Blood Gas Analysis (BGA)

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Last updated: September 22, 2026Bookmark

Table Of Contents

Overview
Definition of terms
Components of a blood gas analysis
Henderson-Hasselbach Equation
Interpretation of a blood gas

Overview

A blood gas analysis evaluates oxygenation, ventilation, and acid-base status. It can be performed on arterial, venous, or capillary blood.

Arterial blood gas (ABG) is the gold standard test for assessing partial pressure of oxygen (PaO2 – oxygenation), partial pressure of carbon dioxide (PaCO2 – ventilation), and pH + HCO3- (acid-base status)

These note focus on assessing the acid-base status.

Indications
- Acute respiratory distress syndrome (ARDS)
- Acute respiratory failure
- Severe sepsis and septic shock
- Hypovolemic shock
- Diabetic ketoacidosis (DKA)
- Renal tubular acidosis
- Heart failure
- Cardiac arrest
- Asthma
- Inborn errors of metabolism

Definition of terms

A patient can have multiple simultaneous acid-base disorders

Term	Definition
Acidaemia	Blood pH < 7.35
Alkalaemia	Blood pH > 7.45
Acidosis	A process that causes the pH to fall
Alkalosis	A process that causes a rise in pH

Components of a blood gas analysis

Parameter	Reference Range	Significance
pH	7.35 – 7.45 (7.4)	Acid-base status
pCO2	35 – 45 mmHg (40)	Ventilation
pO2	75 – 100 mmHg	Oxygenation
HCO₃⁻	22 – 26 mEq/L (24)	Metabolic component
Base Excess	-4 to +2	Metabolic acid-base balance
SaO₂	95 – 100%	Oxygen saturation

The components of a blood gas analysis are represented in the following format:

pH / pCO2 / pO2 / HCO3-

Example:

7.40 / 40 / 90 / 24

Henderson-Hasselbach Equation

The relationship between pH, bicarbonate and PaCO2 can be described by:

pH = 6.1 + log [HCO₃⁻ / (0.03 × PaCO₂)]

In this equation:

HCO3- represents the metabolic component

PaCO2 represents the respiratory components

Putting it simply:

↑ HCO₃⁻ → ↑ pH
↓ HCO₃⁻ → ↓ pH
↑ PaCO₂ → ↓ pH
↓ PaCO₂ → ↑ pH

Interpretation of a blood gas

Step 1: Is there an acid base disorder?

Look at the pH and determine whether there is acidaemia or alkalaemia

pH	Interpretation
7.35 – 7.45	Normal. Use 7.40 as the midpoint
< 7.35	Acidaemia
> 7.45	Alkalaemia

Step 2: What is the primary disorder

Are the pH and PCO2 changing in the same or opposite direction?

Same directions = metabolic disorder

Opposite directions = respiratory disorder

Primary disorder	pH	pCO2	HCO3-
Metabolic acidosis	↓	↓ (compensatory)	↓
Metabolic alkalosis	↑	↑ (compensatory)	↑
Respiratory acidosis (acute)	↓	↑	Slight ↑ (compensatory)
Respiratory acidosis (chronic)	Slight ↓	↑	↑ (compensatory)
Respiratory alkalosis (acute)	↑	↓	Slight ↓ (compensatory)
Respiratory alkalosis (chronic)	Slight ↑	↓	↓ (compensatory)

Step 3: Is compensation appropriate?

Compensation is the body’s attempt to normalize pH. Respiratory compensation happens faster through changes in alveolar ventilation. Metabolic compensation happens slower through changes in renal handling of H+ and HCO3-.

