ABG NCLEX Questions: The 6 Patterns the Exam Keeps Testing

The NCLEX rarely asks you to name a gas. It asks what the gas means and what the nurse does about it, with four options that all sound reasonable. Once you have seen enough acid–base questions you notice they are built from a small number of patterns — six, by our count of the questions we write and audit. Learn the six and the individual questions stop being surprising.

Pattern 1 — Name the gas, then the trap is the pH that looks normal

The straightforward “which disorder is this?” question exists, and it is usually placed on a gas with a normal pH so students who stop at step one miss it. The method: pH direction (using 7.40 as the tie-break), the value that agrees with it, and whether the other value has moved. If you can make the three calls on a random gas ten times in a row, this pattern is free points. The three-step read.

Pattern 2 — Which patient is at risk for this disorder?

You are given four patients and asked which one the nurse expects to develop, say, metabolic alkalosis. This is a causes question wearing a patient costume. The pairings the exam loves: NG suction and vomiting → metabolic alkalosis; diarrhea → metabolic acidosis (normal gap); DKA and shock → metabolic acidosis (high gap); opioids, sedation and COPD → respiratory acidosis; anxiety, pain, fever, hypoxemia and early salicylates → respiratory alkalosis; loop diuretics → metabolic alkalosis with hypokalemia. All four, with causes.

Pattern 3 — What does the nurse do first?

The priority is the patient, not the number. For a respiratory acidosis the first move is the airway and breathing: stimulate, position, bag, reverse the sedative. For a metabolic acidosis it is perfusion and the cause — fluids in DKA before insulin, not bicarbonate. For a respiratory alkalosis it is ruling out hypoxemia before calling it anxiety. The distractor is almost always a medication that treats the number: “administer sodium bicarbonate” is wrong far more often than it is right.

Pattern 4 — What does the nurse anticipate the provider will order?

This pattern tests whether you know the fix follows the cause. For vomiting alkalosis it is 0.9% saline with potassium chloride — chloride is what the kidney needs to let go of bicarbonate. For opioid hypoventilation it is naloxone. For COPD with a rising CO₂ it is controlled oxygen and BiPAP, not high-flow oxygen. For DKA it is isotonic fluid, then an insulin infusion, then potassium once the level is below about 5.3 and urine is flowing. Below is the vomiting patient; read the gas, then look at the bedside cards and notice what is not in the treatment.

Pattern 5 — The electrolyte that moves with the pH

Two couplings, tested constantly. Acidosis raises serum potassium (hydrogen goes into cells, potassium comes out), so the DKA patient reads hyperkalemic and then falls fast once insulin starts — the question asks when to add potassium to the fluids. Alkalosis lowers ionized calcium (more calcium binds to albumin), so the hyperventilating or vomiting patient tingles and spasms with a normal total calcium — the question asks which finding the nurse expects. Both couplings are in the six electrolyte relationships.

Pattern 6 — The finding that means it is getting worse

The exam wants you to know which assessment change is the alarm. In respiratory acidosis it is falling level of consciousness — a COPD patient who is suddenly hard to arouse is retaining CO₂ faster than the kidneys can answer. In metabolic acidosis it is deep Kussmaul respirations giving way to slow, shallow breathing — the patient is tiring out of their own compensation. In a fully compensated gas the number is stable, so the patient is the only monitor you have.

How to prepare for all six

1. Get the read automatic first. Random gases on the simulator until the three calls take seconds. Every other pattern assumes you can do this without thinking.
2. Learn the eight patients, not the four boxes. DKA, diarrhea, vomiting, opioid, COPD, panic, salicylate, shock. Each carries its causes, its findings and its treatment — patterns 2, 3 and 4 in one place.
3. Attach every number to an action. pH under 7.30 in COPD → BiPAP. pH under 7.1 → bicarbonate becomes reasonable. K⁺ under 5.3 in DKA → start replacing.
4. Then answer questions at rising difficulty. Naming the gas is tier one. “What does the nurse do first” with two defensible options is where the exam lives, and it takes volume.

The simulator is free at /tools/acid-base-simulator. The two patterns students find hardest have their own posts: the compensation stages and mixed disorders. The same approach for the electrolyte questions is in the seven fluid and electrolyte patterns.