Interpreting ABG is not a difficult thing, it’s actually very easy. Read the full article to gain a complete idea about ABG Interpretation.


pH – pH signifies free hydrogen ion concentration. An increase in pH means H+ ion concentration is decreasing and decrease pH means H+ ion increasing.

Acid – Acid a substance that can donate H+ ion which means it lowers pH.

Base – A substance that receives H+ ion which means it raises pH.

Acidemia – It means acidic blood, it decreases pH below 7.35.

Alkalaemia – It means alkaline blood, It increases pH above 7.45.

Acidosis – An abnormal process or disease which reduces blood pH due to increased production of acid or decrease in alkali.

Alkalosis – An abnormal process or disease which increase the pH of blood due to decrease in acid or increase in alkali.

ABG (Arterial blood gas) is a laboratory test, which identifies the level of gases as well as the PH of the blood. Basic components of ABG are listed below.

Components Normal Values
pH (Hydrogen ion concentration known as potential of hydrogen) 7.35  to 7.45
PaCO2 ( partial pressure of carbon dioxide ) 35 mmHg  to 45 mmHg
HCO3 (Bicarbonate) 22meq/L to 28meq/L
PaO2 (partial pressure of oxygen) 80mmHg to 100mmHg
SO2 (Saturation of Oxygen) > 90%
Lactate < 2
Base excess & deficit (Its only represent BASE, Here the Base is Bicarbonate.
Positive increasing number is called a base excess and indicates a metabolic alkalosis.
The negative increasing number is called a base deficit and indicates metabolic acidosis.)
+2 to -2

Apart from the above-mentioned values, many Blood gas analyzers can analyze Hematocrit, several electrolytesoxyhemoglobincarboxyhemoglobin, and methemoglobin.

Site for Arterial Blood Gas(ABG) Collection

ABG sample is collected from the artery by a very thin needle with a heparinized syringe. The most appropriate site is lateral side of the wrist(Radial artery) because the radial artery is superficially available. During puncture, there is less risk of collateral damage.

We can collect blood from the femoral artery and brachial artery with a risk of Vasovagal response, bleeding, hematoma, and nerve damage. 1 to 1.5 ml of arterial blood is enough for the analyzer.

You want to know the interesting fact about the Vasovagal response the click below

Vasovagal Response & Vasovagal Syncope CLICK HERE

Site for Arterial Blood Gas(ABG) Collection

ABG Interpretation

At first, we have to know Wheather it is an ABG(Arterial Blood Gas) report or VBG(Venous Blood Gas) report by looking SO2(Saturation of oxygen). It should be above 90% or same with patient saturation which showing on the pulse oximeter.

PaO2( partial pressure of oxygen ) shows the tension of oxygen in the blood (It is the most needed value for health care professionals during providing mechanical ventilation and oxygenation to a patient). In arterial blood, at atmospheric air partial pressure of oxygen should be within 80 to 100 mmHg. If there is low PaO2 then it is an indication that we have to give oxygen therapy to the patient. If it is very high in a patient during Oxygen therapy then we have to reduce the percentage of inspired oxygen otherwise, it will lead to oxygen toxicity.

Partial pressure of oxygen is also necessary to determine ARDS (Acute Respiratory Distress Syndrome) grade in the critical care unit. It is determined by PaO2/FiO2 ratio (FiO2 means a fraction of inspired oxygen)

If you guys are really curious to know how to calculate ARDS grade then please click below link

Acute Respiratory Distress Syndrome (CLICK HERE)

Very first thing we notice when we get an ABG report is the pH value. It should be between 7.35 to 7.45. Any value below 7.35 is known as acidosis and above 7.45 is known as alkalosis. There are commonly two values 1. PaCO2 (Carbon Dioxide) and 2. HCO3 (Bicarbonate) affects the blood PH.

Carbon dioxide(CO2) represents the respiratory status of the body and it acts as an acid, any increasing value of carbon dioxide in the body causes acidosis (Respiratory Acidosis) and decreasing value causes alkalosis (Respiratory Alkalosis). Bicarbonate(HCO3) is representing the metabolic part of the body and it acts as a base. when bicarbonate value increases it causes alkalosis (Metabolic Alkalosis) and decrease value causes acidosis (Metabolic Acidosis).