The value that is inconsistent with the pH usually represents compensation. For example:

Compensation does not completely normalise the underlying disorder. If the measured compensation is outside the expected range a mixed acid-base disorder

6 formulas to determine whether compensation is appropriate:

In metabolic acidosis: Expected pCO2 = 1.5 (HCO3-) + 8 +/- 2
In metabolic alkalosis: Expected pCO2 = 0.7 (HCO3-) + 21 +/- 2
In acute respiratory acidosis: Expected pH = 7.4 – [0.008(pCO2 – 40)]
In acute respiratory alkalosis: Expected pH = 7.4 + [0.008(40 – pCO2)]
In chronic respiratory acidosis: Expected pH = 7.4 – [0.003(pCO2 – 40)]
In chronic respiratory alkalosis: Expected pH = 7.4 + [0.003(40 – pCO2)]

Examples:

7.08/16/115/5 – metabolic acidosis with respiratory compensation
7.16/16/70/60/23 – acute respiratory academia with little compensation

Step 4: Calculate the anion gap in metabolic acidosis

The anion gap (AG) can be used to determine the cause of metabolic acidosis. It estimates unmeasured plasma anionssuch as phosphate, ketones, and lactate

Formula:

AG = (Na+) – (Cl + HCO3-)

A normal AG is 8- 12 mmol/L

Step 5: Calculate Delta-Delta (Delta ratio) in an anion gap metabolic acidosis (AGMA)

The delta ratio identifies an additional metabolic disorder in patients with an anion gap metabolic acidosis

Formula:

Δ Ratio = ΔAG/ΔHCO3-

ΔAG = AG − 12

ΔHCO3- = 24 − measured HCO3-

Therefore Δ Ratio = (AG − 12) / (24 − HCO₃⁻)

Interpretation:

Delta ratio	Interpretation	Physiology
< 1	AGMA + NAGMA	Another process is causing bicarbonate to fall
1 – 2	Pure AGMA
> 2	AGMA + metabolic alkalosis	Another process is causing bicarbonate to rise

Reference Intervals ›

Biochemistry

ACTH	P: <80 ng/L
ALT	P: 5–35 U/L
Albumin	P: 35–50 g/L
Aldosterone	P: 100–500 pmol/L
Alk. phosphatase	P: 30–130 U/L
α-Amylase	P: 0–180 IU/dL
α-Fetoprotein	S: <10 kU/L
Angiotensin II	P: 5–35 pmol/L
ADH	P: 0.9–4.6 pmol/L
AST	P: 5–35 U/L
Bicarbonate	P: 24–30 mmol/L
Bilirubin	P: 3–17 μmol/L
BNP	P: <50 ng/L
CRP	P: <10 mg/L
Calcitonin	P: <0.1 mcg/L
Calcium (ionized)	P: 1.0–1.25 mmol/L
Calcium (total)	P: 2.12–2.60 mmol/L
Chloride	P: 95–105 mmol/L
Cholesterol	P: <5.0 mmol/L
VLDL	P: 0.128–0.645 mmol/L
LDL	P: <2.0 mmol/L
HDL	P: 0.9–1.93 mmol/L
Cortisol AM	P: 450–700 nmol/L
Cortisol Midnight	P: 80–280 nmol/L
CK ♂	P: 25–195 U/L
CK ♀	P: 25–170 U/L
Creatinine	P: 70–100 μmol/L
Ferritin	P: 12–200 mcg/L
Folate	S: 2.1 mcg/L
FSH	P: 2–8 U/L ♂; >25 menopause
GGT ♂	P: 11–51 U/L
GGT ♀	P: 7–33 U/L
Glucose (fasting)	P: 3.5–5.5 mmol/L
Growth hormone	P: <20 mu/L
HbA1C (DCCT)	B: 4–6%
HbA1C (IFCC)	B: 20–42 mmol/mol
Iron ♂	S: 14–31 μmol/L
Iron ♀	S: 11–30 μmol/L
Lactate (venous)	P: 0.6–2.4 mmol/L
Lactate (arterial)	P: 0.6–1.8 mmol/L
LDH	P: 70–250 U/L
LH	P: 3–16 U/L
Magnesium	P: 0.75–1.05 mmol/L
Osmolality	P: 278–305 mosmol/kg
PTH	P: 0.8–8.5 pmol/L
Potassium	P: 3.5–5.3 mmol/L
Prolactin ♂	P: <450 U/L
Prolactin ♀	P: <600 U/L
PSA	P: 0–4 mcg/mL
Protein (total)	P: 60–80 g/L
Red cell folate	B: 0.36–1.44 μmol/L
Renin (erect)	P: 2.8–4.5 pmol/mL/h
Renin (recumbent)	P: 1.1–2.7 pmol/mL/h
Sodium	P: 135–145 mmol/L
TBG	P: 7–17 mg/L
TSH	P: 0.5–4.2 mU/L
T4	P: 70–140 nmol/L
Free T4	P: 9–22 pmol/L
TIBC	S: 54–75 μmol/L
Triglycerides	P: 0.50–2.3 mmol/L
T3	P: 1.2–3.0 nmol/L
Troponin T	P: <0.1 mcg/L
Urate ♂	P: 210–480 μmol/L
Urate ♀	P: 150–390 μmol/L
Urea	P: 2.5–6.7 mmol/L
Vitamin B12	S: 0.13–0.68 nmol/L
Vitamin D	S: 50 nmol/L

