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Blood Gas Analysis

Blood gas analysis	Vessel	Range	Interpretation
Oxygen Partial Pressure (pO₂)	Arterial	80 to 100 mmHg	Normal
	Arterial	<80  mmHg	Hypoxia
	Venous	35 to 40 mmHg	Normal
Oxygen Saturation (SO₂)	Arterial	>95%	Normal
	Arterial	<95%	Hypoxia
	Venous	70 to 75%	Normal
pH	Arterial	<7.35	Acidemia
		7.35 to 7.45	Normal
		>7.45	Alkalemia
	Venous	7.26 to 7.46	Normal
Carbon Dioxide Partial Pressure (pCO₂)	Arterial	<35 mmHg	Low
		35 to 45 mmHg	Normal
		>45 mmHg	High
	Venous	40 to 45 mmHg	Normal
Bicarbonate (HCO₃⁻)	Arterial	<22 mmol/L	Low
		22 to 26 mmol/L	Normal
		>26 mmol/L	High
	Venous	19 to 28 mmol/L	Normal
Base Excess (BE)	Arterial	<−3.4	Acidemia
		−3.4 to +2.3 mmol/L	Normal
		>2.3	Alkalemia
	Venous	−2 to −5 mmol/L	Normal
Osmolar gap = Osmolality – Osmolarity		>10	Abnormal
Anion gap = [[[Sodium\|Na]]⁺] – {[[[Chloride\|Cl]]⁻]+[[[Bicarbonate\|HCO₃]]⁻]} Corrected AG = (measured serum AG) + (2.5 x [4.5 − Alb])		<8	Low
		8 to 16	Normal
		>16	High

There are compensation mechanisms in the body in order to normalizing the pH inside the blood.^[1]
The amount of compensation depends on proper functioning of renal and respiratory systems. However, it is uncommon to compensate completely. Compensatory mechanisms might correct only 50–75% of pH to normal.
Acute respiratory compensation usually occurs within first day. However, chronic respiratory compensation takes 1 to 4 days to occur.
Renal compensation might occur slower than respiratory compensation.

Primary disorder	pH	PaCO₂	[HCO3⁻]	Compensation	Compensation formula
Metabolic acidosis	↓	↓	↓	Respiratory	Expected paCO₂ = 1.5 x serum HCO₃⁻ + 8 ± 2 (Winters' formula) Expected paCO₂ = Serum HCO₃⁻ + 15
Metabolic alkalosis	↑	↑	↑	Respiratory	Expected paCO₂ = 0.5 − 1 increase/ every 1 unit increase in serum HCO₃⁻ from 24
Respiratory acidosis	↓	↑	↑	Renal	Acute: HCO₃⁻ increases by 1mEq/L for every 10 mmHg increase in paCO2 above 40 Chronic: HCO₃⁻ increases by 3.5mEq/L for every 10 mmHg increase in paCO2 above 40
Respiratory alkalosis	↑	↓	↓	Renal	Acute: HCO₃⁻ decreases by 2mEq/L for every 10 mmHg derease in paCO2 above 40 Chronic: HCO₃⁻ decreases by 5mEq/L for every 10 mmHg decrease in paCO2 above 40

Check pH on ABG

pH < 7.35= Acidosis

pH > 7.45= Alkalosis

Check PaCO₂

PaCO₂ > 45mm Hg =
Respiratory acidosis

PaCO₂ Normal or < 35mm Hg =
Metabolic acidosis

Check PaCO₂

PaCO₂ > 45mm Hg =
Metabolic alkalosis

PaCO₂ < 35mm Hg =
Respiratory alkalosis

[HCO₃^-] > 29

Check [HCO₃^-]

Normal or slight decrease =
Acute respiratory alkalosis

Decreased < 24 =
Chronic respiratory alkalosis

Disorder	Key features	Examples
Metabolic acidosis & respiratory alkalosis	High− or normal−AG metabolic acidosis Prevailing PaCO₂ below predicted value	Lactic acidosis Sepsis in ICU
Metabolic acidosis & respiratory acidosis	High− or normal−AG metabolic acidosis Prevailing PaCO₂ above predicted value	Severe pneumonia Pulmonary edema
Metabolic alkalosis & respiratory alkalosis	PaCO₂ does not increase as predicted pH higher than expected	Liver disease Diuretics
Metabolic alkalosis & respiratory acidosis	PaCO₂ higher than predicted pH normal	COPD on diuretics
Metabolic acidosis & metabolic alkalosis	Only detectable with high−AG acidosis ∆AG >> ∆[HCO₃⁻]	Uremia with vomiting
Metabolic acidosis & metabolic acidosis	Mixed high−AG & normal−AG acidosis ∆[HCO₃⁻] accounted for by combined change in ∆AG and ∆Cl⁻	Diarrhea and lactic acidosis Toluene toxicity Treatment of diabetic ketoacidosis

↑ Sood P, Paul G, Puri S (April 2010). "Interpretation of arterial blood gas". Indian J Crit Care Med. 14 (2): 57–64. doi:10.4103/0972-5229.68215. PMC 2936733. PMID 20859488.