C-reactive protein Introduction (What it is)
C-reactive protein is a substance made mainly by the liver that rises in the blood when there is inflammation in the body.
It is measured with a simple blood test.
Clinicians use it as a general signal that the immune system is “active,” not as a diagnosis by itself.
In orthopedics, it is commonly used when evaluating possible infection or inflammatory causes of hip and joint symptoms.
Why C-reactive protein used (Purpose / benefits)
C-reactive protein helps clinicians detect and monitor inflammation when symptoms and physical exam findings are not specific. In the context of hip pain and joint care, many different problems can look similar early on—such as postoperative inflammation, autoimmune arthritis flares, crystal arthritis (like gout), or infection around a joint or implant. A C-reactive protein level can add objective information to the evaluation.
Key purposes include:
- Screening for “inflammatory activity.” A higher result suggests that something inflammatory may be happening somewhere in the body, which can guide the next step in workup.
- Supporting infection evaluation. When infection is on the differential diagnosis (for example, septic arthritis of the hip or infection after hip replacement), C-reactive protein is often used alongside other tests.
- Tracking change over time. Single results can be hard to interpret; repeated measurements (a “trend”) can help show whether inflammation is improving, worsening, or staying the same.
- Complementing imaging and procedures. It can provide context for findings on MRI, ultrasound, X-ray, or joint aspiration (sampling joint fluid).
Because C-reactive protein is non-specific, its main “benefit” is not pinpointing a single cause, but helping clinicians decide how urgently to evaluate, what to test next, and how to interpret multiple data points together.
Indications (When orthopedic clinicians use it)
Orthopedic, sports medicine, and rehabilitation clinicians may order C-reactive protein in scenarios such as:
- New or worsening hip pain with concern for infection (fever, systemic symptoms, severe pain, inability to bear weight, or concerning exam findings)
- Evaluation for septic arthritis (infection inside a joint), often along with joint aspiration and imaging
- Suspected osteomyelitis (bone infection) or deep soft-tissue infection
- Workup of possible periprosthetic joint infection (infection involving a hip implant) after total hip arthroplasty
- Monitoring response when treating an established inflammatory or infectious condition (trend over time)
- Distinguishing inflammatory arthritis patterns from primarily mechanical causes (used with history, exam, and other labs)
- Postoperative assessment when symptoms are out of proportion to expected recovery and clinicians need more objective data
Contraindications / when it’s NOT ideal
C-reactive protein is a blood test and is generally safe, but there are important situations where it may be less useful or where interpretation can be misleading:
- When a specific diagnosis is needed: C-reactive protein does not identify the exact source of inflammation; other tests are usually required.
- Immediately after surgery or major injury: C-reactive protein commonly rises as part of normal healing and tissue inflammation, so a single elevated result may not distinguish expected recovery from a complication. Interpretation depends on timing and trend.
- Chronic low-grade elevation: Some people have persistently elevated levels from non-orthopedic conditions (for example, chronic inflammatory disease). This can reduce specificity for joint-focused questions.
- Conditions that blunt inflammatory responses: Some medications or medical conditions can alter immune signaling; results may not rise as much as expected. Varies by clinician and case.
- When symptoms are clearly mechanical: For issues like isolated tendon overload or osteoarthritis flare without systemic features, C-reactive protein may add limited value; other assessments may be more informative.
- Situations where venipuncture is difficult: Blood draw challenges (poor veins, severe needle anxiety, bleeding disorders, or anticoagulant use) don’t usually prevent testing, but they can affect how and where it’s performed. Varies by clinician and case.
How it works (Mechanism / physiology)
C-reactive protein is an acute-phase reactant—a protein whose blood level changes in response to inflammation. When the immune system is activated (by infection, tissue injury, autoimmune disease, or other inflammatory triggers), signaling molecules called cytokines—especially interleukin-6 (IL-6)—stimulate the liver to produce more C-reactive protein. The level can rise relatively quickly and can also fall as the inflammatory stimulus resolves.
From an orthopedic perspective, the underlying inflammation may involve structures such as:
- Synovium: The lining of the hip joint that can become inflamed in inflammatory arthritis or infection.
- Articular cartilage and subchondral bone: Painful joint conditions can involve bone marrow inflammation or stress changes, even when cartilage wear is the main long-term issue.
- Tendons and bursae: Conditions like trochanteric bursitis and gluteal tendon irritation are often local; they do not always cause a marked systemic C-reactive protein rise.