Respiratory Acidosis pH ↓ (Decreases) PaCO2  ↑ (Increases) HCO3(Normal)
Metabolic Acidosis pH ↓ (Decreases) PaCO2  (Normal) HCO3  (Decreases)
Respiratory Alkalosis pH ↑ (Increases) PaCO2  ↓ (Decreases) HCO3 (Normal)
Metabolic Alkalosis pH ↑ (Increases) PaCO2  (Normal) HCO3–  ↑ (Increases)

If still you have any doubt and having any problem then see my youtube video of ABG interpretation, click below

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Full compensation and Partially compensation of ABG

In responding to ABG abnormalities(I.e Acidosis or Alkalosis) body tries to manage it by natural defense mechanisms. As an example, if there is acidosis then the body tries to create an alkaline condition naturally to neutralize acidosis and to bring pH of blood into a normal range. If there is Alkalosis then the human body neutralizes it by builds up acids and tries to bring blood pH in the normal range. This is called compensation of ABG. It can be full compensation or partial compensation. If the body tries to compensate and initiate action then it is partial compensation, but if body successfully brings down pH to normal range then it is full compensation.

Disorder pH Primary problem Compensation
(Body defence mechanism towads abnormal blood pH)
Metabolic acidosis ↓ in HCO3 ↓ in PaCO2 (Body tries to increase blood pH by creating
alkaline environment because low carbon dioxide causes
alkaline environment)
Metabolic alkalosis ↑ in HCO3 ↑ in PaCO2 (Body tries to increase blood pH by creating
acidic environment because increasing carbon dioxide causes
acidic environment)
Respiratory acidosis ↑ in PaCO2 ↑ in [HCO3] (Body tries to increase blood pH by creating
alkaline environment because increase bicarbonate level cause
alkaline environment)
Respiratory alkalosis ↓ in PaCO2 ↓ in [HCO3] (Body tries to increase blood pH by creating
acidic environment because decrease bicarbonate level in the
body creates acidic environment)

Remember the same direction rule in compensation. See the above table, in the metabolic acidosis bicarbonate level is decreased, to compensate it PaCO2 also decreasing. In metabolic alkalosis bicarbonate is increased, to compensate it PaCO2 also increased. The same condition also applied in respiratory acidosis and alkalosis. This is called the same direction rule.

Remember if you notice the same direction rule, then it is compensation. If you notice the opposite direction rukle then it is mixed acid-base disorder.

Example 1. PH – 7.2   PaCO2 – 56 HCO3 – 25

Look PH is acidic because it’s lower than 7.35, the cause is PaCO2 because it increases and HCO3 is in the normal range. PaCO2 is representing the respiratory part of body So, it is Respiratory Acidosis (non-compensatory)

How the body can compensate/ When we say it is a compensatory ABG

It is non-compensatory because the body is not doing anything to compensate for acidosis. If here HCO3 is increased to an alkaline state, then it will be a Partially compensatory Respiratory acidosis. Because now the body tries to manage acidosis by increasing level of Bicarbonate (By reabsorption of Bicarbonate ion in the kidney)

If here PH will come to normal range then it will be a compensatory Respiratory acidosis because now the body is successful with its defense mechanism.

Remember one thing in compensatory value the cause of the disturbance (Here it is PaCO2) and compensation value(Here it is HCO3 ) will move in the same direction.

Example 2. pH – 7.50 PaCO2 – 40 HCO3 – 40

Here PH is in an alkaline state, and it is caused by HCO3 because It is increased than normal value. HCO3 represents metabolic part So, It is Metabolic alkalosis(non-compensatory).

What happens if it changes non-compensatory to compensatory ABG

If the body adopts a defense mechanism and starts CO2 retention in the lungs by hypoventilation to neutralize pH. Then the value of PaCO2 will increase (It is called Partially compensation, in this stage body tries but pH does not come to normal). In fully compensation pH comes to the normal state.

