11.2 Performing Blood Gas Analysis
Key Takeaways
Analyze ABG samples only on a calibrated, QC-passed analyzer at controlled temperature near 37 °C after remixing the specimen and confirming no clots or residual air.
pH, PCO2, and PO2 are measured (electrode or equivalent sensor); HCO3−, base excess, and many SaO2 displays are calculated—do not treat calculated saturation as measured oxyhemoglobin fraction.
Critical or extreme values require technologist verification: check sample integrity, repeat analysis or redraw per policy, and escalate critical results through the lab’s reporting pathway.
Domain II tasks for analysis map to selecting the pathway (II.A.6), performing the run (II.B.6), and judging validity of the result set (II.C.6).
Document FiO2/device, patient temperature when required for correction policies, and time-to-analysis so clinical interpretation of PO2 and acid–base status is contextualized.
From syringe to report: the analysis procedure
Chapter 3 covered instrumentation (electrodes, warm-up, calibration, QC). Section 11.2 is the Domain II handoff: once a specimen arrives, the RPFT must select the correct analysis pathway (II.A.6), perform the run (II.B.6), and evaluate validity of outputs (II.C.6). Form construction may limit pure “blood gas analysis” scored items (often a small maximum per form), but the procedure still integrates with sampling validity and with clinical data management later in Domain III.
Pre-analytical acceptance at the analyzer
Before introducing blood:
- Instrument ready state: temperature stable near 37 °C, recent calibration valid, QC in control for the analytes you will report, no hard sensor or fluidic faults.
- Specimen identity: labels match order and patient; FiO2 or oxygen delivery mode and flow recorded; draw time known.
- Physical integrity: remix by gentle inversion; inspect for clots, layers, or air; if air remains, expel before aspiration into the analyzer if still within integrity rules—otherwise redraw.
- Volume: meet the analyzer’s minimum for the panel ordered (blood gas only versus blood gas + co-oximetry).
- Time/temperature history: if the sample sat warm beyond policy, or icing rules were violated for your syringe type, reject and redraw rather than reporting questionable gases.
Exam mindset: The technologist is the last gate before a wrong number reaches a chart.
Running the sample
Temperature control
Blood gas pH and gas partial pressures are temperature-dependent. Analyzers measure at a controlled 37 °C (or apply defined corrections). Introducing a cold sample into a warm measuring chamber is expected; the instrument brings the aliquot to measuring temperature. What must not happen is analyzing on a machine that has not reached thermal equilibrium after startup or service—readings will bias across pH, PCO2, and PO2.
Some labs optionally report temperature-corrected values when patient temperature is far from 37 °C (hypothermia/hyperthermia). Know your lab’s policy: many clinical decisions use 37 °C measured values with clinical context rather than corrected sets. If correction is used, patient temperature must be entered correctly.
Mixing and introduction
- Remix immediately before analysis so cells and plasma are uniform (important for co-ox and calculated indices).
- Follow the device workflow: syringe adapter versus capillary mode; select the correct sample type if the menu distinguishes arterial/venous/capillary.
- Avoid introducing foam or residual bubbles into the measuring path.
- After analysis, review flags (air detect, clot, short sample, drift) before releasing results.
Analyzer acceptance of the result
Modern systems may auto-repeat or flag outliers. The technologist still applies validity judgment:
- Internal consistency (e.g., extremely high PO2 on room air with normal clinical context → suspect air contamination or wrong FiO2 documentation).
- Comparison with recent results and SpO2 when available (not as a hard rule, but as a reasonableness check).
- Sensor/QC status for that channel.
