Looking for an ABG calculator or an ABGs calculator you can trust—and quick answers to everyday ABG questions? You’re in the right place. Below you’ll find a concise ABG interpretation chart, step-by-step “how to calculate ABGs,” and answers to common questions pulled from real Reddit threads. When you’re ready to analyze a gas, try our tool: ABG calculator
Quick ABG interpretation chart (keep handy)
- Normal adult ranges (arterial):
pH 7.35–7.45 · PaCO₂ 35–45 mmHg (≈ 4.7–6.0 kPa) · HCO₃⁻ 22–26 mEq/L (mmol/L) · PaO₂ 80–100 mmHg · SaO₂ 95–100% - First decide: acidosis (pH < 7.35) or alkalosis (pH > 7.45). Then look at PaCO₂ (respiratory) and HCO₃⁻ (metabolic).
- Compensation can normalize pH; a “normal” pH doesn’t rule out an acid–base disorder—check CO₂/HCO₃⁻ and the overall pattern.
- Measured vs calculated: pH and PaCO₂ are measured; HCO₃⁻ is calculated from pH and PaCO₂ using the Henderson–Hasselbalch relationship.
Pro tip: Want structured steps? LITFL’s classic approach and StatPearls’ examples pair well with practice.
How to calculate ABGs (what “ABG calculation formula” means)
Clinically, “ABG calculation” usually means interpreting pH, PaCO₂, and HCO₃⁻—not computing pH from scratch. Still, the math under the hood is useful:
- Henderson–Hasselbalch (blood): pH=6.1+log10(0.03×PaCO2HCO3−)
Many analyzers calculate HCO₃⁻ from your measured pH and PaCO₂ with this formula.
Practical steps (fast bedside flow):
- Look at pH → acidemia vs alkalemia.
- Look at PaCO₂ → respiratory component (↑ CO₂ acidifies, ↓ CO₂ alkalinizes).
- Look at HCO₃⁻ → metabolic component (↑ alkalinizes, ↓ acidifies).
- Decide the primary disorder and whether compensation is appropriate (don’t overcall “normal pH”).
If you’d rather skip the math, run the numbers with our ABG interpretation calculator.
Real questions
1) “I’m confused—my ABG has normal pH but weird CO₂/HCO₃⁻. Is that normal?”
It can be mixed or compensated. A normal pH doesn’t exclude an acid–base disorder; compensating or opposing processes can bring pH back to near-normal. Always check PaCO₂ and HCO₃⁻ and decide what’s primary.
Why: Compensation aims to restore pH toward 7.40; analyzers measure pH and PaCO₂ and calculate HCO₃⁻, so “normal pH” with abnormal components is common in mixed states.
2) “Any step-by-step way to get better at ABGs?”
Yes—use a structured approach (pH → CO₂ → HCO₃⁻ → compensation) and practice. Several clinicians share quick frameworks and practice links in Reddit threads; LITFL and StatPearls have systematic guides.
3) “Is there a trick like R.O.M.E. that actually helps?”
R.O.M.E. (Respiratory Opposite, Metabolic Equal) helps beginners remember pH relationships: in respiratory disorders, pH moves opposite PaCO₂; in metabolic, pH moves the same way as HCO₃⁻. Use it as a memory aid—then confirm with the numbers.
4) “How do I tell primary vs compensation?”
Start with pH and ask, “Which change (PaCO₂ or HCO₃⁻) would push pH that way?” That’s usually primary. Then check if the other variable changed in the expected compensatory direction. Threads in r/Residency and r/medicine echo this method.
5) “What’s the lowest pH you’ve seen—and was it high-gap metabolic acidosis?”
Critical care clinicians often discuss severe lactic acidosis with very low pH and incomplete respiratory compensation. Insights: look for anion gap and lactate, and remember Winter’s formula expectations when you suspect metabolic acidosis.
6) “Why does the machine say HCO₃⁻ is X? I thought it was measured.”
Most analyzers measure pH and PaCO₂, then calculate HCO₃⁻ (and base excess) via Henderson–Hasselbalch; the report labels HCO₃⁻ as “calculated.”
7) “I see mmHg in some places and kPa in others. How do I switch?”
1 kPa = 7.50062 mmHg. If you prefer SI units, convert PaCO₂ accordingly. (Our calculator supports both mmHg and kPa.)
