From the Guidelines

Elevated C-reactive protein (CRP) is primarily caused by infection, injury, and inflammation.

Infection: Bacterial, viral, or fungal infections can cause an increase in CRP levels, as the body's immune system responds to the invading pathogens 1.
Injury: Trauma, surgery, or other forms of tissue damage can lead to elevated CRP levels, as the body's inflammatory response is triggered to repair and heal the damaged tissue 1.
Inflammation: Chronic inflammatory conditions, such as rheumatoid arthritis, inflammatory bowel disease, and fatty liver disease, can also cause elevated CRP levels 1.
Other factors: Demographic, behavioral, and technical factors, such as age, smoking, and laboratory measurement errors, can also influence CRP levels 1. It is essential to consider these factors when interpreting elevated CRP levels, as they can provide valuable information about the underlying cause of the inflammation. For example, a clinical index of the acute phase response or screening for infection/injury symptoms can help identify the suspected cause of very high CRP values 1. Additionally, liver function tests, such as AST/ALT, can help rule out fatty liver disease as a competing explanation for elevated CRP levels 1.

From the Research

Elevated CRP levels can be caused by various factors, including:

Infections, mainly bacterial, which account for 88% of episodes 2
Inflammatory conditions, such as appendicitis, pneumonia, pancreatitis, pelvic inflammatory disease (PID), and urinary tract infections 3
Trauma, inflammation, and infection, which can cause a rapid increase in CRP levels 4
Cancer, with elevated CRP levels associated with poor prognosis in patients with several types of solid cancers 5
Chronic inflammation, which can lead to elevated CRP levels and increase the risk of cancer 5

Some specific conditions that can cause elevated CRP levels include:

Meningitis, neonatal sepsis, and occult bacteremia, where CRP is usually elevated 3
Lung diseases, such as pneumonia, where CRP levels can be extremely elevated 2, 6
Cancer, particularly lung cancer, where elevated CRP levels are associated with increased risk and poor prognosis 5
Active malignancies, where mortality is high and CRP levels can be extremely elevated 2

CRP levels can be elevated due to various mechanisms, including:

The innate immune system, where CRP acts as a surveillance molecule for altered self and certain pathogens 4
Pattern recognition, where CRP recognizes altered self and foreign molecules based on pattern recognition 4
Activation of complement, binding to Fc receptors, and acting as an opsonin for various pathogens, which can lead to a proinflammatory signal and activation of the humoural, adaptive immune system 4