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What is Paroxysmal Nocturnal Haemoglobinuria (PNH)?

The medical information provided on this website is by way of general guidance only. You should always contact your clinician for medical advice and do not rely upon the information provided here.

PNH is a very rare blood disease (also sometimes referred to as an ultra-orphan disease) where blood cells are vulnerable to be attacked by a particular part of the body’s immune system called "the complement”. The process whereby the red blood cells are destroyed, is called haemolysis and is responsible for many of the symptoms of the disease. Haemolytic PNH affects between approximately 1 and 9 people in every one million of the population. A slightly larger proportion of the population have PNH but with few symptoms. PNH affects both men and women, all races and all ages. Most patients are diagnosed when they are in their 30’s or 40’s but PNH can develop at any age. PNH is an acquired disease, it cannot be inherited and it is not contagious.

What Causes PNH?

A lot of work is being done to find out why PNH happens. If you have PNH, your bone marrow may not be working normally. Some patients develop PNH after they have had treatment for a bone marrow disorder called aplastic anaemia (AA) which causes the bone marrow to produce fewer blood cells. You develop PNH when your bone marrow produces a gene called PIG-A. This genetic change (mutation) happens after birth which means you acquire PNH rather than inheriting it. The PIG-A gene causes your bone marrow to produce blood cells without a special protein known as the GPI anchor which protects normal blood cells against your own immune system. Without the GPI anchor, PNH blood cells are not protected against a series of complex reactions called “complement activation” which is part of your body’s normal immune response to help fight infections. It is this lack of protection which allows your immune system to destroy your PNH blood cells and results in haemolysis which is the main cause of the symptoms and complications related to PNH.

Not everyone who has PNH is affected by it in the same way. Some people may have no symptoms and others may have many as well as other complications. If you have a large PNH clone (50% or more) you are more likely to have symptoms and are at a greater risk of complications such as blood clots. The group of blood cells affected by the genetic defect that causes PNH is known as a PNH clone. The extent to which your blood cells are affected by PNH is often described in terms of a clone size. Your doctor can tell you how large your PNH clone is because there is a test that can be done to measure this. Generally, if you have more than 50% of PNH blood cells, this is referred to as a large clone, 10 to 50% of PNH blood cells is a moderate size clone and less than 10% is a small clone.

It is possible for PNH to disappear however this is not very common.


There is no cure yet for PNH, however there are treatments which can alleviate the symptoms.

What does the phrase “Paroxysmal Nocturnal Haemoglobinuria” mean?

The name given to this disease is actually the latin description of one of the symptoms (from which not all PNH patients suffer). “Paroxysmal” means intermittently. “Nocturnal” means at night and “Haemoglobinuria” means haemoglobin is present in the urine. Therefore, the name refers to haemoglobin (which is the pigment inside red blood cells) being present in urine at night. Patients will know that it is actually usually present in the morning. Due to the presence of the haemoglobin in the urine, it can vary from a red to a dark brown colour.

What triggers haemolysis?

If you have PNH, your red blood cells are always being destroyed (chronic haemolysis). Sometimes a large number of red blood cells can suddenly be destroyed (acute haemolysis). Haemolysis tends to increase if you get an infection and in women, if you have a period or during pregnancy. Some patients have increased haemolysis after air travel.

During haemolysis, you are encouraged to drink more fluids to help your kidneys flush out the excess haemoglobin and iron which your body is producing. If an episode of haemolysis continues for more than 48 hours or you have severe abdominal pains or pain in your loin/back, you should seek urgent medical advice. 

What should I do if something else happens to my health apart from PNH?

Should a PNH patient need dental treatment or treatment for another, perhaps unrelated, medical problem (including cosmetic surgery) they should ensure they tell their medical team in case these treatments may impact on the PNH.

PNH and Pregnancy see separate section on Pregnancy and Fertility.

Information for Medical Professionals

Please see the information for medical professionals on the PNH National Service website here:




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