CRP- Infection vs. Inflammation

(Before continue reading it has to be stressed that this blog is only to be used as a reference. If the reader has any questions regarding this test for their health, they must consult a licensed physician. The reference range of <5mg/L is being used only as an example as different analyzers and populations will have different reference ranges. The Labvocate’s purpose is for education and starting a dialogue about issues affecting the medical laboratory)

What exactly is CRP, or as it is also known as C - reactive protein and what role does it have as a diagnostic tool?
To start, what’s with the name? The C that this protein reacts with is the C-polysaccharide cell wall of Streptococcus pneumoniae, the bacteria that causes pneumonia. In 1930, two researchers, Tillet and Francis, discovered that when serum from patients with pneumonia was added to the C-polysaccharide cell wall of Streptococcus pneumoniae, a precipitate formed. That led to the discovery of C-Reactive protein, a protein synthesized by the liver that has been found to be increased in both infections and inflammation. It works to help remove pathogenic bacteria and/or dead damaged cells.
That’s the good news. An elevated CRP in a patient is a strong indicator of a pathological process occurring in the patient. The bad news is that it won’t tell you what the problem is. It’s like the body is calling ‘911’, and just as the operator answers and says, ‘This is 911, what is the state of your emergency?’ the caller hangs up, and the operator has to send someone in to investigate.
The first step is to be aware of the difference between ‘inflammation’ and ‘infection’.
Inflammation is the body’s response to repairing dead tissue. It can be acute or chronic. Symptoms include redness, heat, pain and swelling. Infection is a pathogenic condition where the body has been invaded by an organism.
Let’s illustrate this by a patient coming in with a sore knee. Upon examination it is noted that the joint in question has some of the hallmarks of inflammation and infection. Besides being sore, it is also swollen and red.
One of the tests done is a CRP and as expected it is elevated, answering the question that yes, a pathogenic process is occurring, probable due to the patient’s sore knee. The question is the elevated CRP due to an inflammation, an infection or both?
A CBC would also be needed and careful attention paid to the WBC and differential, doe it indicates an acute infection? Does the patient have a fever? Or is the patient a jogger who is developing early the early stages of osteoarthritis?
How about throwing this monkey wrench into the mix, maybe the patient just has is a sprained knee and the CRP is actually elevated due an increased risk of coronary artery disease? So instead of answering questions, the CRP has made more.
So how can the CRP be used as a useful diagnostic tool?
First of all, compare the value to the reference range. The reference range is dependent on the analyzer, but for arguments sake we’ll use a reference range of up to 5 mg/L. Values less then the reference range mean that the level of inflammation is non pathogenic. Levels over 5 mg/L mean that there the level of inflammation is pathogenic. If the CRP has spiked two to four times greater than the upper level of the reference range, there is acute inflammation occurring.
Going back to our patient with the sore knee a CRP is ordered and has a result of 6 mg/L. Compared to 5mg/L as a reference point, it can be stated that the patient probable has a significant amount of inflammation probable due to a chronic condition.
But what if the result is higher, 20 mg/L? This is indicative of a more acute inflammation. Did the patient do more than sprain the knee? Are imaging studies (ie MRI for soft tissue damage) indicated? Is the WBC elevated accompanied by fever, indicating a possible septic joint infection?
One advantage of the CRP is that it does not require special collection like other tests used to investigate sepsis. Lactic acid is a useful marker for sepsis, but it requires specialized collection. CRP testing can be added to routine testing.
CRP has been found to be a good marker of neonatal sepsis. In one study (The Role of C-Reactive Protein in the Evaluation and Management of Infants With Suspected Sepsis,Joan M. Hengst, RN,MSN, Posted 04/16/2003; Adv Neonatal Care), an increase of CRP levels taken 24 hrs apart could be an indicator of neonatal sepsis.
So even if CRP can be a vague test, it is still valuable as an indication of inflammation. The challenge is to determine and treat the source of the inflammation.