- Bone and soft tissues after surgery or trauma: Healing tissues naturally generate inflammatory signals.
C-reactive protein does not act like a medication, implant, or injection, so concepts like “reversibility” or “duration of effect” apply differently. Instead, what matters is how the lab value changes over time—often used as a trend rather than a one-time answer.
C-reactive protein Procedure overview (How it’s applied)
C-reactive protein is not a procedure performed on the hip. It is a blood test used as part of clinical evaluation. A typical workflow looks like this:
- Evaluation/exam: A clinician reviews symptoms (pain location, severity, onset), checks for systemic signs (fever, chills), and performs a focused hip and neurologic exam.
- Preparation: If C-reactive protein is ordered, the patient typically has routine blood draw preparation. Fasting is not usually required for C-reactive protein alone, but testing panels vary by clinician and case.
- Intervention/testing: Blood is drawn from a vein (venipuncture) and sent to a laboratory (or, in some settings, tested with point-of-care equipment).
- Immediate checks: The care team reviews the result in context with other data—often including complete blood count (CBC), erythrocyte sedimentation rate (ESR), temperature, imaging, and sometimes joint aspiration.
- Follow-up: If needed, C-reactive protein may be repeated to assess the trend (rising, falling, or stable). Trend interpretation depends on the clinical question, timing, and concurrent events (like surgery).
This information is used to support clinical decision-making, not to provide a stand-alone diagnosis.
Types / variations
C-reactive protein testing is commonly discussed in a few variations:
- Standard (routine) C-reactive protein: Designed to detect broader, clinically significant inflammation. This is the form most often used in infection and postoperative or acute inflammatory evaluation.
- High-sensitivity C-reactive protein (hs-CRP): A more sensitive assay that can detect smaller increases. It is often discussed in cardiovascular risk contexts, but it may also appear in lab panels for other reasons. Clinicians choose based on the question being asked.
- Single measurement vs serial measurements (trending): Orthopedic clinicians often value repeat measurements when monitoring postoperative recovery concerns or suspected infection, because the direction and speed of change can be more informative than one value.
- Laboratory method differences: Different labs and analyzers can have different reference ranges, reporting units, and turnaround times. Interpretation should follow the lab’s reference interval and the clinician’s clinical context.
Pros and cons
Pros:
- Helps detect systemic inflammation when symptoms are non-specific
- Useful as a supporting test in evaluating possible joint or bone infection
- Can be repeated to assess a trend over time
- Widely available and relatively quick to obtain in many settings
- Interpretable alongside other orthopedic data (exam, imaging, aspiration results)
- May help differentiate “inflammatory pattern” presentations from primarily mechanical ones when used appropriately
Cons:
- Non-specific: does not identify the exact cause or location of inflammation
- Can rise from many non-orthopedic issues (respiratory infections, chronic inflammatory conditions), reducing specificity
- Can be elevated after surgery or injury as part of normal healing, complicating interpretation
- A normal result does not fully exclude localized problems; clinicians consider the whole picture. Varies by clinician and case.
- Results depend on timing relative to symptom onset and interventions
- Reference ranges and clinical thresholds can differ by lab and scenario; interpretation is context-dependent
Aftercare & longevity
Because C-reactive protein is a lab value rather than a treatment, “aftercare” mainly means understanding what influences results over time and how follow-up is typically handled.
Factors that can affect how C-reactive protein behaves include:
- Timing: Early testing may miss a rise if inflammation is just beginning; later testing may reflect evolving disease or postoperative physiology.
- Recent surgery or injections: Procedures can provoke inflammatory responses that temporarily elevate C-reactive protein.
- Infection vs non-infectious inflammation: Both can raise C-reactive protein; the degree and pattern can differ, but overlap is common.
- Comorbidities: Autoimmune disease, chronic infections, obesity, and other systemic conditions can influence baseline levels.
- Medications: Anti-inflammatory drugs or immune-modulating therapies can change inflammatory signaling and lab patterns. Varies by clinician and case.
- Follow-up plan: Clinicians may recheck C-reactive protein if symptoms change, if infection remains a concern, or to correlate with other findings (imaging changes, wound status, functional recovery).
In general terms, the “longevity” of a C-reactive protein result is short: it reflects the body’s inflammatory state around the time of testing. That is why trends and clinical context matter.
