ABGs Don't Have to Be Scary: A Three-Step Reading Order

Why ABGs feel harder than they are

Most people's first instinct when they see an ABG panel is to look at the pH, the CO2, and the HCO3 all at once and try to hold the whole picture in their head. That is exactly what makes it feel overwhelming. The fix is boring but effective: read the three values in a fixed order, one at a time, and let the pattern reveal itself instead of trying to see it all in one glance.

Step one: pH, and only pH

Normal pH is 7.35 to 7.45. If it is below 7.35, the client is acidotic. If it is above 7.45, the client is alkalotic. Write down "acidotic" or "alkalotic" before you look at anything else. This single number tells you the direction of the problem — everything after this step is just figuring out the cause.

Step two: CO2 (PaCO2)

Normal PaCO2 is 35 to 45 mmHg. CO2 is acidic — more of it pushes pH down, less of it pushes pH up. If your CO2 moved in the SAME direction as your acid/base conclusion from step one (high CO2 with acidosis, or low CO2 with alkalosis), you likely have a respiratory problem. If CO2 is normal or moved the opposite direction, look to step three instead.

Step three: HCO3 (bicarbonate)

Normal HCO3 is 22 to 26 mEq/L. Bicarbonate is a base — more of it pushes pH up, less of it pushes pH down. If HCO3 moved in the SAME direction as your pH conclusion (low HCO3 with acidosis, or high HCO3 with alkalosis), the root cause is metabolic, not respiratory.

Put it together

Once you know the direction (acidotic or alkalotic) and which value is driving it (respiratory or metabolic), you have your primary classification: respiratory acidosis, respiratory alkalosis, metabolic acidosis, or metabolic alkalosis. Compensation questions build on this same three-step read — you are just checking whether the OTHER system (the one not driving the primary problem) has started moving in a direction that would correct the pH back toward normal.

Why the order matters more than memorizing a chart

Trying to memorize every possible ABG combination as a lookup table is fragile — there are too many combinations, and NGN case-study items will hand you the values embedded in a larger clinical picture, not a clean isolated panel. Reading in a fixed order (pH, then CO2, then HCO3) is a process you can run under pressure on any panel you are handed, which is worth more than a memorized table you might blank on.