Acid Base Disorders Arterial Blood Gas (ABG) Analyzer

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    The principal investigators of the study request that you use the official version of the modified score here.

    Arterial Blood Gas (ABG) Analyzer

    Interprets ABG.


    This analyzer should not substitute for clinical context. Sodium and chloride are required for anion gap calculation.
    While the analyzer can often help with analysis, the history of the patient is critical for accurate interpretation.

    mm Hg






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    Next Steps
    Creator Insights


    A venous blood gas often correlates well with arterial blood gas findings (except for PaO2) unless values are extremely abnormal, and can often be used successfully as a screening tool.


    This tool, developed by Jonathan Chen, MD first determines the primary process by looking at the pH and the PCO2. It then calculates compensations to determine chronicity, compensatory, and co-existing acid-base disturbances.

    Facts & Figures

    Metabolic Acidosis (Anion Gap)


    • Methanol
    • Uremia
    • Diabetic Ketoacidosis (check serum ketones)
    • Propylene Glycol (in BZD drips) or Paraldehydes
    • Isoniazid
    • Lactic Acidosis (check serum lactate)
    • Ethylene Glycol (anti-freeze)
    • Salycylates


    • Glycols (ethylene or propylene)
    • Oxoporin (reflects fatty liver damage from glutathione consumption, e.g. acetaminophen toxicity)
    • L-Lactate
    • D-Lactate (bacterial form)
    • Methanol
    • Aspirin (salycylate)
    • Renal Failure (BUN uremia)
    • Ketoacidosis

    Metabolic Acidosis (Non-Anion Gap)

    GI Loss

    • Diarrhea / Laxatives
    • Fistula, (pancreatic, biliary)
    • Uretero-intestinal diversion (ileal conduit)

    Renal Loss

    • Renal Tubular Acidosis (Type 1 Distal or Type 2 Proximal)
    • Renal Failure
    • Hyper-kalemia

    Exogenous Acid

    • HCl
    • Amino Acids


    • Fistula (pancreatic, biliary)
    • Uretero-gastric conduit
    • Saline admin (dilutional acidosis)
    • Endocrine (hyper-PTH)
    • Diarrhea
    • Carbonic anhydrase inhibitor (acetazolamide)
    • Ammonium chloride
    • Renal tubular acidosis
    • Spironolactone

    Metabolic Alkalosis

    Alkaline Input

    • Bicarbonate Infusion
    • Hemodialysis
    • Calcium Carbonate
    • Parenteral Nutrition

    Proton Loss

    • GI Loss (vomiting, NG suction)
    • Renal loss
    • Diuretics
    • Mineralocorticoids

    Respiratory Acidosis

    Airway Obstruction

    • Foreign body, aspiration
    • OSA (obstructive sleep apnea)
    • Laryngo- or broncho-spasm


    • Myasthenia gravis
    • Hypokalemic periodic paralysis
    • Guillain-Barre
    • Botulism, Tetanus
    • Hypo-kalemia, hypo-phosphatemia
    • Cervical spine injury
    • Morbid obesity
    • Polio, MS, ALS


    • Drugs (opiates, sedatives)
    • Oxygen treatment in acute hypercapnia
    • Brain trauma or stroke


    • Pulmonary edema
    • Asthma
    • Pneumonia
    • ARDS
    • COPD
    • Pulmonary Fibrosis

    Mechanical Ventilation

    Respiratory Alkalosis


    • High altitude
    • CHF
    • Pulmonary Embolism

    Lung Disease

    • Pulmonary fibrosis
    • Pulmonary edema
    • Pneumonia


    • Progesterone
    • Nicotine

    Stimulation of Respiratory Drive

    • Psychogenic
    • Neurologic (pontine tumor)
    • Sepsis
    • Pregnancy
    • Mechanical ventilation


    Original/Primary Reference

    Research PaperBaillie JK. Simple, easily memorised 'rules of thumb' for the rapid assessment of physiological compensation for respiratory acid-base disorders. Thorax 2008;63:289-290 doi:10.1136/thx.2007.091223


    Research PaperKaufman DA. Interpretation of Arterial Blood Gases (ABGs). Written for Retrieved 12/9/2014.

