ABG Calculator
Enter a pH, a pCO2 and a bicarbonate and get the acid-base diagnosis, the expected compensation, and whether there is a second disorder hiding underneath the first one.
Acid-base interpretation pH, pCO2, HCO3
These three values are all you need for the diagnosis. Fill them in and the interpretation updates as you type.
Arterial blood gas
Normal 7.36 to 7.44
Normal 36 to 44 mm Hg
Normal 22 to 26 mEq/L. Use the calculated bicarbonate from the gas, or the CO2 from the chemistry panel.
Interpretation
•
Enter valuesThe calculator also reports the pH that the Henderson-Hasselbalch equation predicts from your pCO2 and bicarbonate. If the measured pH and the predicted pH are more than about 0.03 apart, the three numbers are internally inconsistent, and you are usually looking at a sampling error, a delay in running the sample, or a venous sample being reported as arterial. Redraw before you act on it.
Anion gap Works on its own
The anion gap is a separate calculation from the blood gas, and this one runs independently. You do not need a pH or a pCO2 for it. All it needs is a chemistry panel.
Chemistry panel
Optional, but do not skip it in a sick patient. A low albumin hides a real gap.
Anion gap
•
Enter valuesFor the delta-delta, the causes behind each pattern and the full explanation, see the dedicated anion gap calculator.
How to read an ABG in four steps
Every blood gas yields to the same sequence. Do it in this order and you will not get lost.
Look at the pH alone. Under 7.36 is acidemia, over 7.44 is alkalemia. If the pH is normal you are either normal or you have two disorders cancelling each other out, so keep going.
Which value moved in the direction that explains the pH? A high pCO2 with a low pH is a respiratory acidosis. A low bicarbonate with a low pH is a metabolic acidosis. If both moved in the same direction, you have two processes running at once.
Use the formula for the primary disorder and compare the predicted value against the measured one. Compensation that falls short or overshoots means there is a second disorder. The body never overcorrects past normal on its own.
Even when the pH is normal. A patient with an anion gap acidosis and a metabolic alkalosis can present with a pH of 7.40 and a bicarbonate of 24 and still be in serious trouble. The gap is the only thing that will tell you.
Compensation formulas
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | Expected pCO2 = (1.5 x HCO3) + 8, plus or minus 2. Winter's formula. |
| Metabolic alkalosis | Expected pCO2 = (0.7 x HCO3) + 21, plus or minus 2. |
| Acute respiratory acidosis | HCO3 rises about 1 for every 10 mm Hg the pCO2 rises. |
| Chronic respiratory acidosis | HCO3 rises about 3.5 for every 10 mm Hg the pCO2 rises. |
| Acute respiratory alkalosis | HCO3 falls about 2 for every 10 mm Hg the pCO2 falls. |
| Chronic respiratory alkalosis | HCO3 falls about 5 for every 10 mm Hg the pCO2 falls. |
The acute versus chronic distinction on the respiratory side is the one that changes management. A COPD patient living at a pCO2 of 60 with a bicarbonate of 33 and a pH of 7.36 is fully compensated and does not need to be intubated. The same three numbers appearing over four hours in someone who was normal this morning is a different emergency entirely. The calculator above tells you which pattern the numbers fit.
The body does not fully normalize the pH through compensation, and it never overshoots into the opposite abnormality. If the numbers suggest compensation has carried the pH past normal in the other direction, you are not looking at compensation. You are looking at a second primary disorder.
Causes worth memorizing
Anion gap metabolic acidosis
The mnemonic worth keeping is GOLDMARK: Glycols (ethylene and propylene), Oxoproline (chronic acetaminophen, classically a malnourished woman), L-lactate, D-lactate (short gut), Methanol, Aspirin, Renal failure, Ketoacidosis. In the hospital it is almost always lactate, ketones or uremia, and lactate is the one that kills people the same day.
Non-gap metabolic acidosis
Diarrhea and renal tubular acidosis account for most of it, and large-volume normal saline resuscitation accounts for a surprising share of the rest. A urine anion gap separates them: negative means the kidney is excreting ammonium appropriately, so the loss is gastrointestinal. Positive points at the kidney.
Metabolic alkalosis
Vomiting, nasogastric suction and diuretics cover the overwhelming majority. A urine chloride under 20 means it is chloride-responsive and saline will fix it. Over 20 points you toward mineralocorticoid excess or ongoing diuretic use.
Respiratory acidosis
Anything that drops minute ventilation: opiates and sedatives, COPD, neuromuscular weakness, obesity hypoventilation, and a ventilator that is set wrong. If it is the ventilator, see the guide to adjusting vent settings from an ABG.
Respiratory alkalosis
Pain, anxiety, fever, sepsis, pulmonary embolism, salicylates, pregnancy and hepatic failure. In a patient who is short of breath with a clear chest and a respiratory alkalosis, think about pulmonary embolism before you settle on anxiety.
This is a teaching tool, not a clinical decision system. It does not know the history, the medication list, or how the sample was drawn. Confirm every interpretation against the patient in front of you, and follow your own institution's protocols.