What is the most appropriate initial investigation for a patient with Diabetes Mellitus (DM), Hypertension (HTN), and Peripheral Artery Disease (PAD) presenting with sudden onset leg pain, paresthesia, and a diminished pulse?

Most Appropriate Initial Investigation for Acute Limb Ischemia

Direct Answer

For this patient presenting with sudden onset leg pain, paresthesia, and diminished pulse—classic signs of acute limb ischemia—the most appropriate initial investigation is bedside ankle-brachial index (ABI) measurement with handheld Doppler, followed immediately by CT angiography if the patient is stable. 1

Clinical Reasoning and Algorithm

Step 1: Immediate Bedside Assessment (ABI/Doppler)

Start with ABI measurement and handheld Doppler at the bedside as your first diagnostic step. 1 This approach is recommended by the American College of Radiology because it:

• Provides immediate confirmation of arterial occlusion within minutes 1
• Establishes objective baseline measurements for severity assessment 1
• Can be performed rapidly without delaying definitive therapy 1
• Does not require contrast administration, making it particularly suitable for patients with DM who may have underlying chronic kidney disease 1
• Helps determine the Rutherford classification severity, guiding subsequent management decisions 1

Step 2: Assess Severity Using Clinical Findings

While performing the ABI, evaluate for the "6 Ps" to determine urgency: Pain, Pallor, Pulselessness, Poikilothermia (coldness), Paresthesias, and Paralysis. 1, 2

Critical decision point: If the patient has profound sensory loss AND paralysis (Rutherford Class III—irreversible ischemia), proceed directly to surgery without delay for imaging. 1 In this scenario, imaging would only delay limb-saving intervention.

Step 3: Proceed to CT Angiography

If the patient is stable (Rutherford Class I, IIa, or IIb), immediately obtain CT angiography of the entire lower extremity. 1, 3, 2 The American College of Radiology rates CTA as "usually appropriate" because it:

• Provides excellent anatomic detail of the entire arterial tree from aorta to pedal vessels 3, 2
• Identifies the exact level of occlusion and degree of atherosclerotic disease 2
• Allows rapid revascularization planning in a single study 3, 2
• Is widely available in emergency settings 2

Why NOT the Other Options Alone

Doppler Ultrasound Alone (Option C)

Duplex ultrasound is NOT recommended as the primary investigation for acute limb ischemia. 1 The American College of Radiology specifically notes critical limitations:

• Requires significant operator expertise that may not be immediately available 1
• Poor vessel accessibility in emergency situations 3
• Heavy calcification interference, particularly problematic in diabetic patients with PAD 1, 3
• Cannot provide the comprehensive anatomic detail needed for urgent revascularization planning 3

ABI Alone (Option B)

While ABI is the correct first step, it is insufficient as the only investigation. 3 The American College of Cardiology emphasizes that:

• ABI only confirms the presence of arterial occlusion but does not provide information on location or cause 3
• ABI is useful for screening chronic PAD, not for acute presentations requiring urgent revascularization 3
• You need anatomic imaging (CTA) to plan definitive treatment 3, 2

Practical Implementation

Your immediate action sequence should be:

1. Bedside ABI/handheld Doppler (takes 5-10 minutes) 1
2. Start IV unfractionated heparin immediately to prevent thrombus propagation 3, 2
3. Consult vascular surgery before obtaining imaging 3
4. Order CT angiography of entire lower extremity (aortoiliac through tibial-pedal vessels) 2
5. Proceed to urgent revascularization based on CTA findings 3, 2

Critical Caveat for This Patient

This patient has DM, which increases contrast-induced nephropathy risk. 3 However, the benefit of rapid diagnosis and limb salvage outweighs the risk of worsening kidney function in acute limb ischemia. 3 Modern reduced-contrast-dose techniques minimize nephrotoxicity risk. 1, 3

Time-Critical Nature

The principle of "time is tissue" applies—delays beyond 4-6 hours significantly increase the risk of permanent damage and limb loss. 3 This is why the sequential approach (ABI first, then immediate CTA) is superior to relying on Doppler ultrasound alone, which is time-consuming and operator-dependent in emergency settings. 1, 3

Answer: B (ABI) as the initial bedside test, but this must be immediately followed by A (CT Angiography) for definitive diagnosis and treatment planning. If forced to choose only one answer from the options given, choose C (Doppler) only if it refers to handheld Doppler for ABI measurement, not duplex ultrasound. However, the complete correct answer is ABI followed by CTA. 1, 3, 2