What is the most likely diagnosis for a 63-year-old man with a history of hypertension, type 2 diabetes, and obesity, who recently underwent knee arthroplasty and now presents with swelling, pain, and warmth in his left calf?

Deep Vein Thrombosis (DVT)

The most likely diagnosis is deep vein thrombosis (DVT), given the classic presentation of unilateral calf swelling, pain, warmth, and erythema in a high-risk patient who is 10 days post-knee arthroplasty with prolonged immobility. 1

Clinical Reasoning

This patient presents with the quintessential features of DVT in a prototypical high-risk scenario:

• Post-surgical state: 10 days after knee arthroplasty, which is a well-established risk factor for venous thromboembolism 2
• Prolonged immobility: A critical risk factor for thrombus formation 1
• Multiple comorbidities: Diabetes mellitus, hypertension, and obesity all contribute to increased thrombotic risk 3, 4
• Classic clinical signs: Unilateral calf swelling with increased circumference, pain worsening with ambulation, warmth, and erythema 1

The combination of these factors makes DVT the overwhelming diagnostic consideration.

Key Differential Considerations to Exclude

While DVT is most likely, several conditions must be considered and ruled out:

Cellulitis

• Requires at least two inflammatory signs (redness, warmth, swelling, tenderness) 5
• However, cellulitis typically presents with fever and more diffuse erythema with poorly demarcated borders
• The unilateral presentation and post-surgical timing strongly favor DVT over infection 1

Charcot Neuro-osteoarthropathy (CNO)

• Critical pitfall: In diabetic patients with neuropathy presenting with a unilateral red, warm, swollen foot, CNO must always be considered 2, 5
• However, CNO typically affects the foot and ankle rather than the calf 2
• The patient's pain that worsens with activity argues against CNO, as pain is typically absent or mild due to neuropathy 2, 5
• The 10-day post-operative timing and calf-specific symptoms make DVT far more likely 1

Ruptured Baker's Cyst

• Can mimic DVT with calf pain and swelling
• However, this diagnosis should not delay DVT evaluation given the high pretest probability

Immediate Management Algorithm

Initiate the following steps immediately while confirming the diagnosis: 1

1. Calculate clinical probability score (Wells score or DAYS score) to stratify risk 6

   • DVT as most likely diagnosis: +2 points
   • Calf swelling >3 cm compared to contralateral leg: +1 point
   • Recent surgery: +1 point
   • Immobilization: +1 point
   • This patient likely has a high probability score

2. Order D-dimer testing if clinical probability is low-to-moderate 6

   • However, given this patient's high pretest probability, proceed directly to imaging

3. Obtain compression ultrasonography of the affected leg immediately 1

   • This is the diagnostic test of choice for suspected DVT
   • Do not delay imaging in high-probability patients

4. If DVT is confirmed, initiate anticoagulation immediately with low-molecular-weight heparin or direct oral anticoagulant 1

   • Target INR 2.0-3.0 if using warfarin 7
   • Consider rivaroxaban 15 mg twice daily for 21 days, then 20 mg once daily 8

Critical Risk Factors in This Patient

Diabetes Mellitus

• Diabetes significantly increases DVT risk after total knee arthroplasty: The incidence of DVT within 14 days post-TKA is 74.3% in diabetic patients versus 50.7% in non-diabetic patients 3
• Diabetes confers a 2.71-fold increased risk of DVT after TKA (95% CI 1.183-6.212) 3

Metabolic Syndrome

• This patient has at least 3 components: diabetes, hypertension, and obesity 4
• Metabolic syndrome increases cardiovascular complications by 29-32% in patients undergoing total joint arthroplasty 4
• The clustering effect of multiple risk factors poses substantially increased risk 4

Post-Operative Period

• Orthopedic surgery, particularly knee arthroplasty, is an intermediate-to-high risk procedure for venous thromboembolism 2
• The 10-day post-operative timeframe is a classic window for DVT development 1

Duration of Anticoagulation

For DVT secondary to surgery (a transient risk factor), treat with anticoagulation for 3 months 7

• Maintain target INR 2.0-3.0 throughout treatment duration 7
• Reassess risk-benefit periodically, particularly given diabetes-related bleeding risks 3, 9

Common Pitfalls to Avoid

• Do not dismiss the diagnosis based on absence of fever: DVT does not typically cause fever unless complicated by septic thrombophlebitis 1
• Do not delay imaging while waiting for D-dimer results in high-probability patients: This patient's clinical presentation warrants immediate compression ultrasonography 1, 6
• Do not assume aspirin provides adequate DVT prophylaxis: While the patient is on aspirin, this is insufficient for post-operative DVT prevention in high-risk patients 2
• In diabetic patients, do not automatically attribute all unilateral leg swelling to CNO: The calf-predominant symptoms and post-surgical timing make DVT the primary concern 2, 5