C-reactive protein (CRP) is a substance in the blood that acts as a general indicator of inflammation within the body. Polymyalgia rheumatica (PMR) is an inflammatory condition that causes pain and stiffness, predominantly in the shoulders, neck, and hips. Because PMR involves inflammation, measuring CRP levels is a common practice in its assessment and management.
Understanding CRP and PMR
C-reactive protein is an acute-phase protein produced by the liver, with its levels increasing rapidly in the bloodstream during inflammation or infection. It is a non-specific marker, indicating inflammation but not its specific cause or location. CRP concentrations rise due to signaling molecules released by immune cells in response to inflammation.
Polymyalgia rheumatica is a systemic inflammatory disease that primarily affects adults over 50 years old, causing pain and stiffness in the shoulder and hip regions. The pain and stiffness associated with PMR are often most pronounced in the morning and can limit daily activities. Inflammation is a core characteristic of PMR, involving the activation of immune cells in the affected joints and surrounding tissues.
Typical CRP Levels in PMR
Individuals with active Polymyalgia Rheumatica typically have significantly elevated C-reactive protein levels. While a “normal” CRP level is generally considered below 3.0 mg/L, or often below 5 mg/L, in active PMR, levels are usually much higher. Most patients with PMR present with CRP levels greater than 10 mg/L at diagnosis, and often these levels can exceed 30-40 mg/L. Approximately 90% of individuals with newly diagnosed PMR show CRP levels above 10 mg/L.
While elevated CRP is a common finding, a small percentage of PMR patients, around 10-20%, may have normal inflammatory markers despite experiencing typical symptoms. Another inflammatory marker often measured alongside CRP in PMR is the Erythrocyte Sedimentation Rate (ESR), which is also typically elevated. CRP is considered a more sensitive indicator of disease activity than ESR in PMR because it responds more quickly to changes in inflammation and is less influenced by factors like age or gender.
Interpreting CRP Levels in PMR
CRP levels play a supportive role in the diagnosis of Polymyalgia Rheumatica, but are not diagnostic alone. Elevated CRP, especially above 10 mg/L, combined with typical symptoms such as bilateral shoulder and hip pain and morning stiffness in an adult over 50, strongly suggests PMR. However, since elevated CRP can stem from many causes, medical professionals consider it alongside a patient’s symptoms, physical examination, and other blood tests to rule out similar conditions. For instance, while high CRP supports a PMR diagnosis, specific tests are also performed to exclude conditions like rheumatoid arthritis, which can present with similar symptoms.
Monitoring CRP levels is also important for assessing disease activity and treatment effectiveness in PMR. A decrease in CRP levels often indicates that treatment, typically with corticosteroids, is working. Conversely, a rise in CRP might signal a disease flare-up, inadequate treatment, or a potential relapse. Healthcare providers typically measure CRP at diagnosis, then regularly during the initial treatment phase, and periodically during maintenance and tapering of medication. A rapid normalization of CRP, often within one to two weeks of starting glucocorticoid therapy, is a positive sign of treatment response.
Factors Influencing CRP Beyond PMR
Elevated C-reactive protein levels are not exclusive to Polymyalgia Rheumatica and can be caused by various other conditions. Infections, whether bacterial, viral, or fungal, are common causes of a significant rise in CRP. Bacterial infections, in particular, can lead to very high CRP levels. Other inflammatory diseases, such as rheumatoid arthritis, lupus, and inflammatory bowel disease, also commonly result in elevated CRP.
Trauma, surgery, and certain cancers can also cause an increase in CRP. Even factors like pregnancy, hormone replacement therapy, smoking, and obesity can influence CRP levels. Therefore, an elevated CRP test result always requires careful interpretation by a medical professional to determine the underlying cause.