What is the differential diagnosis for a 69-year-old patient with an eight‑day fever, right hip pain, bilateral thigh pain, hemoglobin 11.4 g/dL, and C‑reactive protein 224 mg/L?

Differential Diagnosis

This 69-year-old patient with 8 days of fever, right hip pain, bilateral thigh pain, hemoglobin 11.4 g/dL, and markedly elevated CRP (224 mg/L) most likely has either septic arthritis/prosthetic joint infection (if a prosthesis is present), pyogenic spondylodiscitis, psoas abscess, or less likely polymyalgia rheumatica with giant cell arteritis—and requires urgent diagnostic arthrocentesis and blood cultures before initiating antibiotics.

Primary Diagnostic Considerations

1. Prosthetic Joint Infection (PJI) - If Hip Prosthesis Present

The combination of fever, hip pain, and markedly elevated CRP (224 mg/L) strongly suggests PJI if this patient has a hip prosthesis 1. The elevated CRP combined with ESR provides the best sensitivity and specificity for PJI diagnosis 1.

Immediate actions required:

• Obtain blood cultures for aerobic and anaerobic organisms given fever presence 1
• Perform diagnostic arthrocentesis with synovial fluid analysis including cell count with differential and cultures (aerobic/anaerobic) 1
• Withhold antimicrobials until cultures obtained unless patient is hemodynamically unstable 1
• Plain radiographs of the hip 1

2. Native Septic Arthritis - If No Prosthesis

Acute bacterial arthritis presents with fever, joint pain, and markedly elevated inflammatory markers. The 8-day duration and bilateral thigh pain suggest possible extension or multiple sites of involvement.

Key diagnostic steps:

• Urgent arthrocentesis before antibiotics 2
• Blood cultures (positive in 50% of cases) 1
• Staphylococcus aureus and Streptococcus species are most common with CRP >500 mg/L 3

3. Psoas Abscess

This diagnosis must be considered given the hip pain and fever combination 4. The bilateral thigh pain could represent referred pain or bilateral involvement.

Clinical clues to identify:

• Painful passive hip extension and internal rotation 4
• Painful hip flexion stress test 4
• May have gastrointestinal symptoms if secondary form (Crohn's, diverticulitis) 4

Diagnostic approach:

• Ultrasound as initial imaging 4
• CT scan if ultrasound inconclusive 4
• Blood cultures (Staphylococcus aureus most common in primary form) 4

4. Pyogenic Spondylodiscitis with Epidural Extension

Spinal infection can present with hip/thigh pain due to nerve root involvement or referred pain, along with fever and elevated inflammatory markers.

Diagnostic steps:

• MRI of lumbar spine (most sensitive)
• Blood cultures before antibiotics

5. Polymyalgia Rheumatica (PMR) with Superimposed Infection

The bilateral thigh pain and hip involvement in a 69-year-old could suggest PMR, but the 8-day fever and CRP of 224 mg/L is atypically high for uncomplicated PMR and suggests concurrent infection.

Critical Laboratory Interpretation

CRP 224 mg/L is highly significant:

• CRP >200 mg/L strongly suggests bacterial infection 3
• In one study, CRP ≥500 mg/L was associated with 27% 30-day mortality and predominantly bacterial infections (respiratory 63%, urinary 17%, abdominal 16%) 3
• Most common organisms: Streptococcus pneumoniae, Staphylococcus species, E. coli 3

Hemoglobin 11.4 g/dL:

• Mild anemia consistent with chronic inflammation or acute infection
• In post-arthroplasty patients, hemoglobin drop correlates with fever risk 5

Immediate Diagnostic Algorithm

1. Determine prosthesis status - Review surgical history
2. Obtain blood cultures (2 sets, aerobic/anaerobic) before antibiotics 1
3. Perform hip arthrocentesis unless surgery imminent and antibiotics can be safely withheld 1
4. Order ESR to complement CRP (combination has best diagnostic accuracy) 1, 6
5. Plain radiographs of hip and pelvis 1
6. CT abdomen/pelvis if psoas abscess suspected based on physical exam findings 4
7. MRI lumbar spine if neurological symptoms or spinal tenderness present

Critical Pitfalls to Avoid

• Do not start antibiotics before obtaining cultures in a stable patient—this significantly reduces organism recovery 1
• Do not rely on Gram stain alone to rule out infection (moderate evidence against this) 6
• Do not assume PMR without excluding infection given the fever duration and CRP magnitude
• Do not overlook intra-abdominal pathology (diverticulitis with retroperitoneal perforation can present with hip pain and fever) 7
• Consider immunosuppression status as 52% of patients with CRP ≥500 mg/L were immunocompromised 3

Less Likely but Important Considerations

• Deep vein thrombosis with pulmonary embolism: Can rarely cause fever, though typically with normal inflammatory markers 8
• Malignancy: Lymphoma or metastatic disease, but less likely with acute 8-day presentation
• Crystalline arthropathy: Gout or pseudogout can cause fever and elevated CRP, but bilateral thigh involvement is atypical

The clinical urgency here is high—with 8 days of fever and CRP 224 mg/L, this patient requires same-day evaluation with cultures and imaging to identify a source requiring source control (abscess drainage, joint washout) in addition to antimicrobials 2, 3.