Measured versus calculated parameters
Memorize this distinction—it is high-yield for RPFT and for avoiding over-interpretation of printouts.
| Parameter | Typically | Notes for the exam |
|---|---|---|
| pH | Measured | Glass electrode or equivalent; primary acid–base value |
| PCO2 | Measured | Severinghaus-type or equivalent CO2 sensor |
| PO2 | Measured | Clark polarographic or equivalent O2 sensor |
| HCO3− (bicarbonate) | Calculated | From pH and PCO2 via Henderson–Hasselbalch relationship |
| Base excess (BE) / base deficit | Calculated | Algorithm using pH, PCO2 (±Hb); reflects metabolic component |
| SaO2 / O2 sat on many ABG panels | Estimated/calculated | Often from PO2 and pH (oxyhemoglobin dissociation assumptions)—not the same as multiwavelength FO2Hb |
| FO2Hb, FCOHb, FMetHb, etc. | Measured on CO-oximeter | Requires hemoximetry module/device (Section 11.3) |
Critical teaching point: A “normal” calculated saturation can coexist with high carboxyhemoglobin because pulse oximeters and PO2-based estimates do not correctly quantify COHb. When dyshemoglobins matter, order CO-oximetry, not ABG alone.
Immediate critical value recognition
Laboratories define critical (panic) values—examples often include very low or high pH, extreme PCO2, life-threatening PO2, or unexpected extremes relative to age and support. Exact numeric cutoffs are lab-specific; the RPFT skill is the process:
- Stop and verify before reflexive charting: sample integrity, patient ID, FiO2, analyzer flags, possible air/clot/heparin issues.
- Repeat analysis on the same specimen if volume and policy allow and integrity looks intact.
- Redraw if pre-analytical error is likely (air, delay, mislabel risk, clot).
- Notify per critical-result policy (licensed provider, documented read-back).
- Do not “average away” an extreme with a normal without a documented reason—investigate first.
Extreme patterns that should trigger technologist suspicion:
- Room-air PO2 > ~120–150 mm Hg without supplemental O2 documentation → air contamination or wrong clinical data.
- PCO2 near 0 or impossibly low with normal clinical status → air, dilution, or sensor failure.
- pH incompatible with life without matching clinical emergency → instrument or sample error until proven otherwise.
- Sudden large change from a sample drawn minutes earlier without clinical event → pre-analytical or analytic fault.
Putting II.A.6 / II.B.6 / II.C.6 together
| Outline lens | Technologist behaviors |
|---|---|
| Select (II.A.6) | Choose ABG versus CBG pathway; decide whether co-ox is also required; select correct analyzer mode and panel |
| Perform (II.B.6) | Remix, introduce sample, run at 37 °C on in-control instrument, document context |
| Validity (II.C.6) | Accept/reject based on flags, consistency, QC status, critical-value verification, and redraw rules |
Related practice sets: /practice/rpft. Clinical interpretation of acid–base patterns is expanded in Domain III chapters; here the job is produce a defensible measured panel.
On a standard blood gas report without co-oximetry, which trio is measured directly by the blood gas sensors rather than calculated?
pH, PCO2, and PO2
HCO3−, base excess, and SaO2
FO2Hb, FCOHb, and FMetHb
pH, HCO3−, and total hemoglobin
An ABG on a patient documented as room air returns PO2 185 mm Hg, PCO2 22 mm Hg, and the technologist sees a small residual bubble that was not expelled. What is the best next action?
Report the results immediately because high PO2 is always beneficial
Suspect air contamination, verify sample handling, and repeat or redraw per policy before releasing
Enter FiO2 1.0 in the computer so the values look consistent
Correct PO2 downward by 50 mm Hg as a standard bubble adjustment
Why must a blood gas analyzer be at thermal equilibrium near 37 °C before patient samples are analyzed?
Only co-oximetry absorbance is temperature dependent; electrodes are not
Warm-up is cosmetic and required only for manufacturer warranty
Electrode equilibria and gas solubility are temperature dependent, so cold sensors bias pH, PCO2, and PO2
37 °C is required solely to kill pathogens in the sample path
A blood gas analyzer flags a critical low pH. The technologist’s first priority under Domain II validity practice is to:
Average the critical pH with the previous hour’s normal result and report the mean
Change QC limits so the value is no longer critical
Delete the result because critical values are never released from PFT labs
Verify sample integrity and analyzer status, repeat or redraw per policy, then follow critical notification procedures
Sections you finish are checked off in the contents.