La CRP et la SAA
What is inflammation?
SAA
SAA blood test
Quand faut-il doser la CRP et la SAA?
En pratique
What is inflammation?
Inflammation is the body's response to danger, infection, irritation or aggression. It is a complex biological process designed to protect the body from external agents, repair damaged tissue and promote healing. When a part of the body is attacked by various agents such as bacteria, viruses, trauma, burns or chemical irritants, the immune system reacts by triggering an “inflammatory reaction”.
This inflammatory reaction is reflected in the blood by an increase in molecules produced mainly by the liver:
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C-Reactive Protein (CRP)
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fibrinogen
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Serum Amyloid A protein (SAA)
These molecules activate the production of proteins known as cytokines by white blood cells, mainly monocytes, macrophages and neutrophils. These cytokines can cause symptoms such as fever, fatigue and pain in muscles, stomach, head or joints, as well as skin rashes.
La CRP
C-Reactive Protein, abbreviated “CRP”, is produced in response to a danger detected in the body. It is one of the so-called “acute phase” proteins of inflammation. Its concentration in the blood rises rapidly (within a few hours) in response to danger: it is therefore used to assess inflammation in the body. An elevated CRP is a sign of inflammation, without prejudging the cause.
It can be measured by a simple blood test, and results are available within a few hours. This test can be performed by any city laboratory, any day of the week.
It is not necessary to fast to measure CRP in the blood. It usually returns to normal after 7 days, once the danger signal that triggered its production has disappeared.
CRP blood test
It can be used to determine whether inflammation is present either permanently (chronic), or during flare-ups (attacks) with clinical signs such as fever, pain or rash. The advantage of this assay is that it can be performed in any laboratory, and results are available within a few hours. In autoinflammatory diseases, CRP is measured to :
1/ confirm the inflammatory nature of an attack or disease
2/ check that treatment dosage is appropriate. In fact, CRP should be negative outside a relapse if the treatment is well adapted
3/ detect a relapse of the disease
SAA
Serum Amyloid A protein (SAA) is a protein of the acute phase of inflammation. Elevated SAA is a sign of inflammation, without prejudging the cause.
It can be measured by a simple blood test. Its increase in the blood is rapid (within a few hours) in the event of inflammation. What sets it apart from CRP is that it can lead to inflammatory amyloidosis (see below).
Results are available after several days, as they are sent to specialized laboratories. It is therefore more expensive than CRP. It is not necessary to fast to measure SAA in the blood. It usually returns to normal after 7 days, once the danger signal that triggered its production has disappeared.
SAA blood test
When SAA is in excess in the blood for a long time, it can misfold, leading to the formation of AA amyloidosis fibrils and plaques. SAA is therefore responsible for inflammatory amyloidosis (AA), which can lead to renal failure.
In autoinflammatory diseases, SAA is measured to :
1/ look for chronic inflammation in the presence of renal amyloidosis
2/ check that treatment of chronic inflammatory disease is sufficient/effective to normalize SAA levels and limit the formation of amyloid plaques
When should CRP and SAA be measured?
It is advisable to measure CRP twice a year, outside an inflammatory flare-up (acute attack) of the disease, to check that it is normal, otherwise treatment will have to be increased, even in the absence of symptoms experienced by the patient.
It is not compulsory to measure SAA in the city. It can be measured once at the same time as CRP, outside an attack, to check that CRP and SAA are evolving in the same way.
If inflammatory amyloidosis (AA) is diagnosed, SAA should be measured at the time of diagnosis and when treatment is initiated. Subsequently, its measurement will enable us to monitor the efficacy of treatment, by watching for its decrease and normalization.
In practice
CRP is easily measured in all laboratories, and results are rapid. The normal level is less than 5 mg/L.
SAA cannot be measured in all laboratories, and results are not available for several days. Normal levels are below 10 mg/L.