Chart — Labs & Diagnostics
Dyshemoglobins Compared: Oxy-, Carboxy-, Met-, and Sulfhemoglobin
The four hemoglobin species a co-oximeter measures - oxyhemoglobin, carboxyhemoglobin, methemoglobin, and sulfhemoglobin - compared by how each one fools the pulse oximeter, the clue that gives it away on co-oximetry, and how it is treated.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
A pulse oximeter does not measure how much oxygen the blood is actually carrying — it estimates the fraction of hemoglobin that is saturated using only two wavelengths of light. Any abnormal hemoglobin species (a dyshemoglobin) that absorbs light at those same wavelengths can therefore drive the SpO₂ in the wrong direction while the patient is genuinely hypoxic. Co-oximetry, which reads four or more wavelengths, is the tool that separates the species apart and reports the true SaO₂.
The table below contrasts the four hemoglobin species a co-oximeter quantifies — oxyhemoglobin, carboxyhemoglobin, methemoglobin, and sulfhemoglobin — by what causes each one, what it does to the pulse oximeter, the bedside or co-oximetry clue that unmasks it, and the treatment.
The Four Hemoglobin Species Compared
| Hemoglobin species | Cause | Pulse-ox SpO₂ effect | Co-oximetry / clue | Treatment |
|---|---|---|---|---|
| Oxyhemoglobin (O₂Hb) | Normal oxygen-bound hemoglobin. | Accurate. | Reports the true SaO₂. | None. |
| Carboxyhemoglobin (COHb) | Carbon monoxide exposure - smoke inhalation, faulty heaters, vehicle exhaust. | Falsely normal or high. CO absorbs light like O₂Hb at 660/940 nm, so SpO₂ overreads; the PaO₂ is also normal because dissolved oxygen is unaffected. | COHb is measured directly: >3% in a nonsmoker, up to ~10-15% in a smoker, severe poisoning >25%. | 100% oxygen to shorten the CO half-life; hyperbaric oxygen for severe poisoning. |
| Methemoglobin (MetHb) | Oxidizing drugs - benzocaine and other topical/local anesthetics, dapsone, nitrates and nitrites. | Trends toward a fixed ~85% regardless of the true saturation, so it neither reliably falls nor rises. | MetHb is measured directly; blood is chocolate-brown; cyanosis appears at ~1.5 g/dL (~10-15%). | Methylene blue - but it is ineffective and risks hemolysis in G6PD deficiency, where ascorbic acid or exchange transfusion is used instead. |
| Sulfhemoglobin (SulfHb) | Certain sulfonamides and oxidizers; the change is irreversible. | Low or variable; may be misreported as methemoglobin. | Cyanosis is present; there is no specific antidote. | Supportive only; resolves only with red-cell turnover. |
Clinical Notes
A standard pulse oximeter uses only two wavelengths — 660 nm and 940 nm — and cannot distinguish dyshemoglobins from one another, so it reports only the functional saturation: the ratio of oxyhemoglobin to oxyhemoglobin plus deoxyhemoglobin, as if no abnormal species were present. A co-oximeter uses four or more wavelengths and reports the fractionalsaturation — the true SaO₂ measured against all hemoglobin species, including COHb, MetHb, and SulfHb.
Suspect a dyshemoglobinemia when the saturation gap — the pulse-ox SpO₂ minus the co-oximeter SaO₂ — exceeds about 5%, or when cyanosis fails to improve despite supplemental oxygen. Either finding is a prompt to send blood for co-oximetry rather than trusting the bedside SpO₂.
Related Resources
Sources
- American Association for Respiratory Care. AARC Clinical Practice Guideline: Blood Gas Analysis and Hemoximetry. Respir Care. 2013;58(10):1694-1703.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Hemoximetry and abnormal hemoglobins.