Arterial blood gas

An arterial blood gas (ABG) is a blood test that is performed specifically on blood from an artery. It involves puncturing an artery with a thin needle and syringe and drawing a small volume of blood. The most common puncture site is the radial artery at the wrist, but sometimes the femoral artery in the groin or other sites are used. It is usually somewhat more uncomfortable and difficult to perform compared to the regular technique of drawing blood from a vein (venipuncture). The blood can also be drawn from a previously placed arterial catheter.

Combinations of disorders can be complex and difficult to interpret, so calculators ^[1], nomograms, and rules of thumb^[2] are commonly used.

Extraction and analysis

The syringe is pre-packaged and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again. Once the sample is obtained, care is taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results. The sealed syringe is taken to a blood gas analyzer. If the sample cannot be immediately analyzed, it is chilled in an ice bath in a glass syringe to slow metabolic processes which can cause inaccuracy. Samples drawn in plastic syringes should not be iced and should always be analyzed within 30 minutes.^[4]

The machine used for analysis aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide. The bicarbonate concentration is also calculated. These results are usually available for interpretation within five minutes.

Reference ranges and interpretation

These are typical reference ranges, although various analysers and laboratories may employ different ranges.

Analyte	Range	Interpretation
pH	7.35 - 7.45	The pH or H⁺ indicates if a patient is acidemic (pH < 7.35; H⁺ >45) or alkalemic (pH > 7.45; H⁺ < 35).
H⁺	35 - 45 nmol/l (nM)	See above.
PO₂	9.3-13.3 kPa or 80-100 mmHg	A low O₂ indicates that the patient is not respiring properly, and is hypoxemic. At a PO₂ of less than 60 mm Hg, supplemental oxygen should be administered. At a PO₂ of less than 26 mm Hg, the patient is at risk of death and must be oxygenated immediately.
PCO₂	4.7-6.0 kPa or 35-45 mmHg	The carbon dioxide and partial pressure (PCO₂) indicates a respiratory problem: for a constant metabolic rate, the PCO₂ is determined entirely by ventilation.^[5] A high PCO₂ (respiratory acidosis) indicates underventilation, a low PCO₂ (respiratory alkalosis) hyper- or overventilation. PCO₂ levels can also become abnormal when the respiratory system is working to compensate for a metabolic issue so as to normalize the blood pH.
HCO₃^-	22?26 mmol/l	The HCO₃^- ion indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO₃^- indicates metabolic acidosis, a high HCO₃^- indicates metabolic alkalosis. HCO₃^- levels can also become abnormal when the kidneys are working to compensate for a respiratory issue so as to normalize the blood pH.
SBC_e	21 to 27 mmol/l	the bicarbonate concentration in the blood at a CO₂ of 5.33 kPa, full oxygen saturation and 37 degrees Celsius.^[6]
Base excess	-2 to +2 mmol/l	The base excess indicates whether the patient is acidotic or alkalotic. A negative base excess indicates that the patient is acidotic. A high positive base excess indicates that the patient is alkalotic. \|- \| HPO₄^2? \| 0.8 to 1.5 ^[7] mM \| \|- \| total CO₂ (tCO₂ (P)_c) \| 25 to 30 mmol/l \| This is the total amount of CO₂, and is the sum of HCO₃^- and PCO₂ by the formula: tCO₂ = [HCO₃^-] + ?PCO₂, where ?=0.226 mM/kPa, HCO₃^- is expressed in molar concentration (M) (mol/l) and PCO₂ is expressed in kPa ^[8] \|- \| total O₂ (tO_2e)* \| \| This is the sum of oxygen solved in plasma and chemically bound to hemoglobin. ^[9]

Contamination with room air will result in abnormally low carbon dioxide and (generally) normal oxygen levels. Delays in analysis (without chilling) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.

Lactate level analysis is often featured on blood gas machines in neonatal wards, as infants often have elevated lactic acid.