“Your blood is thick” - What should you know about high hemoglobin?

Most people are familiar with anemia and referral to a hematologist when they’re told they have anemia but did you know that high hemoglobin i.e erythrocytosis is also a very common reason for referral to a hematologist.

When a male is noted to have hemoglobin >16.5 g/dl or female has hemoglobin >16 g/dl persistently, they are labeled as having erythrocytosis or polycythemia. While these terms are often used interchangeably, I strongly prefer to reserve the term polycythemia for patients who have been diagnosed with PV or polycythemia vera to avoid confusion.

I’d like to focus this post on secondary erythrocytosis as it is wayyyy more common than primary polycythemia vera. Why would the body need to make more red blood cells or more hemoglobin? As mentioned in my previous post, the primary job of hemoglobin is to carry oxygen throughout the body. So if the body feels there is not enough oxygen, its going to signal to the bone marrow to make more red blood cells to carry the oxygen everywhere!

This feedback mechanism goes through the kidneys. Kidneys have oxygen sensing capabilities (yep. they don’t just make urine..) and if they sense that there is not enough oxygen, they start making more of the hormone called erythropoietin!

The above phenomenon leads to a condition called secondary erythrocytosis. Secondary, because it is in response to something and not a problem primarily with the bone marrow.

Common causes of secondary erythrocytosis

These are the common ones but there is a longer list here for further reference.

Of note, the erythropoietin level may or may not be elevated in these patients. According to this study, a high erythropoietin level (>15.1 mU/mL) was specific (98%) but had poor sensitivity (47%) for the diagnosis of secondary erythrocytosis.

Largely speaking, no. There is no definitive evidence that the thrombotic risk is elevated to the same degree as patients with primary polycythemia vera. The treatment is directed at the underlying cause - smoking cessation, use of CPAP for OSA, stopping exogenous testosterone use, using oxygen for hypoxic lung conditions.

However, if the hematocrit is persistently >55, some experts do suggest phlebotomy to reduce thrombotic risk.