Hardest Topics8 min read

ABGs in 90 Seconds: The Tic-Tac-Toe Method, Fully Worked

Arterial blood gases were taught to you as chemistry — buffers, the Henderson-Hasselbalch equation, a lecture slide with a seesaw on it. That's why they feel impossible. On the exam an ABG is three numbers and a tic-tac-toe grid, and once you've seen the grid you can't unsee it. Ninety seconds, every time, no exceptions.

The three numbers that matter

You'll be handed four or five values. Only three go on the grid. Memorize these ranges cold — not "roughly," cold — because the whole method is about which side of the range each number lands on.

Value	Normal	What it is	Low means	High means
pH	7.35–7.45	The verdict	Acidosis	Alkalosis
PaCO2	35–45 mmHg	The respiratory number. CO2 is an acid.	Alkalosis (blowing off acid)	Acidosis (retaining acid)
HCO3	22–26 mEq/L	The metabolic number. Bicarb is a base.	Acidosis (lost base)	Alkalosis (gained base)
PaO2	80–100 mmHg	Oxygenation. Not acid-base.	Hypoxemia	—

PaO2 does not go on the grid. Ever. A question will give you a PaO2 of 60 next to a perfectly normal pH, CO2 and bicarb and ask you to name the acid-base imbalance. The answer is "there isn't one — the client is hypoxemic." Two different problems. The grid only sees three numbers.

The grid

Draw three columns: ACID on the left, NORMAL in the middle, BASE on the right. Now place each of the three values in the column it belongs to.

pH goes where it says. Below 7.35 is ACID. Above 7.45 is BASE. In range is NORMAL — but remember which side of 7.40 it's on, because you'll need that in step 5.
PaCO2 is an acid, so it goes in the column that matches what it's doing. High CO2 (above 45) is more acid — ACID column. Low CO2 (below 35) is less acid — BASE column. Normal goes in the middle.
HCO3 is a base. High bicarb (above 26) is more base — BASE column. Low bicarb (below 22) is less base — ACID column. Normal goes in the middle.
Find the match. Whichever value is in the same column as pH is the cause. If it's PaCO2, the problem is respiratory. If it's HCO3, the problem is metabolic. The pH column tells you whether it's acidosis or alkalosis. You now have a name: respiratory acidosis, metabolic alkalosis, and so on.
Check compensation. Look at the third value, the one that isn't pH and isn't the cause. If it's in the NORMAL column, the body hasn't started fixing anything: uncompensated. If it's in the opposite column from pH, the body is fighting back. Then look at pH again: still out of range means partially compensated; back in range means fully compensated.

ROME. Respiratory Opposite — pH and PaCO2 move in opposite directions when the lungs are the cause. Metabolic Equal — pH and HCO3 move in the same direction when the kidneys are the cause. Use it as a check after the grid, not instead of it.

Fully worked: four gases, four grids

Gas 1 — pH 7.30 · PaCO2 55 · HCO3 24

ACID	NORMAL	BASE
pH 7.30
PaCO2 55 (high CO2 = more acid)
	HCO3 24

pH is in ACID. PaCO2 is in ACID with it — the match. So it's respiratory, and it's acidosis. HCO3 sits in NORMAL, so the kidneys haven't started helping. Uncompensated respiratory acidosis. Clinically: the client isn't ventilating. Think opioid oversedation, COPD exacerbation, a post-op client who won't take a deep breath, pneumonia. Total time: fifteen seconds.

Gas 2 — pH 7.48 · PaCO2 30 · HCO3 20

ACID	NORMAL	BASE
		pH 7.48
		PaCO2 30 (low CO2 = less acid)
HCO3 20 (low bicarb = less base)

pH is in BASE. PaCO2 is in BASE with it — respiratory alkalosis. HCO3 has moved to the opposite column, so the kidneys are dumping bicarb to pull the pH back down. Is the pH normal yet? No — 7.48. Partially compensated respiratory alkalosis. Clinically: the client is blowing off CO2 — hyperventilation from anxiety, pain, early sepsis, a pulmonary embolus, or a ventilator set too fast.

