Evaluating Oxygenation, Blood Gas Analyzers & QA/QC

Objective 1.3.1 — Distinguish hypoxemia from hypoxia and stage PaO₂ severity (with age adjustment); calculate CaO₂ and the A–a gradient using the alveolar air equation; describe the SANZ, Severinghaus, and Clark electrodes and lab procedures for ABG analysis; explain co-oximetry and the four common hemoglobin species; apply point-of-care testing (POCT / i-STAT); and follow regulatory, calibration, and quality-control protocols (CLIA · CAP · Levey-Jennings · Westgard rules).

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Evaluating Oxygenation

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Assess for Hypoxemia — PaO₂ Ranges

Normal PaO₂ ranges for patients less than 60 years old, breathing 21% O₂ at sea level.

Age Adjustment — Patients Over 60

Rule of thumb: for patients over 60, subtract additional years from 80.

Example: 66-year-old patient → 80 − 6 = 74 mmHg PaO₂ is the expected normal.

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Assess for Hypoxia

Hypoxia is evaluated using two main calculations:

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Arterial Oxygen Content (CaO₂)

The best measurement of oxygen delivered to the tissues — and the best index of oxygen transport. CaO₂ estimates O₂ carried by hemoglobin plus O₂ dissolved in plasma.

CaO₂ = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.003)

First term = O₂ in the RBC · Second term = O₂ in plasma

Normal value: 17 – 20 vol % (mL/dL)

Worked Example

Calculate the CaO₂ for a patient with: SaO₂ 98%, PaO₂ 90 torr, Hb 14 g/dL.

CaO₂ = (14 × 1.34 × 0.98) + (90 × 0.003)
     = (18.388) + (0.27)
     ≈ 18.66 vol % (mL/dL)

Within the normal range of 17–20 vol %.

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A–a Gradient (A–aDO₂, P(A–a)O₂)

The A–a gradient measures the difference (gradient) between alveolar and arterial PO₂.

Best done after the patient has been on 100% O₂ for 20 minutes or more.

A–aDO₂ = PAO₂ − PaO₂

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Alveolar Air Equation — PAO₂

Calculates the partial pressure of O₂ in the alveoli.

PAO₂ = (P_B − P_H₂O) × FiO₂ − (PaCO₂ ÷ 0.8)

Normal value varies directly with the patient's FiO₂ and barometric pressure (P_B).

Worked Example

Calculate the PAO₂ for a patient on 50% O₂ with PaCO₂ 40 torr and P_B 747 torr.

PAO₂ = (747 − 47) × 0.50 − (40 ÷ 0.8)
     = (700)(0.50) − 50
     = 350 − 50
     = 300 torr

A–a Gradient Worked Example

What would the A–a gradient be for the patient above if the PaO₂ was 80 torr?

A–aDO₂ = PAO₂ − PaO₂
       = 300 − 80
       = 220 torr   →  V/Q mismatch range

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Blood Gas Analyzers — Electrode Functional Operation

Electrode Terminology

The Three ABG Electrodes

Mnemonic: "Some Senior Clinicians" — SANZ → pH, Severinghaus → PCO₂, Clark → PO₂.

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Laboratory Procedures for ABG Analysis

1. Obtain the sample.
2. Ensure all air bubbles are removed from the syringe.
3. Mix the sample thoroughly.
4. Introduce sample into the analyzer.
5. As the sample runs through the analyzer, watch for air bubbles or clots.
6. Print, record, and report results in accordance with local policy.
7. Check results for errors:
   • If inaccurate → run sample through another analyzer to compare results.
   • If another analyzer is not available → place sample on ice.
   • Run a control through the analyzer to check for accuracy.
8. Temperature correction — results CAN be corrected if patient temperature is other than 37 °C (98.6 °F).

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Co-oximeter

Spectrophotometry — How It Works

Light is transmitted at specific wavelengths through the blood and received by a photodetector on the opposite side.

Light Transmission Behavior

When light hits blood it can be:

• Absorbed (optical density)
• Pass through (inverse to light absorbed)
• Reflected (backscatter oximetry)

Lambert-Beer Law

The Lambert-Beer law determines hemoglobin saturation and concentration.

Each form of hemoglobin has its own unique absorption / transmission spectrum (isobestic points).

Isobestic points — used to determine hemoglobin concentration regardless of its saturation.

Co-Oximetry — 8 Wavelengths, Multiple Hemoglobin Species

Co-oximetry uses eight light wavelengths to differentiate between various hemoglobin species:

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SaO₂, Total Hemoglobin, and Functional vs. Fractional

SaO₂ is the percentage of hemoglobin bound with oxygen.

Total Hemoglobin (tHb, g/dL) is comprised of functional and fractional SaO₂:

Normal Total Hemoglobin Values

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Carboxyhemoglobin (HbCO) and Methemoglobin (MetHb)

Carboxyhemoglobin — HbCO

• Caused by smoke inhalation / CO exposure
• Normal range: 1.5 – 9.0 % of tHb

Methemoglobin — MetHb

• Normal: 1 % or less
• Methemoglobinemia = abnormal blood state where MetHb is > 1%

Treatment of methemoglobinemia: METHYLENE BLUE.

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Point-of-Care Testing (POCT)

Definition: Laboratory testing performed outside the central laboratory (at the patient's bedside).

Classified as "Moderately Complex" according to CLIA.

