Methemoglobin (MHGB) Lab Values

Hey there. This is Abby from nursing.com. In this lesson, we’ll discuss methemoglobin. We’ll talk about its normal values and what in the clinical picture might cause an increase or decrease in this lab value. Let’s get started. 

Methemoglobin is a blood test to evaluate the percentage of methemoglobin within a blood sample. What is methemoglobin? Well it’s malformed hemoglobin. They both start with M. I think that might help us remember. Methemoglobin being malformed, is incapable of transporting oxygen. It’s formed when hemoglobin within the red blood cell oxidizes from exposure to free radicals. Now, if the red blood cell can’t carry oxygen, what type of presentation from our patient will we see? Let’s take a look. The condition where methemoglobin is high in the blood is called methemoglobinemia and the patient will present with signs of symptoms of hypoxemia. Like this individual in this picture, can you see the cyanosis? Patients will also be tachycardic and they might even have neurologic symptoms or pain like headache, dizziness, even anxiety. 

Of course, they’re gonna be breathless. Their cells, red blood cells are not carrying O2 out to the tissues. They’re definitely hypoxic. Now, they could even be fatigued and in truly terrible cases, they can go into a coma or suffer from seizures. Now, one of the craziest things about this is that the blood is actually brown. Some studies call it chocolate-colored, which is pretty strange. In normal values, the methemoglobin will be about 1 to 2%. That oxidative process within the red blood cell of the oxygen is actually common. It happens all the time. It’s just when it’s an elevated, uh, value that it’s a big issue. However, 10 to 15% can actually be well tolerated. This would be someone with chronic methemoglobinemia. Now, look at this picture. Did I tell you it was weird? Or did I tell you it was weird? That is chocolate brown blood. It almost looks like it’s old blood, right? But, it’s that malformation of hemoglobin. We collect to take this, uh, lab value or to test it in a heparinized syringe.

Lab values will be increased from either there being an acquired issue of oxidant or free radical, uh, exposure, or it could be congenital, which is a rare or recessive inheritance. Exposure can happen through inhalation, ingestion and even absorption. It can come from materials or chemicals like anesthetics, exhaust fumes, herbicides, and pesticides, and also nitrate contaminated water. We’ll see a decrease in the methemoglobin as we monitor for the effectiveness of treatment. So, it would be when there’s already a preexisting increase. Now, look at this, uh, picture here. We can see that the test is being compared chromatically for color against this chart. We know that when the value is increased above 70%, that equals death. It’s fatal, not good. 

Now some linchpins for this lesson. Methemoglobin is tested to evaluate the percentage of the methemoglobin that is in the blood. Remember, it’s a malformation of hemoglobin. 

Maybe we can remember that with our M’s. 1 to 2%, a small amount is normal. That oxidative process is constantly going on. That treating of electrons. An increased value would come from either a congenital or inherited disorder, or it can be acquired by exposure to chemicals or contaminated water like we talked about. A decreased value is going to be evaluated when treatment has been effective for the value that has already been increased, so hopefully we’re not having the malformation of those red blood cells and instead, our hemoglobin is properly structured and carrying oxygen around, and we no longer are cyanotic or breathless. That’s a good thing. 

Now, you all did great on this lesson and this wraps it up for methemoglobin. We love you guys, now go out and be your best self today and as always, happy nursing.

References:

• https://acutecaretesting.org/en/articles/methemoglobin
• https://www.uptodate.com/contents/approach-to-cyanosis-in-children?search=methemoglobin%20lab%20value&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1
• https://www.uptodate.com/contents/methemoglobinemia?search=methemoglobin%20lab%20value&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2
• https://www.uptodate.com/contents/arterial-blood-gases?search=methemoglobin%20lab%20value&source=search_result&selectedTitle=5~150&usage_type=default&display_rank=5
• https://acutecaretesting.org/en/articles/methemoglobin
• https://www.uptodate.com/contents/structure-and-function-of-normal-hemoglobins?sectionName=Oxygen%20affinity&search=methemoglobin%20lab%20value&topicRef=7094&anchor=H6&source=see_link#H6
• https://www.uptodate.com/contents/methemoglobinemia?topicRef=1648&source=related_link#H4166800317
• http://Methemoglobinemia: Causes, Diagnosis, and Treatment
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071541/#:~:text=Some%20drugs%2C%20such%20as%20dapsone,after%20successful%20methylene%20blue%20therapy
• https://www.ncbi.nlm.nih.gov/books/NBK537317