What are the possible causes of acute left hip arthritis in a 55-year-old woman with a C-reactive protein of 112 mg/L, recent right-knee total knee replacement, and no fever?

Differential Diagnosis: Prosthetic Joint Infection Must Be Ruled Out First

In a 55-year-old woman with acute left hip arthritis, CRP 112 mg/L, recent right knee TKR, and no fever, prosthetic joint infection (PJI) of the contralateral knee is the most critical diagnosis to exclude first, followed by native septic arthritis of the left hip, crystal arthropathy, and less likely non-infectious inflammatory conditions.

Primary Diagnostic Considerations

1. Prosthetic Joint Infection of Right Knee (Most Critical)

The recent right knee TKR makes PJI your highest priority diagnosis, even though her symptoms are in the contralateral hip 1. Here's why this matters:

• CRP 112 mg/L is significantly elevated and falls into the intermediate range where both PJI and other serious pathology overlap 2
• While CRP normally remains elevated up to 14 days post-TKA (even reaching 40-47 mg/L) 3, a CRP of 112 mg/L "few months" after surgery is abnormal and concerning
• Absence of fever does NOT exclude PJI - many PJI cases present without fever, particularly in chronic/subacute presentations 1, 4
• The IDSA guidelines explicitly state to suspect PJI with "acute onset of a painful prosthesis" at any time after implantation 1

Critical action: Examine the right knee carefully for pain, effusion, warmth, or any wound issues. The hip pain could be referred pain or a separate process, but you cannot miss an evolving PJI.

2. Native Septic Arthritis of Left Hip (Second Priority)

This is your second critical diagnosis because:

• CRP 112 mg/L falls in the diagnostic gray zone (90-200 mg/L) where both septic arthritis and crystal arthropathy occur 2
• The study by 2 showed that all patients with CRP >200 mg/L had septic arthritis, but the 90-200 mg/L range contained a mix of both septic and crystal arthropathy
• Mean CRP in confirmed septic arthritis was 308 mg/L, but this doesn't exclude SA at lower values 2
• No fever is present in many SA cases, particularly in immunocompromised or elderly patients 4

Key point: No patients with CRP <90 mg/L had septic arthritis in the study 2, but at 112 mg/L, this patient is above that threshold.

3. Crystal Arthropathy (Gout/Pseudogout)

This remains a strong consideration:

• Mean CRP in crystal arthropathy was 100 mg/L 2, nearly identical to this patient's value
• Crystal arthropathy commonly affects the hip and can present acutely
• Post-surgical stress and inflammation can precipitate gout attacks

4. Hematogenous Seeding from Occult Source

Consider that bacteremia from any source could seed either the native hip joint or the prosthetic knee:

• Recent surgery increases risk of bacteremia
• The IDSA guidelines recommend blood cultures when there's acute symptom onset 1

Immediate Diagnostic Algorithm

Step 1: Assess Both Joints Clinically

• Right knee (TKR site): Check for pain, effusion, warmth, wound drainage, sinus tract
• Left hip: Assess range of motion, pain with movement, effusion if palpable

Step 2: Obtain Blood Cultures

Given acute presentation and elevated CRP, obtain aerobic and anaerobic blood cultures before any antibiotics 1

Step 3: Arthrocentesis of BOTH Joints

For the symptomatic left hip 1:

• Synovial fluid cell count with differential
• Gram stain and culture (aerobic/anaerobic)
• Crystal analysis
• Consider synovial CRP if available (cutoff ~3.0 mg/L for chronic hip infection, ~9.05 mg/L for acute <90 days) 5

For the right knee prosthesis 1:

• Same studies as above
• Critical: If patient is stable and not septic, ideally withhold antibiotics for 2 weeks before aspiration to maximize culture yield 1
• However, if clinically unstable or high suspicion, do not delay

Step 4: Imaging

• Plain radiographs of both hip and knee 1
• Advanced imaging (MRI, bone scan) is NOT routinely recommended for PJI diagnosis 1 but may help if native hip osteomyelitis is suspected

Step 5: Additional Labs

• ESR (combination of ESR + CRP provides best sensitivity/specificity for PJI) 1
• Complete metabolic panel, urinalysis (to exclude other infection sources that could cause bacteremia)

Critical Pitfalls to Avoid

1. Don't assume absence of fever excludes infection - many PJI and SA cases are afebrile 1, 4

2. Don't be falsely reassured by "normal" post-operative CRP elevation - while CRP can remain elevated 14 days post-TKA, values of 112 mg/L months after surgery are abnormal 3, 6

3. Don't start antibiotics before obtaining cultures unless the patient is septic - this dramatically reduces diagnostic yield 1

4. Don't ignore the prosthetic joint just because symptoms are elsewhere - hematogenous seeding can occur, and referred pain patterns exist

5. Don't rely on CRP alone - at 112 mg/L, you're in the overlap zone between multiple diagnoses 2. Synovial fluid analysis is mandatory.

Risk Factors Present in This Patient

• Recent joint surgery (increases PJI risk and bacteremia risk)
• Age 55 (moderate risk for both SA and crystal disease)
• Female sex (slightly higher risk for certain crystal arthropathies)
• Presence of prosthetic material (permanent infection risk)

The bottom line: Treat this as presumed infection until proven otherwise. Aspirate both joints, obtain blood cultures, and hold antibiotics until cultures are obtained unless the patient becomes systemically ill.