Gas 3 — pH 7.36 · PaCO2 58 · HCO3 32

ACID	NORMAL	BASE
	pH 7.36 (normal — but below 7.40, leaning acid)
PaCO2 58
		HCO3 32

This is the one that fails people. The pH is normal, but both of the other values are abnormal, in opposite columns. That's full compensation — the body has completely corrected the pH — and now you need to know which one started it. Use the 7.40 rule: a pH below 7.40 leans acid, above 7.40 leans base. 7.36 leans acid. Which value is in the ACID column? PaCO2. Fully compensated respiratory acidosis. Clinically: this is your long-standing COPD client. His kidneys have spent years hoarding bicarb to balance the CO2 he can't exhale. He's chronic, he's compensated, and he's not the one you see first.

Gas 4 — pH 7.25 · PaCO2 38 · HCO3 15

ACID	NORMAL	BASE
pH 7.25
	PaCO2 38
HCO3 15

pH in ACID, HCO3 in ACID with it — metabolic acidosis. PaCO2 sits in NORMAL, so the lungs haven't started blowing off acid yet. Uncompensated metabolic acidosis. Clinically: DKA, kidney failure, severe diarrhea, lactic acidosis from shock. And if the next set of gases shows PaCO2 dropping to 28 with the pH creeping up, that's the lungs kicking in — the deep, rapid Kussmaul breathing you'll see at the bedside in DKA is compensation you can watch happen.

The 90-second budget

Seconds	Step
0–10	Place pH. Note which side of 7.40.
10–20	Place PaCO2. High = ACID, low = BASE.
20–30	Place HCO3. High = BASE, low = ACID.
30–45	Match. Same column as pH = cause. Name it.
45–60	Third value: NORMAL = uncompensated · opposite column = partial (pH abnormal) or full (pH normal).
60–90	Now — and only now — read the scenario and ask what caused it and what the nurse does about it.

What the exam actually asks

Here's the part nobody warns you about: NCLEX rarely asks "name this imbalance." It gives you the gas, expects you to name it silently in your head, and then asks what you'd do. So the grid is step one, and step two is knowing the nursing move for each of the four.

Imbalance	Classic causes	What you'll see	Nursing priority
Respiratory acidosis	Hypoventilation: oversedation, COPD flare, pneumonia, splinting after surgery, neuromuscular weakness	Drowsy, confused, headache, flushed, shallow slow breathing	Get air moving. Head of bed up, stimulate, cough and deep breathe, incentive spirometer, suction if needed, prepare for ventilatory support if worsening
Respiratory alkalosis	Hyperventilation: anxiety, pain, fever, early sepsis, PE, vent rate too high	Lightheaded, tingling fingers and lips, carpopedal spasm, rapid breathing	Slow the breathing. Coach slow breaths, treat the cause (pain, fear), stay with the client. No paper bags.
Metabolic acidosis	DKA, renal failure, severe diarrhea, shock, lactic acidosis	Kussmaul respirations, lethargy, confusion, warm flushed skin, high K+	Treat the cause, monitor cardiac rhythm for the potassium, prepare for fluids and provider-directed correction
Metabolic alkalosis	Vomiting, NG suction, excessive antacids, diuretic-related volume loss	Muscle twitching, tremor, confusion, slow shallow breathing, low K+	Stop the loss, watch the potassium and the rhythm, seizure precautions if severe

Acidosis and potassium travel together. When the blood is acidic, hydrogen ions move into cells and potassium moves out — so metabolic acidosis usually comes with a high K+, and metabolic alkalosis with a low one. If a question gives you a gas and a potassium, they want you to connect them, and the priority is usually the cardiac monitor.

pH 7.35–7.45 · PaCO2 35–45 (acid) · HCO3 22–26 (base). Same column as pH = cause. Third value NORMAL = uncompensated · opposite = compensating · pH back in range = full. PaO2 stays off the grid. Normal pH with two abnormal values: use 7.40 to pick the side.

Go deeper. The Hardest Topics Survival Pack has the full ABG chapter — twelve more worked gases, the mixed disorders, the electrolyte crossovers, and a bank of practice questions with rationales that explain why each wrong option is wrong. Same grid, every possible way the exam can twist it.

Want the Sunday email? One hardest-topic breakdown, one NCLEX trap, one thing to study this week. Get the free cheat sheet and you're in.