Blood Gas Analysis Calculator

Interpret arterial blood gas values and calculate A-a gradient, bicarbonate, and compensation.

pCO2 (mmHg) (mmHg)

pO2 (mmHg) (mmHg)

HCO3- (mEq/L, or calculate) (mEq/L)

0 = calculate from pH and pCO2

FiO2 (fraction inspired O2)

0.21 = room air, 1.0 = 100% O2

Please check your inputs and try again.

The Six-Step Interpretation

Blood gas interpretation is a fixed sequence. Working through it in order prevents the commonest failure, which is stopping after identifying the primary disorder and missing a second one underneath.

Step	Question	Finding
1	Is pH acidaemic or alkalaemic?	< 7.35 or > 7.45
2	Is pCO₂ or HCO₃⁻ the driver?	Which moves in the same direction as pH change
3	Is compensation appropriate?	Use the expected formula
4	If metabolic acidosis, what is the anion gap?	Na − (Cl + HCO₃)
5	If raised gap, check the delta ratio	Reveals a second disorder
6	Is oxygenation adequate?	A–a gradient

Step 3 is where most errors occur. The body never overcompensates, so if measured compensation exceeds the expected range, a second primary disorder is present rather than unusually vigorous compensation.

Primary disorder	Expected compensation
Metabolic acidosis	pCO₂ = 1.5(HCO₃) + 8 ± 2 (Winter's formula)
Acute respiratory acidosis	HCO₃ rises 1 per 10 mmHg pCO₂
Chronic respiratory acidosis	HCO₃ rises 3.5 per 10 mmHg pCO₂

The acute versus chronic distinction matters clinically: renal compensation takes two to three days, so a patient with markedly raised bicarbonate for their pCO₂ has had the problem for days, not hours.

The A–a Gradient

A–a = [FiO₂(P_atm − 47) − pCO₂/0.8] − pO₂

The 47 mmHg subtracted is water vapour pressure at body temperature, and the gradient normally runs about (age/4) + 4. Its value is in distinguishing why a patient is hypoxic:

A–a gradient	Cause of hypoxia
Normal	Hypoventilation or low inspired oxygen — lungs are fine
Raised	V/Q mismatch, shunt, or diffusion impairment

An opioid overdose gives hypoxia with a normal gradient; a pulmonary embolism gives hypoxia with a raised one. The pO₂ alone cannot distinguish them.

Worked Examples

Example 1: Normal ABG

pH=7.38, pCO2=40, pO2=95, HCO3=24

Result: Normal, A-a gradient ~10 mmHg

Reference values for interpretation

Example 2: Respiratory acidosis: pH=7.25, pCO2=65

HCO3 starts rising (compensation)

Result: Hypoventilation, COPD exacerbation

Treat underlying cause

Example 3: Detecting a mixed disorder

HCO₃ 14, measured pCO₂ 20, Winter's predicts 29 ± 2

Result: Concurrent respiratory alkalosis

pCO₂ is far below the expected compensation, so a second primary disorder is driving it down — classically salicylate poisoning or sepsis.

Example 4: Same hypoxia, different cause

pO₂ 55 with normal A–a versus pO₂ 55 with raised A–a

Result: Hypoventilation versus lung pathology

Opioid overdose gives the first, pulmonary embolism the second. The gradient makes the distinction that pO₂ alone cannot.

Common Mistakes

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Stopping at the primary disorder

Mixed disorders are common. Always check whether compensation falls within the expected range and calculate the anion gap in metabolic acidosis.

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Interpreting excess compensation as vigorous physiology

The body does not overcompensate. Compensation beyond the expected range means a second primary disorder is present.

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Ignoring the acute versus chronic distinction

Renal compensation takes two to three days. Bicarbonate far above the acute expectation indicates a long-standing problem.

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Assessing oxygenation by pO₂ alone

The A–a gradient distinguishes hypoventilation from lung pathology. Both give hypoxia, but they need entirely different treatment.

Frequently Asked Questions

6-step ABG interpretation?

1. Is pH normal? 2. What is primary disturbance (acidosis/alkalosis)? 3. What is the primary cause (respiratory: pCO2, metabolic: HCO3)? 4. Is there compensation? 5. If metabolic acidosis: calculate anion gap. 6. Is there an elevated A-a gradient?

A-a gradient significance?

Normal A-a = ~(age/4)+4 mmHg. Elevated A-a: V/Q mismatch (PE, COPD), shunt (pneumonia, ARDS), diffusion impairment (interstitial fibrosis). Normal A-a with hypoxemia: hypoventilation only.

How do I know if compensation is appropriate?

Use the expected formula for the primary disorder. Winter's formula covers metabolic acidosis; respiratory disorders have separate acute and chronic expectations.

What does it mean if compensation exceeds expectation?

A second primary disorder is present. The body does not overcompensate, so an excessive value always indicates a mixed picture.

What does the A–a gradient tell you?

Whether hypoxia comes from the lungs or from inadequate ventilation. A normal gradient points to hypoventilation; a raised one indicates V/Q mismatch or shunt.

How do I distinguish acute from chronic respiratory acidosis?

By the degree of bicarbonate rise. Acutely it increases about 1 per 10 mmHg of pCO₂; chronically about 3.5, since renal compensation takes days.