CRNA School Interview ABG Questions: Explain Each Step
In This Article (4 sections)
When a panel hands you a blood gas, the order for CRNA school interview ABG questions is pH first, then the primary change in PaCO2 or bicarbonate, then whether the other value is compensating as expected. Look at oxygenation and the patient context separately. These three cases show you how to say that reasoning out loud.
Updated October 2026.
An ABG is sooo easy to misread when you jump straight to “respiratory” or “metabolic.” So keep your eyes on the numbers in the same order every time (even when three faculty members are watching and your brain wants to skip ahead). And if the pH looks normal, don’t stop there. The patient might still have a mixed acid-base problem.
What’s the order for CRNA school interview ABG questions?
Run this sequence every time the panel hands you a blood gas:
- pH: Is the blood acidemic (below 7.35), alkalemic (above 7.45), or in the reference range?
- Primary process: Does PaCO2 move in the direction that explains the pH? Or does bicarbonate?
- Expected compensation: Does the other value change by about the expected amount?
- Anion gap when relevant: With a metabolic acidosis and electrolytes on hand, calculate
Na - (Cl + HCO3)and use the lab’s reference range. - Oxygenation and context: Read PaO2 alongside the oxygen being delivered, the saturation, and the clinical situation. A PaO2 without FiO2 only tells you part of the story.
The Merck Manual acid-base reference gives the classification, expected compensation ranges, and the anion-gap method. Keep in mind compensation rules are estimates. If a result lands outside the expected range, consider another primary process, or go back and check the data and the timing.
When you speak, don’t try to compress the whole gas into one breath. Say the pH, say which value explains it, and pause. Then calculate compensation. That pause sounds like thoughtfulness because it is. A sample opening is, “The pH is 7.25, so this is acidemia. The bicarbonate is low and explains that direction. I want to check whether the low PaCO2 fits the expected response.” You’ll have already told the panel how you are organizing the problem.
Can you interpret these three ABGs out loud?
Cover the explanation first. Give your answer in one or two sentences, then do the compensation math (on paper, please, because mental math under pressure gets ugly fast).
Case 1: pH 7.28, PaCO2 60, HCO3 27
The pH shows acidemia. PaCO2 is high, which pushes pH down, so the primary process is respiratory acidosis. Bicarbonate is slightly high. If this developed acutely, Merck’s expected range is roughly a 1 to 2 mEq/L bicarbonate rise for each 10 mm Hg rise in PaCO2. PaCO2 is 20 above 40, so a bicarbonate near 26 to 28 fits an acute compensatory response from a starting value near 24.
What I’d say: “This is acidemia from a primary respiratory acidosis. The bicarbonate is about where I’d expect for an acute process. I’d want to know what changed in ventilation, and I’d want to see the patient as well as the gas.”
Follow-up question: “What if the PaCO2 had been high for days?” Then the expected bicarbonate response would be different because renal compensation takes time. The single gas does not tell you the full timeline. Ask for prior gases and the patient’s history before saying “acute” as if it were established. If the patient is ventilated, ask about the airway, ventilator readings, and recent changes; don’t turn a calculation into a ventilator order from a vignette.
Case 2: pH 7.25, PaCO2 25, HCO3 11
The pH shows acidemia. Low bicarbonate explains the direction, so this is a primary metabolic acidosis. The low PaCO2 might be respiratory compensation. Check it with Winter’s formula: expected PaCO2 = (1.5 × HCO3) + 8 ± 2. Here, 1.5 × 11 + 8 = 24.5 mm Hg, with an approximate range of 22.5 to 26.5. The measured PaCO2 of 25 fits. Nice and tidy.
What I’d say: “This is metabolic acidosis with an appropriate respiratory response by Winter’s formula. I’d need electrolytes, lactate, ketones when relevant, and the clinical picture to investigate the cause.” Merck Manual lists the formula and the next checks.
Follow-up question: “Would you call the PaCO2 of 25 a second respiratory disorder?” Not from these numbers. It falls in the expected range, so it fits compensation. If an interviewer changes PaCO2 to 35 while leaving bicarbonate at 11, run Winter’s formula again. Thirty-five is higher than the approximate expected 22.5 to 26.5 range, so a concurrent respiratory acidosis becomes a concern. Tell the panel you would verify the gas and assess ventilation, not just apply a label.
Case 3: pH 7.40, PaCO2 20, HCO3 12, Na 140, Cl 103
This one’s sneaky. The pH is in range, but both PaCO2 and bicarbonate are low. The anion gap is 140 - (103 + 12) = 25 mEq/L, above many lab reference ranges. That points toward a high-anion-gap metabolic acidosis, though you’d still check the lab range and albumin. If the bicarbonate of 12 represented a simple metabolic acidosis, Winter’s formula predicts PaCO2 near 1.5 × 12 + 8 = 26 ± 2. The measured PaCO2 is 20, lower than expected. So there’s more going on, and it suggests an additional primary respiratory alkalosis.
What I’d say: “The normal pH hides two processes. There’s a high-gap metabolic acidosis, and the PaCO2 is lower than expected for compensation alone. I’d investigate both the acid source and the reason for excess ventilation.” The Merck acid-base guide explains how a near-normal pH might accompany a mixed disorder.
