What are the likely diagnoses and initial work‑up/management for a 25‑year‑old non‑pregnant woman with acute right foot swelling, no trauma, following a respiratory illness a week ago?

Acute Unilateral Foot Swelling Following Respiratory Illness

Immediate Priority: Rule Out Deep Vein Thrombosis

In a 25-year-old woman with acute unilateral foot swelling and no trauma, deep vein thrombosis (DVT) must be excluded first, particularly given the recent respiratory illness which may have caused immobilization or a hypercoagulable state. 1

Initial Risk Stratification

• Calculate the Wells score immediately to determine DVT probability before proceeding with further testing 1

  • Recent immobilization from respiratory illness adds points
  • Unilateral leg swelling adds points
  • Absence of alternative diagnosis adds points

• If Wells score ≥2 ("DVT likely"): Proceed directly to complete duplex ultrasound from inguinal ligament to ankle without waiting for D-dimer 1

• If Wells score <2 ("DVT unlikely"): Obtain high-sensitivity D-dimer first; if negative, DVT is excluded and you can proceed with cellulitis evaluation 1

Critical Physical Examination Findings

Look specifically for:

• Pitting edema confined to the symptomatic leg (presence increases Wells score) 1
• Localized tenderness along deep venous distribution (suggests DVT) 1
• At least 2 inflammatory signs for cellulitis: erythema >0.5 cm, warmth, tenderness, pain, or induration 1
• Calf swelling ≥3 cm compared to the contralateral side (measured 10 cm below tibial tuberosity) 1

Secondary Differential: Cellulitis vs. Post-Viral Reactive Arthritis

Cellulitis Diagnosis

Diagnose cellulitis if at least 2 classic inflammatory signs are present: erythema, warmth, tenderness, pain, or induration 1

• Classify severity to guide treatment intensity: 1

  • Mild: erythema 0.5-2 cm, skin/subcutaneous tissue only, no systemic signs
  • Moderate: erythema >2 cm OR deeper tissue involvement, no systemic signs
  • Severe: any local infection PLUS systemic inflammatory response syndrome (SIRS)

• Initiate empirical oral antibiotics targeting Gram-positive organisms (cephalexin or dicloxacillin) for uncomplicated cellulitis 1

• Treatment duration is 5-10 days for uncomplicated cases 1

Post-Viral Reactive Arthritis Consideration

• Recent respiratory illness (particularly viral) can trigger reactive inflammatory arthritis affecting joints including the foot 2
• Respiratory viruses are the predominant detectable aetiological agents in most hospitalized adults with acute respiratory illness (44% detection rate), with picornaviruses most common 2
• This typically presents with joint swelling, warmth, and pain but without purulent discharge or rapidly progressive erythema that characterizes infection

Mandatory Imaging

Obtain three-view plain radiographs (AP, oblique, lateral) of the affected foot to identify fractures, dislocations, bone destruction, or foreign bodies, even without reported trauma 3

• Stress fractures can occur from normal activity and may not be recalled as "injury" by the patient 3
• Early pathology may show normal radiographs; maintain high clinical suspicion 3

Red Flags Requiring Urgent Intervention

Seek immediate surgical consultation if any of the following develop: 1

• Rapid progression of infection
• Crepitus or tissue gas
• Bullae, especially hemorrhagic
• Pain out of proportion to findings
• New neurologic deficits
• Failure to improve with appropriate therapy within 48-72 hours

Algorithmic Approach Summary

1. Calculate Wells score → If ≥2, order duplex ultrasound immediately 1
2. If Wells <2, order D-dimer → If positive, order duplex ultrasound 1
3. If DVT excluded, examine for inflammatory signs → If ≥2 signs present, diagnose cellulitis and start antibiotics 1
4. Obtain foot radiographs regardless of trauma history 3
5. If no infection or DVT, consider post-viral reactive arthritis as diagnosis of exclusion

Common Pitfall

Do not assume the absence of pitting edema rules out DVT—while its absence reduces the Wells score by one point, DVT can still be present 1. The complete duplex ultrasound from inguinal ligament to ankle is the definitive test and reliably excludes DVT when negative 1.