidosis plus metabolic alkalosis) that cancel out, leaving pH normal.

Suspect a mixed disorder when: (1) pH is more extreme than expected for the primary disorder, (2) compensation does not match expected formulas, (3) PaCO₂ and HCO₃⁻ move in the same direction (both high or both low suggests two primary disorders), (4) delta-delta is not ~1 in high anion gap acidosis, or (5) clinical context suggests multiple processes (e.g., sepsis with vomiting, COPD with diuretic use). Mixed disorders are common in ICU patients.

Use the mnemonic GOLDMARK: Glycols (ethylene glycol, propylene glycol), Oxoproline (acetaminophen toxicity), L-lactate (lactic acidosis from shock, seizures, ischemia), D-lactate (short bowel syndrome), Methanol, Aspirin (salicylates), Renal failure (uremia), Ketoacidosis (diabetic, alcoholic, starvation). Lactic acidosis is by far the most common cause in hospitalized patients, followed by ketoacidosis and renal failure.

In mechanically ventilated patients, PaCO₂ is controlled by the ventilator (minute ventilation = tidal volume × respiratory rate). An elevated PaCO₂ suggests inadequate ventilation settings or high CO₂ production. A low PaCO₂ may indicate over-ventilation or the ventilator compensating for metabolic acidosis. Adjust ventilator settings based on the ABG and the patient's underlying disorder. Target pH 7.35-7.45 in most cases, but permissive hypercapnia (accepting higher PaCO₂) may be appropriate in ARDS to avoid ventilator-induced lung injury.

How to Interpret Your Result

The ABG interpreter identifies the primary acid-base disorder based on the relationship between pH, PaCO2, and HCO3. A pH below 7.35 indicates acidemia, while a pH above 7.45 indicates alkalemia. The primary disorder is the process driving the pH change: if PaCO2 is abnormal in the direction that explains the pH, the primary disorder is respiratory; if HCO3 is abnormal in the direction that explains the pH, the primary disorder is metabolic.

Compensation is the body's attempt to normalize pH by adjusting the system not primarily affected. In metabolic acidosis, the lungs compensate by hyperventilating (lowering PaCO2). In respiratory acidosis, the kidneys compensate by retaining bicarbonate. The interpreter checks whether the degree of compensation matches expected values — if it does not, a mixed acid-base disorder is present. Identifying mixed disorders is clinically important because it may reveal an additional underlying process requiring treatment.

When to Use This Tool

Interpret an ABG whenever you need to assess a patient's acid-base status and ventilatory function. Common clinical scenarios include: acute dyspnea or respiratory failure, altered mental status, suspected sepsis or shock, diabetic ketoacidosis, suspected toxic ingestion, monitoring of mechanically ventilated patients, and any critically ill patient in the emergency department or ICU.

ABG interpretation is also essential in the perioperative setting, during cardiopulmonary resuscitation, and when managing patients with chronic respiratory conditions (COPD, asthma exacerbations). The systematic approach ensures that primary disorders, compensation, and mixed processes are all identified rather than just the most obvious abnormality.

Limitations

ABG interpretation from pH, PaCO2, and HCO3 alone does not provide the complete acid-base picture. A full assessment often requires additional data: the anion gap (to identify unmeasured acids), the delta-delta ratio (to detect concurrent normal anion gap acidosis or metabolic alkalosis), serum lactate, ketones, and the osmolar gap (for toxic alcohol ingestion). This interpreter provides the framework but should be supplemented with these additional calculations.

The interpreter uses standard compensation rules (e.g., Winter's formula for metabolic acidosis) that apply to acute or chronic steady-state conditions. During the transition period as compensation develops (hours for respiratory compensation, days for renal compensation), expected values may not yet be reached, and the results should be interpreted cautiously.

Venous blood gases (VBGs) are sometimes used as a substitute for ABGs. Venous pH is typically 0.03–0.05 lower and venous PCO2 is 3–8 mmHg higher than arterial values. While venous gases can be useful for screening, the compensation formulas and interpretation rules used here are validated for arterial samples.

Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.

Related Tools

Clinical

Anion Gap

Calculate the anion gap and albumin-corrected anion gap to help evaluate metabolic acidosis. Essential for the ER and ICU workup.

Clinical

A-a Gradient

Calculate the alveolar-arterial oxygen gradient to evaluate the cause of hypoxemia. Differentiates lung pathology from hypoventilation.

Clinical

Corrected Calcium

Calculate corrected calcium adjusted for albumin levels. Essential for accurate interpretation of total calcium in hypoalbuminemic patients.

Frequently Asked Questions

Respiratory acidosis results from CO₂ retention (hypoventilation), causing PaCO₂ to rise and pH to fall. Causes include COPD, asthma, pneumonia, and neuromuscular weakness. Metabolic acidosis results from loss of bicarbonate or accumulation of acids, causing HCO₃⁻ to fall and pH to fall. Causes include lactic acidosis, ketoacidosis, renal failure, and diarrhea. The primary disorder is the one driving the pH change.

Respiratory alkalosis results from hyperventilation, causing PaCO₂ to fall and pH to rise. Causes include anxiety, pain, sepsis, and hypoxemia. Metabolic alkalosis results from loss of acid or gain of bicarbonate, causing HCO₃⁻ to rise and pH to rise. Causes include vomiting, nasogastric suction, diuretic use, and alkali administration. Treatment targets the underlying cause rather than the ABG abnormality itself.

Online Medical Tools — ABG Interpreter

Printed on 3/17/2026

For informational purposes only. This is not medical advice.

Clinical

ABG Interpreter

Compare with Anion Gap

Formula: Systematic analysis using pH, PaCO₂, HCO₃⁻, Winter's formula, and compensation rules