Arterial Blood Gases

pH	7.35–7.45
PaCO₂	4.7–6.0 kPa
PaO₂	>10.6 kPa
Base excess	±2 mmol/L

Urine

Cortisol (free)	<280 nmol/24h
Hydroxyindole acetic acid	16–73 μmol/24h
Hydroxymethylmandelic acid	16–48 μmol/24h
Metanephrines	0.03–0.69 μmol/mmol cr.
Osmolality	350–1000 mosmol/kg
17-Oxogenic steroids ♂	28–30 μmol/24h
17-Oxogenic steroids ♀	21–66 μmol/24h
17-Oxosteroids ♂	17–76 μmol/24h
17-Oxosteroids ♀	14–59 μmol/24h
Phosphate (inorganic)	15–50 mmol/24h
Potassium	14–120 mmol/24h
Protein	<150 mg/24h
Protein/creatinine ratio	<3 mg/mmol
Sodium	100–250 mmol/24h

Haematology

WCC	4.0–11.0 ×10⁹/L
RBC ♂	4.5–6.5 ×10¹²/L
RBC ♀	3.9–5.6 ×10¹²/L
Hb ♂	130–180 g/L
Hb ♀	115–160 g/L
PCV ♂	0.4–0.54 L/L
PCV ♀	0.37–0.47 L/L
MCV	76–96 fL
MCH	27–32 pg
MCHC	300–360 g/L
RDW	11.6–14.6%
Neutrophils	2.0–7.5 ×10⁹/L (40–75%)
Lymphocytes	1.0–4.5 ×10⁹/L (20–45%)
Eosinophils	0.04–0.44 ×10⁹/L (1–6%)
Basophils	0–0.10 ×10⁹/L (0–1%)
Monocytes	0.2–0.8 ×10⁹/L (2–10%)
Platelets	150–400 ×10⁹/L
Reticulocytes	0.8–2.0% / 25–100 ×10⁹/L
Prothrombin time	10–14 s
APTT	35–45 s

Paediatric

Pulse Rate (bpm)
Neonate	140–160
Infant <1yr	120–140
1–5 years	110–130
5–12 years	80–120
>12 years	70–100
Respiratory Rate (tachypnoea)
0–2 months	≥60/min
2–12 months	≥50/min
1–5 years	≥40/min
>5 years	≥30/min
Blood Pressure (mmHg)
Term	65/45
1 year	75/50
4 years	85/60
8 years	95/65
10 years	100/70
Weight Formulas
3–12 months	(a + 9)/2 kg
1–6 years	2a + 8 kg
>6 years	(7a − 5)/2 kg
Haemoglobin (g/dL)
Term newborn	13–20
1 month	11–18
2 months	10–15
1–2 years	10–13
>2 years	11–14
MUAC (6 months–5 years)
Obese	>17.5 cm
Normal	13.5–17.4 cm
At risk	12.5–13.4 cm
Moderate malnutrition	11.5–12.4 cm
Severe malnutrition	<11.5 cm
Developmental Milestones
Social smile	1.5 months
Head control	4 months
Sits unsupported	7 months
Crawls	10 months
Stands unsupported	10–12 months
Walks	12–13 months
Talks	18 months
CSF WBC (/mm³)
Term newborn	0–25
>2 weeks	0–5

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