Alternatives / comparisons
C-reactive protein is one tool among many. Depending on the question, clinicians may compare or pair it with other approaches:
- Observation and monitoring: For mild symptoms without red flags, clinicians may prioritize history, exam, and functional monitoring rather than immediate lab testing. This depends on clinical context.
- ESR (erythrocyte sedimentation rate): Another inflammation marker. ESR often changes more slowly than C-reactive protein and may remain elevated longer. Many clinicians order ESR and C-reactive protein together for joint infection or inflammatory arthritis workups.
- CBC (white blood cell count): Can support infection evaluation, but it may be normal in some orthopedic infections or inflammatory states.
- Procalcitonin: Sometimes used to support bacterial infection assessment in broader medical contexts; its role in orthopedic questions varies by clinician and case.
- Imaging (X-ray, ultrasound, MRI, CT): Imaging evaluates anatomy—bone, cartilage, muscle, and fluid collections. C-reactive protein evaluates systemic inflammation; they answer different questions and are often complementary.
- Joint aspiration and synovial fluid analysis: For suspected septic arthritis or periprosthetic joint infection, sampling joint fluid can provide more specific data (cell counts, cultures, crystal analysis). C-reactive protein may support the decision to aspirate and helps interpret the overall likelihood of infection.
- Culture and biopsy (when indicated): If deep infection is suspected, identifying an organism or tissue pathology may be necessary. C-reactive protein cannot replace these tests.
No single test is definitive in every case; clinicians generally integrate symptoms, exam, labs, and imaging.
C-reactive protein Common questions (FAQ)
Q: What does C-reactive protein actually measure?
It measures the level of C-reactive protein circulating in the blood, which tends to increase when the body is experiencing inflammation. It does not directly measure pain, tissue damage, or a specific disease. It is best understood as a general signal that the immune system is activated.
Q: Why would C-reactive protein be ordered for hip pain?
Hip pain can come from mechanical causes (like osteoarthritis or tendon irritation) or from inflammatory/infectious causes (like inflammatory arthritis or septic arthritis). C-reactive protein can help clinicians decide whether a broader inflammatory process may be present. It is usually interpreted alongside the exam and, when needed, imaging or additional labs.
Q: Does an elevated C-reactive protein mean I have an infection?
Not necessarily. Infection can raise C-reactive protein, but so can surgery, autoimmune disease, recent injury, and many non-orthopedic illnesses. Clinicians look for supporting evidence such as fever, wound changes, imaging findings, and sometimes joint fluid results.
Q: Can C-reactive protein be normal even if something is wrong in the hip?
Yes. Some localized orthopedic problems do not trigger a strong systemic inflammatory response. Also, timing matters—testing very early in an inflammatory process may not show a rise yet. Clinicians use the result as one part of a larger assessment.
Q: How is the test performed, and does it hurt?
It is performed with a standard blood draw from a vein, typically in the arm. Discomfort is usually brief and related to the needle stick. Some people may have minor bruising afterward.
Q: Do I need to fast before a C-reactive protein test?
Fasting is not typically required for C-reactive protein alone. However, clinicians often order multiple labs at once, and some of those tests may have different preparation requirements. The exact instructions vary by clinician and case.
Q: How long do C-reactive protein results “last,” and will I need repeat testing?
A C-reactive protein value reflects inflammation around the time the blood sample is taken, and it can change over days as inflammation changes. Repeat testing may be used to evaluate a trend, especially when monitoring suspected infection or postoperative concerns. Whether repeat testing is helpful varies by clinician and case.
Q: Is C-reactive protein used after hip replacement surgery?
It can be. After surgery, C-reactive protein often rises as part of normal healing, so clinicians focus on timing and whether the level is trending as expected. If symptoms are concerning for complications, it may be used alongside other tests to assess for infection or other inflammatory issues.
Q: Can I drive, work, or bear weight normally after the test?
Because C-reactive protein testing is a blood draw, it typically does not restrict driving, work, or weight-bearing. Exceptions might relate to how you personally feel after venipuncture (for example, lightheadedness) or to the underlying hip condition being evaluated. Activity guidance is individualized and varies by clinician and case.
Q: What determines the cost of a C-reactive protein test?
Cost depends on the care setting (clinic vs hospital), whether it is bundled with other labs, insurance coverage, and the laboratory used. Additional costs may come from related testing (imaging, aspiration, cultures) that is ordered based on the clinical scenario. Pricing varies by clinician and case.