Example 3. pH 7.35, PaCO2 – 20, HCO3 – 15

It is a Fully compensatory Metabolic acidosis (PaCO2 is compensating and pH is in the normal range)

Metabolic acidosis leads to low pH. Low pH stimulates the respiratory center and causes hyperventilation. Hyperventilation leads to CO2 washout and decreases PaCO2.

You can say it is metabolic acidosis compensated by respiratory alkalosis.

Example 4. pH – 7.24, PaCO2 – 20 , HCO3 – 10

It is partially compensatory Metabolic acidosis(Because PaCO2 is compensating but pH is still in the acidic state)

If you still have any doubt the see below video.

Note – these are the basic knowledge of compensation. Please follow the books for extra knowledge.

Etiology/Cause and Treatment Of Acidosis and Alkalosis

Respiratory Acidosis

When the Concentration of Carbon dioxide increase in the blood causes blood acidic(Decrease blood PH) is known as respiratory acidosis. It is mainly two types 1. Acute Respiratory Acidosis 2. Chronic Respiratory Acidosis

Acute Respiratory Acidosis is a medical emergency needs immediate treatment. it is caused by airway blockage or hypoventilation which facilitates carbon dioxide retention. It may be due to underlying causes I.e Severe Asthma, COPD, Coma, Seizures, Airway obstruction, Cardiac arrest, Neuromuscular disease(myasthenia gravis, Guillain barre syndrome) which affects respiration and pneumonia.

Treatment includes hyperventilation by mechanical ventilation to wash out carbon dioxide, infusion of Bicarbonate and treatment of underlying causes.

Chronic Respiratory Acidosis is a long term disease caused by an underlying respiratory disease like COPD. In Chronic Respiratory Acidosis pH is in the normal range but PaCO2 always on the higher side, because the body used to it and kidneys produce more Bicarbonate to maintain Blood pH.

Metabolic Acidosis

Metabolic acidosis refers to when the body produces more acid like ketone and lactate or when the HCO3– level is very low as evidenced by low blood pH.

  1. Diabetic ketoacidosis (When the body does not have enough insulin to convert energy from carbohydrate then body depend on fat for energy. during fat metabolism ketone bodies are produced which causes ketoacidosis. It is a complication of Diabetes). To know more about ketoacidosis click below link ( )
  2. Lactic acidosis (When there is hypoxia in the tissue or in tissue death lactate is produced by the body, increase lactate can dramatically reduce prognosis of the patient)
  3. Kidney disease(When the kidney is not able to excrete acids then it can be the cause of acidosis)
  4. Diarrhea and vomiting also can causes acidosis because of Hydrogen ion loss.
  5. Gi or intestinal fluid loss
  6. Congestive heart failure

Respiratory Alkalosis

Respiratory alkalosis is caused by excessive CO2 washout by Hyperventilation. The underlying cause may be Respiratory diseases, Fever, Anxiety disorder, Pain, Seizures, Comma, Etc.

Metabolic Alkalosis

When bicarbonate level increase in the body, it causes metabolic alkalosis. Normally it is resolved by increased excretion of Bicarbonate through kidneys if you have a healthy kidney.

The underlying causes are Excessive chloride loss, Diuretics, Diarrhoea, vomiting, Posthypercapnia, etc.

Mixed Acid-Base Disorder

Disorder Characteristics Selected situations
Respiratory acidosis with Metabolic acidosis ↓in pH
↓ in HCO3
↑ in PaCO2  
•Cardiac arrest •Intoxications •Multi-organ failure
Respiratory alkalosis with Metabolic alkalosis ↑in pH
↑ in  HCO3–  
↓ in PaCO2     
•Liver cirrhosis with diuretics
•Pregnancy with severe vomiting
•Over ventilation of Chronic obstructive
pulmonary disease

If you have any doubt then visit my youtube channel

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Biswa Prakash

Hi, I am Biswa Prakash Swain, a Public Health expert and author of this Article. I have Done BSc in Nursing and Masters in Public Health (Epidemiology), Now Preparing for Ph.D. in Epidemiology and Clinical Research. I have more than 7 years of experience in Healthcare sectors like Hospitals and almost 4 years of experience in the Public health field and Clinical research.


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