8) “Any practice sources for ABG interpretation?”
Several Redditors share curated practice links—some love step-wise guides, others prefer practice quizzes. Use credible resources and check normal ranges as you work.
9) “When should I take an ABG vs a VBG?”
Threads in r/Residency and r/medicine point out that ABG is preferred for precise oxygenation and ventilation assessment; VBG can be reasonable for trends in some settings but has limits. Know what question you’re answering.
10) “Do you have an advanced deep-dive?”
Yes—community favorites include long-form videos/notes and LITFL’s advanced pages for compensation and mixed disorders.
Interpreting ABGs made easy (nursing & bedside perspective)
- Start with pH. Is it acidemia (< 7.35) or alkalemia (> 7.45)?
- Then PaCO₂: lungs control CO₂ (acid). ↑ CO₂ lowers pH; ↓ CO₂ raises pH.
- Then HCO₃⁻: kidneys control bicarbonate (base). ↑ HCO₃⁻ raises pH; ↓ HCO₃⁻ lowers pH.
- Primary vs compensation: If pH is low and PaCO₂ is high, primary respiratory acidosis; if pH is low and HCO₃⁻ is low, primary metabolic acidosis (expect CO₂ to fall if compensating), etc.
- Remember oxygenation (PaO₂/SaO₂) and the patient—ABG numbers live in a clinical context.
Want to check a result right now? Open the ABG interpretation calculator.
Worked examples (practice the pattern)
- pH 7.52 | PaCO₂ 30 mmHg | HCO₃⁻ 24
Alkalemia. Low PaCO₂ fits primary respiratory alkalosis; HCO₃⁻ near normal → likely acute process. (Check for hyperventilation, pain/anxiety, early sepsis, pregnancy, etc.) - pH 7.28 | PaCO₂ 60 mmHg | HCO₃⁻ 28
Acidemia. High PaCO₂ points to respiratory acidosis with ↑ HCO₃⁻ compensation. Look for hypoventilation/CO₂ retention and treat the cause (airway, ventilation). - pH 7.40 | PaCO₂ 50 mmHg | HCO₃⁻ 30
pH is normal, but PaCO₂ ↑ and HCO₃⁻ ↑ suggest a mixed/compensated picture (respiratory acidosis with metabolic alkalosis or full compensation). Don’t be fooled by a “normal” pH—interpret the pattern.
FAQ
What are the normal ABG values again?
pH 7.35–7.45; PaCO₂ 35–45 mmHg (≈ 4.7–6.0 kPa); HCO₃⁻ 22–26 mEq/L; PaO₂ 80–100 mmHg; SaO₂ 95–100%.
How do I switch my ABG units between mmHg and kPa?
Multiply kPa × 7.50062 to get mmHg (and divide by that for kPa). Our calculator supports both.
Is HCO₃⁻ measured or calculated on an ABG?
Calculated from pH and PaCO₂ using Henderson–Hasselbalch (machines label it as “calculated”).
Any single “rule” to memorize?
R.O.M.E. helps: Respiratory Opposite, Metabolic Equal (direction of pH vs PaCO₂/HCO₃⁻). It’s a shortcut—always verify with full interpretation.
Where can I see a full, step-wise guide?
LITFL’s acid–base pages and StatPearls’ ABG chapter walk through examples and pitfalls.
Practice makes permanent—then use the tool
Work a few examples daily, then confirm your reading with the ABG calculator. It summarizes your pH/PaCO₂/HCO₃⁻ pattern in plain English and supports US & SI units—ideal for ABG interpretation practice, nursing learners, and clinicians who just want a reliable double-check.
Sources
- Normal ranges / stepwise interpretation: NCBI Bookshelf tables; ATS quick guide; LITFL ABG series.
- What analyzers measure/calculate: LITFL and Clinical Methods (HCO₃⁻ via Henderson–Hasselbalch).
- Henderson–Hasselbalch overview: StatPearls PCO₂; peer-reviewed open-access discussion and classic teaching PDFs.
- Reddit threads quoted/paraphrased: r/StudentNurse (ABG confusion/practice), r/nursing (ELI5, R.O.M.E.), r/medicine & r/Residency (primary vs compensation, resources), r/IntensiveCare & r/respiratorytherapy (edge cases and teaching tips).