If you only drill one case before your interview, make it this one. Trust me on this one.
Follow-up question: “How do you know it isn’t simple compensation?” Show the calculation: expected PaCO2 is roughly 24 to 28, and the measured value is 20. The gap between expected and measured is the reason for suspecting a second primary process. A normal pH is not the reason by itself. If you forgot the exact formula, say you would check the expected range rather than guessing at a diagnosis.
Which ABG interview traps should you practice before the panel?
Start with the trap of seeing two abnormal numbers and calling it “mixed.” Compensation makes two numbers abnormal in a simple disorder too. Your job is to test whether the second value is more or less changed than expected. In Case 2, the low PaCO2 belongs with the low bicarbonate. In Case 3, the PaCO2 falls beyond the expected response. Same direction on the page, different interpretation after the calculation.
The other trap is treating oxygenation as if it were explained by the acid-base label. If a prompt includes PaO2, ask what oxygen the patient was receiving when the sample was drawn. PaO2 of 70 mm Hg on room air means something different from 70 on a high FiO2. Also check whether the sample and the clinical picture fit one another. The Merck acid-base reference advises using the history and other laboratory results to interpret the numbers; the gas is one piece of the patient.
For a practice session, write the five-step sequence at the top of a blank page, then cover it. Work Case 1 in 60 seconds and explain it aloud. Work Case 2 with the formula visible. Finally, do Case 3 without looking, then check your arithmetic. If you missed the compensation range, write that exact miss on your study list. The CRNA Club membership includes ABG and ventilation lessons if you need to review the underlying physiology before your next spoken run-through.
Ask your practice partner to interrupt with “What caused it?” after you classify each gas. The numbers alone cannot answer that. In Case 2, a metabolic acidosis is clear; whether lactate, ketones, renal dysfunction, or another process explains it needs history and more data. Practice saying which data you would ask for, and resist filling the silence with a diagnosis the vignette never gave you.
You can also rehearse the moment when the interviewer withholds a value. If they give pH, PaCO2, and bicarbonate but no electrolytes, you can classify the primary acid-base process and check expected compensation; you cannot calculate an anion gap yet. Say exactly what you need next: sodium and chloride from the same clinical period. That sounds steadier than inventing a gap because you remember a typical number.
What should you say after naming the disorder?
Connect the gas back to the patient. PaCO2 is about ventilation. PaO2 is about oxygenation, and you’ve got to read it with the oxygen setting. Bicarbonate and anion gap point you toward metabolic causes. Then say which value you’d trend next and why.
And if an interviewer changes one number in the case? Start the sequence over instead of defending your first answer (it’s a fresh gas now, so treat it like one).
The CRNA Club’s interview question hub has more practice prompts. Work one ABG on paper, then explain it without looking down during a free mock interview. Missed a mixed disorder? Repeat the compensation step until it’s a natural part of your spoken answer.
Frequently Asked Questions
What's the first number to check on an ABG in a CRNA interview?
The first number to check on an ABG is pH. A pH below 7.35 is acidemia, and a pH above 7.45 is alkalemia. Then ask whether PaCO2 or bicarbonate explains the direction of the pH, and check expected compensation before you call it a simple disorder. A pH inside the reference range doesn’t end the analysis when PaCO2 and bicarbonate are both abnormal. The Merck Manual acid-base guide lays out the same classification. The CRNA Club’s interview question hub has more clinical prompts once you’ve practiced this sequence.
How do I know whether a metabolic acidosis has appropriate respiratory compensation?
Winter’s formula tells you whether a metabolic acidosis has appropriate respiratory compensation. It estimates expected PaCO2 as 1.5 times bicarbonate plus 8, with an approximate margin of 2 mm Hg. If bicarbonate is 11 mEq/L, expected PaCO2 is about 24.5, or roughly 22.5 to 26.5 mm Hg. A measured value well outside that range raises concern for an additional respiratory process. Say the calculation out loud and check the patient’s context. Compensation estimates are guides, and they don’t replace examining the patient or reviewing the timing of the gas.
Can an ABG have a normal pH and still be abnormal?
A normal pH doesn’t guarantee a normal ABG. A near-normal pH might hide opposing primary processes. If bicarbonate is low and PaCO2 is also low, check whether the PaCO2 matches expected compensation for metabolic acidosis. A PaCO2 lower than expected suggests a concurrent respiratory alkalosis. Electrolytes help you calculate the anion gap when metabolic acidosis is present. So don’t label the gas “normal” based on pH alone. The CRNA Club’s free mock interview is a place to practice explaining that distinction out loud.
Should I interpret PaO2 without knowing FiO2?
PaO2 is more useful when you know how much oxygen the patient is receiving. The same PaO2 means different things on room air and on high supplemental oxygen. So say you’d check FiO2, saturation, the oxygen delivery device or ventilator settings, and the trend. Keep the oxygenation assessment separate from the acid-base classification so neither one gets lost. And if the question gives no oxygen setting, state that limit plainly instead of making one up. Naming the gap out loud shows the panel you know what the number does and doesn’t tell you.
