Can Blood be Blue?

Image Source: The New England Journal of Medicine ©

Would you believe if someone comes up with a claim that his blood turned blue?

Isn’t blood red? What makes it turn blue?

Here is the answer for all those wondering whether it is to be believed or not.
A 25-year-old woman from Rhode Island had a similar claim. She presented to the emergency room with complaints of malaise, fatigue, weakness, shortness of breath and bluish discolouration of the skin, for the past 1 day. She also gave a history of toothache the previous night for which she used topical benzocaine in excessive quantity to get her self relieved.

On physical examination, the patient looked cyanotic. Her respiratory rate was 22 breaths per minute and SaO2 (saturation of oxygen) was 88% on room air. Supplemental oxygen was given but it didn’t improve the saturation of oxygen on pulse oximetry.

The amazement came when the blood samples were withdrawn from both artery ( Figure B: smaller syringe) and vein (Figure B: Larger syringe). Results of ABGs (arterial blood gases) revealed PaO2 (partial pressure of arterial oxygen)of 120 mm Hg and SaO2 of 100%. Another result was obtained using CO-oximetry, revealing SaO2 of 67% and methemoglobin was 44%. A diagnosis of Methemoglobinemia was made.

Treatment with intravenous methylene blue was started which showed beneficial effects. Discolouration of the skin gradually resolved and her shortness of breath considerably improved. Her symptoms resolved completely. She was referred to a dental clinic for her toothache.

What is Methemoglobinemia?
Normally, haemoglobin contains iron in the ferrous form (Fe+). When ferrous of haemoglobin oxidizes to ferric form (Fe+++), its called methemoglobin, which is unable to reversibly bind oxygen, decreasing the oxygen availability for the tissues. In other words, it’s a useless oxygen carrier as it cannot deliver oxygen. Think of it as you ordered food, the rider picked it up from the restaurant, but the food never reached you. The rider roams around the streets with the food you need! Is it of any use?

When methemoglobin concentration in the red blood cells (RBCs) crosses the threshold of 1%, methemoglobinemia occurs. The clinical features are proportional to the amount of methemoglobin in the blood:

≤ 15%: Skin discolouration and changes in blood colour are seen
≥ 15%: Hypoxia ensues giving neurologic and cardiac features
≥ 70%: Considered fatal

What causes the rider to roam around but not deliver; What causes methemoglobinemia:
It can either be a congenital trait due to genetic defects or it can occur secondarily to toxins or drugs. The latter is the cause in the patient in the discussion here. The local anaesthetic she applied for her toothache is the culprit in her case. Overall, nitrites, nitrates, and chlorates are known causes. People drinking water contaminated with these or dialysis with these compounds can lead to methemoglobinemia. Other causative factors include:

  • Drugs such as analgesics, antipyretics, dapsone, local anaesthetic agents (lidocaine, benzocaine, prilocaine), silver sulfadiazine, antibiotics (sulfonamides, nitrofurans), antimalarials (chloroquine, primaquine, etc. ), antineoplastic agents (cyclophosphamide, flutamide, etc.), metoclopramide, etc.
  • Liver cirrhosis
  • Consumption of vegetables contaminated with methemoglobinemia causing bacteria
  • Fava bean ingestion and Methylene blue treatment if G6PD deficiency is present
  • Recreational drugs
  • Insecticides
  • Systemic acidosis

Longterm management revolves around avoidance of triggers. Ascorbic acid can help in reducing oxidative stress. In acute severe cases, since its an emergency, ABC protocol is to be followed along with an infusion Dextrose and methylene blue for drug-induced-methemoglobinemia.


Josef T Prchal, M. ( 2018, November 28). Clinical features, diagnosis, and treatment of methemoglobinemia. Retrieved from UpToDate:

Mary Denshaw-Burke, M. (2018, December 09). Methemoglobinemia. Retrieved from

Rehman H. U. (2001). Methemoglobinemia. The Western journal of medicine, 175(3), 193–196.

Ludlow JT, Wilkerson RG, Nappe TM. Methemoglobinemia. [Updated 2020 Jan 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from:

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Dr. Arsia Hanif has been a meritorious Healthcare professional with a proven track record throughout her academic life securing first position in her MCAT examination and then, in 2017, she successfully completed her Bachelors of Medicine and Surgery from Dow University of Health Sciences. She has had the opportunity to apply her theoretical knowledge to the real-life scenarios, as a House Officer (HO) serving at Civil Hospital. Whilst working at the Civil Hospital, she discovered that nothing satisfies her more than helping other humans in need and since then has made a commitment to implement her expertise in the field of medicine to cure the sick and regain the state of health and well-being. Being a Doctor is exactly what you’d think it’s like. She is the colleague at work that everyone wants to know but nobody wants to be. If you want to get something done, you approach her – everyone knows that! She is currently studying with Medical Council of Canada and aspires to be a leading Neurologist someday. Alongside, she has taken up medical writing to exercise her skills of delivering comprehensible version of the otherwise difficult medical literature. Her breaks comprise either of swimming, volunteering services at a Medical Camp or spending time with family.


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