11.3 CO-oximetry / Hemoximetry Procedures

Hemoximetry versus “the sat on the ABG”

CO-oximetry (also called hemoximetry) is a multiwavelength spectrophotometric measurement of hemoglobin species in a blood sample. On the NBRC PFT Detailed Content Outline it appears as a distinct procedure cluster: select co-oximetry (II.A.7), perform it (II.B.7), and evaluate validity (II.C.7). Form construction often limits pure co-ox analysis items (commonly discussed as a maximum of about 2 on a form), but RPFT candidates still need operational fluency because dyshemoglobin errors are classic clinical traps.

Do not confuse three different “saturations”:

Fractional versus functional saturation language appears on some reports: know that FO2Hb is a fraction of total hemoglobin species, while functional saturation excludes dyshemoglobins from the denominator—read the report legend rather than assuming labels are interchangeable.

Multiwavelength measurement: what is reported

A laboratory CO-oximeter passes light of multiple wavelengths through a hemolyzed or optically prepared blood sample and solves for concentrations of major hemoglobin species. Exam-level parameters:

Because each species has a characteristic absorption spectrum, multiwavelength systems can separate O2Hb from COHb—something two-wavelength pulse oximeters cannot do. That is why smoke-inhalation assessment requires blood CO-oximetry, not SpO2 reassurance alone.

Many blood gas platforms include a co-ox module in the same cartridge or sample path; others are standalone. Either way, QC and calibration for the optical module must be in control (Domain I knowledge from Chapter 3) before patient results are released.

When CO-oximetry is required (selection — II.A.7)

Select hemoximetry when the clinical question involves hemoglobin species, not merely dissolved oxygen tension:

1. Smoke inhalation / known or suspected carbon monoxide exposure — headache, cherry-red skin is unreliable; measure FCOHb.
2. Unexplained low SpO2 with ABG PO2 that is not correspondingly low (or clinical suspicion of dyshemoglobin) — consider MetHb or COHb.
3. Therapy monitoring — e.g., following COHb decline after oxygen therapy; monitoring methemoglobinemia treatment when ordered.
4. Discrepancy workups — SpO2 versus calculated SaO2 versus clinical cyanosis that does not fit PO2.
5. Orders specifying co-ox panel — do not substitute “ABG only” when the order or indication needs fractions.

Selection trap: Running only PO2/calculated sat in a fire victim and reporting “oxygenation adequate” because PO2 is normal. PO2 can be normal while FO2Hb is reduced by COHb occupying heme sites.

Performing the procedure (II.B.7)

Pre-analytical sample for hemoximetry

• Prefer arterial samples when simultaneous blood gases are needed; venous co-ox may be acceptable for COHb/MetHb screening in some protocols because those dyshemoglobins are not primarily diffusion-limited the way PO2 is—but follow lab policy and the clinical order.
• Use the same anaerobic, heparinized, adequately filled, promptly analyzed standards as ABG when gases are also reported.
• Mix thoroughly immediately before analysis; co-ox measures Hb species in whole blood—settling causes error.
• Ensure volume meets co-ox minimum (often higher than a minimal pH/gas-only aspiration).
• Avoid prolonged tourniquet and traumatic draws that hemolyze when possible.

Instrument steps (exam-level)

1. Confirm co-ox module ready (wavelength checks/QC per manufacturer).
2. Identify sample type in software if prompted.
3. Introduce well-mixed specimen without bubbles/clots.
4. Review fraction sum approximately totaling ~100% (device-dependent presentation) and flag messages.
5. Report FO2Hb, FCOHb, FMetHb, FHHb, tHb as ordered; attach clinical context (e.g., smoke exposure).

Smokers may have mildly elevated baseline COHb (often a few percent); toxic exposures are much higher—know that interpretation cutoffs are clinical and lab-defined, while the technologist’s job is accurate measurement and validity.

Validity threats (II.C.7)

Validity vignette pattern: Normal SpO2, patient from house fire, ABG PO2 normal, no CO-ox performed → incomplete testing. Conversely, CO-ox with clot flag and wild tHb → do not release; fix pre-analytics.

Integrating sampling, ABG, and co-ox on the exam

Think of one clinical story across Chapter 11:

1. 11.1 — Obtain an anaerobic arterial sample safely (Allen test, technique, heparin, complications).
2. 11.2 — Run pH, PCO2, PO2 on an in-control 37 °C analyzer; treat HCO3/BE/calculated sat as calculated; verify criticals.
3. 11.3 — Add multiwavelength fractions when dyshemoglobins or total Hb species matter.

Domain II verbs stay constant: select → perform → validate. Instrumentation QC remains Domain I; clinical pattern interpretation expands in Domain III.

/practice/rpftPractice questions with detailed explanations