What is the differential diagnosis for a swollen left ring finger in a patient with a history of arthritis, impaired renal function (eGFR 46), and no trauma, presenting with 9 days of swelling?

Differential Diagnosis for Swollen Left Ring Finger

The most likely differential diagnoses for a swollen left ring finger without trauma in a patient with arthritis history and eGFR 46 include inflammatory arthritis flare (rheumatoid arthritis or psoriatic arthritis), crystal-induced arthropathy (gout or pseudogout), septic arthritis, flexor tenosynovitis, and dactylitis.

Primary Diagnostic Considerations

Inflammatory Arthritis

• Rheumatoid arthritis typically affects small joints of the hands symmetrically, with prolonged morning stiffness (>30-60 minutes), palpable synovitis, and improvement with activity 1
• The 2010 ACR/EULAR criteria assign 2 points for 1-3 small joint involvement, and a score ≥6/10 indicates definite RA classification 2, 1
• Given the patient's arthritis history, an RA flare affecting a single digit is possible, though monoarticular presentation is less typical 1
• Psoriatic arthritis characteristically presents with dactylitis ("sausage digit"), which can affect a single finger and may occur without skin manifestations 2

Crystal-Induced Arthropathy

• Gout can present as monoarticular swelling in the fingers, particularly in patients with renal impairment (eGFR 46) 2
• The reduced renal function significantly increases risk for hyperuricemia and gout, as impaired kidney function limits uric acid excretion 2
• Pseudogout (calcium pyrophosphate deposition disease) should also be considered, though it more commonly affects larger joints 2

Septic Arthritis

• Septic arthritis must be urgently excluded in any patient with acute monoarticular swelling, as delay in diagnosis leads to irreversible joint destruction 3, 4
• Fever may be absent in immunocompromised or elderly patients 3
• Risk factors include pre-existing joint disease (arthritis), which increases susceptibility to infection 2

Other Considerations

• Flexor tenosynovitis presents with fusiform swelling along the digit, pain with passive extension, and tenderness along the flexor tendon sheath 2
• Localized soft tissue infection (cellulitis) typically shows erythema, warmth, and spreading margins 2

Immediate Diagnostic Workup

Laboratory Testing

• ESR and CRP should be performed immediately, as significantly elevated markers indicate inflammatory arthritis or infection, while normal or mildly elevated markers suggest non-inflammatory causes 2, 1, 4
• RF and ACPA testing if RA is suspected, though negative tests do not exclude RA progression 2, 1
• Serum uric acid level to assess for gout, particularly given the renal impairment 2
• Complete blood count with differential and comprehensive metabolic panel 1

Arthrocentesis

• Arthrocentesis should be performed urgently if septic arthritis cannot be excluded clinically, with synovial fluid sent for cell count with differential, Gram stain, aerobic/anaerobic culture, and crystal analysis 4
• The Infectious Diseases Society of America recommends withholding antimicrobials for at least 2 weeks prior to aspiration if the patient is medically stable, to increase organism recovery 4
• Synovial fluid white blood cell count >50,000 cells/μL suggests septic arthritis, while presence of crystals confirms crystal-induced arthropathy 4

Imaging Studies

• Plain radiographs of the affected hand should be obtained to identify calcific deposits, exclude fractures, assess for erosive changes, and evaluate for soft tissue swelling 1, 4
• Ultrasound is superior to clinical examination for detecting synovitis and can identify tenosynovitis or fluid collections 1
• MRI is reserved for persistent symptoms unresponsive to treatment or diagnostic uncertainty 4

Critical Clinical Pitfalls

Renal Function Considerations

• The patient's eGFR of 46 mL/min/1.73m² (CKD stage 3a) requires careful medication selection, as NSAIDs should be avoided or used with extreme caution due to risk of further renal deterioration 2, 5
• Methotrexate and certain biologics may require dose adjustment in renal impairment 5
• Patients with RA and reduced kidney function have increased cardiovascular disease risk, particularly when eGFR <45 mL/min/1.73m² 6, 7

Arthritis-Related Renal Disease

• Renal impairment in RA patients may be due to the disease itself (glomerulonephritis, amyloidosis), medication toxicity (NSAIDs, DMARDs), or cardiovascular risk factors 5, 8, 9
• The presence of proteinuria or hematuria on urinalysis would suggest glomerular involvement requiring nephrology referral 8, 9
• Elevated ESR correlates with persistent proteinuria and worse renal outcomes in RA patients 9, 6

Red Flags Requiring Urgent Action

• Fever with joint pain requires urgent evaluation for septic arthritis until proven otherwise 3
• Progressive swelling, severe pain, or systemic symptoms warrant immediate assessment 3
• Constitutional symptoms such as weight loss or night sweats may indicate malignancy or systemic disease 3

Algorithmic Approach

1. Immediately assess for septic arthritis: Check vital signs, examine for warmth/erythema, and perform arthrocentesis if any concern for infection 4
2. Order inflammatory markers (ESR, CRP) and basic labs (CBC, CMP including creatinine, uric acid) 1, 4
3. Obtain plain radiographs of the hand to assess for structural changes 1, 4
4. If arthrocentesis performed: Analyze synovial fluid for cell count, crystals, Gram stain, and culture 4
5. Based on results:
   • If septic arthritis confirmed: Initiate IV antibiotics and orthopedic consultation
   • If crystals present: Treat crystal-induced arthropathy (avoiding NSAIDs given renal function)
   • If inflammatory markers elevated without infection: Consider inflammatory arthritis flare and rheumatology referral 1
6. Monitor renal function closely given baseline impairment and avoid nephrotoxic medications 2, 5