Benefits of POCT

• Shortened turnaround time
• More timely intervention
• Correlates results with patient status at the bedside
• Reduced errors related to handling, labeling, and reporting
• Small sample size — only 2–3 drops of blood

POCT Equipment — Handheld Analyzer

• Cartridge port
• Numerical keypad
• 50-patient sample storage
• IR transmitter
• Battery powered
• Printer
• Calibration card
• Cartridges

i-STAT Procedure

Obtaining the Sample

1. Collect blood specimen.
2. Syringe with anticoagulant → test within 5 minutes.
3. Capillary tubes → test within 3 minutes.
4. Gently mix sample in syringes or collection tubes.
5. Practice sample-handling precautions per organizational policy.

Analyzing the Specimen

1. Fill the cartridge via the sample well using slow, steady pressure — 2–3 drops.
2. Close the cartridge, press on the tab until it snaps.

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Quality Assurance (QA)

Quality assurance — a systematic process designed to:

• Monitor
• Document
• Regulate the accuracy and reliability of laboratory measurements

Regulatory and Accreditation Agencies

Preventive Maintenance

• Clean blood spills with appropriate cleaning solution to eliminate contamination hazard
• Replace waste bottles
• Check supply levels and replace if necessary:
  • Wash and cleaning solutions
  • Calibration buffer solutions
  • Calibration gas tank pressure (CO₂)

Always wear gloves and eye protection.

Non-Scheduled Maintenance — Replace as Required

• Electrodes
• Electrode solution
• Electrode membrane
• Pump tubing
• Clean dust filters — replace as needed

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Calibration

Calibration — equipment is adjusted/corrected to match the control standards. Performed on electrodes before analyzing samples to establish the accuracy of results.

Calibration standards = specific blood gas values used to set the machine to read linearly over the expected range of results.

Calibration Levels

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Quality Control (QC)

Quality Control — a test performed to determine the accuracy and precision of a device against a known standard.

Internal Quality Control (Analyzer Controls)

• Controls = samples with known values run to ensure the analyzer is operating correctly.
• Assures the reliability of blood-sample values.
• Values should be within the manufacturer's standard deviation.

Control Statistics

Control results are maintained in statistical databases and evaluated to detect changes in analyzer performance.

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Levey-Jennings Charts & Westgard Rules

Levey-Jennings Chart

A graphic representation of each control run on each electrode, plotted around the mean with ±1, ±2, ±3 standard-deviation lines.

Westgard Rules

Specific criteria and actions to be taken based on the results of quality control samples.

Random Error

• One measurement outside of 2 SD → monitor closely.

"Out of Control" — Perform Corrective Action

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External Quality Control (Proficiency Testing)

A system that compares the accuracy of results from a lab with results obtained from other labs.

• Identical blind (unknown) samples are sent from an outside agency (CAP) to participating labs.
• Samples are analyzed and returned to the sending agency.
• Results are compared to the known values and to results from other labs.

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College of American Pathologists (CAP)

Outside agency introduced in the late 1940s.

CAP's Purpose

• Perform impartial and fair inspections
• Assess the state of the art in all clinical laboratory practice
• Promote laboratory improvement through peer comparison
• Satisfy regulatory requirements of credentialing agencies
• Grant accreditation

CAP Accreditation Requirements

• Quality management program
• Laboratory safety plan
• Document control plan
• Competency assessment program
• Laboratory director oversight documentation
• Specifies requirements for a lab information system
• Standardizes the operation of all laboratories

CAP inspections are every TWO YEARS.

CAP External Quality Control

Systematically compares accuracy of results obtained from multiple laboratories:

• Identical blind samples sent from CAP.
• Consists of 5 – 10 control samples.
• Limits and standard deviations are only known to CAP.
• Samples analyzed and returned to CAP.
• Results compared to known values and to other labs.

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Quick Reference

Hypoxemia Stages (under 60 y/o, RA, sea level)

80–100 Normal · 60–79 Mild · 45–59 Moderate · <45 Severe

Age-Adjusted PaO₂

Over 60 → 80 minus (years over 60). A 70-year-old: 80 − 10 = 70 mmHg expected.

CaO₂ Formula

CaO₂ = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.003) — normal 17–20 vol %.

Alveolar Air Equation

PAO₂ = (P_B − 47) × FiO₂ − (PaCO₂ / 0.8).

A–a Gradient on 100% O₂

25–65 normal · 66–300 V/Q mismatch · >300 shunt.

Three Electrodes

SANZ → pH · Severinghaus → PCO₂ · Clark → PO₂.

Anode vs. Cathode

Anode = positive (Ag/AgCl) · Cathode = negative (platinum). Oxidation = LOSE electrons · Reduction = GAIN electrons (OIL RIG).

Hb Species (Co-oximetry)

Hb · HbO₂ · MetHb (Fe³⁺) · HbCO · SHb — differentiated by 8 wavelengths via the Lambert-Beer law.

Hb-Carrying Capacity

1.34 mL O₂ per gram of Hb.

Normal tHb

Male 13.5–18 · Female 12–16 · Newborn 14–24 g/dL.

HbCO and MetHb

HbCO normal 1.5–9.0% of tHb (smoke / CO). MetHb normal ≤1%; >1% = methemoglobinemia. Causes: iNO, Hurricane spray, nitroglycerin. Treatment: methylene blue.

POCT

Outside the central lab · CLIA "moderately complex" · 2–3 drops · syringe sample tested <5 min, capillary <3 min.

Calibration Cadence

Accuracy vs. Precision

Accuracy = closeness to true value · Precision = dispersion of repeated values.

Westgard Cheat Sheet

CAP At-A-Glance

Established late 1940s · grants accreditation · inspections every 2 years · external proficiency = 5–10 blind samples sent to labs.

Regulatory Alphabet Soup

CLIA · CAP · TJC · CLIP (military CLIA) · CMS · NCCLS.