11.2 Performing Blood Gas Analysis

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:

1. 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.
2. Specimen identity: labels match order and patient; FiO2 or oxygen delivery mode and flow recorded; draw time known.
3. 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.
4. Volume: meet the analyzer’s minimum for the panel ordered (blood gas only versus blood gas + co-oximetry).
5. 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.

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:

1. Stop and verify before reflexive charting: sample integrity, patient ID, FiO2, analyzer flags, possible air/clot/heparin issues.
2. Repeat analysis on the same specimen if volume and policy allow and integrity looks intact.
3. Redraw if pre-analytical error is likely (air, delay, mislabel risk, clot).
4. Notify per critical-result policy (licensed provider, documented read-back).
5. 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

/practice/rpftPractice questions with detailed explanations