    Dr. Jonathan Chen

    About the Creator

    Jonathan Chen, MD, PhD is a research fellow in medical informatics, based at the Veteran Affairs Hospital in Palo Alto and Stanford University. He completed the Stanford Internal Medicine residency program and was in the Medical Scientist Training Program (MSTP) and Biomedical Informatics Training (BIT) program at UC Irvine. Dr. Chen co-founded Reaction Explorer, LLC, which offers a unique system for teaching complex problem-solving in organic chemistry with the aid of expert system technology.

    To view Dr. Jonathan Chen ' s publications, visit PubMed

    Related Calcs
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    About the Creator

    Dr. Jonathan Chen
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    Home | Investigations [tests] | Acid Base Disorders

    Acid Base Disorders

    by , Last updated

    Arterial blood gas analysis is used to determine the adequacy of oxygenation and ventilation, assess respiratory function and determine the acid–base balance. These data provide information regarding potential primary and compensatory processes that affect the body’s acid–base buffering system.

    • Metabolic Acidosis
    • Metabolic Alkalosis
    • Respiratory Acidosis
    • Respiratory Alkalosis

    Interpret the ABGs in a stepwise manner:

    1. Determine the adequacy of oxygenation (PaO2)
      • Normal range: 80–100 mmHg (10.6–13.3 kPa)
    2. Determine pH status
      • Normal pH range: 7.35–7.45 (H+ 35–45 nmol/L)
      • pH <7.35: Acidosis is an abnormal process that increases the serum hydrogen ion concentration, lowers the pH and results in acidaemia.
      • pH >7.45: Alkalosis is an abnormal process that decreases the hydrogen ion concentration and results in alkalaemia.
    3. Determine the respiratory component (PaCO2)
    4. Primary respiratory acidosis (hypoventilation) if pH <7.35 and HCO3– normal.
      • Normal range: PaCO2 35–45 mmHg (4.7–6.0 kPa)
      • PaCO2 >45 mmHg (> 6.0 kPa): Respiratory compensation for metabolic alkalosis if pH >7.45 and HCO3– (increased).
      • PaCO2 <35 mmHg (4.7 kPa): Primary respiratory alkalosis (hyperventilation) if pH >7.45 and HCO3– normal. Respiratory compensation for metabolic acidosis if pH <7.35 and HCO3– (decreased).
    5. Determine the metabolic component (HCO3–)
      • Normal HCO3– range 22–26 mmol/L
      • HCO3 <22 mmol/L: Primary metabolic acidosis if pH <7.35. Renal compensation for respiratory alkalosis if pH >7.45.
      • HCO3 >26 mmol/L: Primary metabolic alkalosis if pH >7.45. Renal compensation for respiratory acidosis if pH <7.35.

    Additional definitions

    • Osmolar Gap
      • Use: Screening test for detecting abnormal low MW solutes (e.g. ethanol, methanol & ethylene glycol [ Reference ])
      • An elevated osmolar gap (>10) provides indirect evidence for the presence of an abnormal solute which is present in significant amounts [ Reference ]
      • Osmolar gap = Osmolality – Osmolarity
      • Osmolality (measured)
        • Units: mOsm/kg
        • Measured in laboratory and returned as the plasma osmolality
      • Osmolarity (calculated)
        • Units: mOsm/l
        • Osmolarity = (1.86 x [Na+]) + [glucose] + [urea] + 9  (using values measured in mmol/l)
        • Osmolarity = (1.86 x [Na+]) + glucose/18 + BUN/2.8 + 9 (using US units of mg/dl)
      • NOTE: even though the units of measured (mOsm/kg) and calculated (mOsm/l) are different [ Reference ], strictly they cannot be subtracted from one another… However, the value of the difference is clinically useful so the problem is usually overlooked!

    Rules and Resources

    • Acid Base disorders worksheet – Joshua Steinberg MD
    • ABG walk though (2003 FACEM exams) below
    Arterial Blood Gas Interpretation Chart
    Arterial Blood Gas (ABG) Interpretation Chart

    1 2 3 4 5 Rule

    Simple table to calculate metabolic compensation in respiratory acidosis and alkalosis (aka the 1-2-3-4-5 rule)

    Simple calculation to predict changes in HCO3– from PaCO2

    (Baseline 24 mmol/L)

    Every 10 mmHg change in PaCO2
    from baseline 40 mmHg









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