A 58-year-old man with diabetes and hypertension presents with claudication pain in the left calf after walking 200 metres. On examination, the left dorsalis pedis and posterior tibial pulses are diminished. What is the investigation of choice to confirm the diagnosis and assess the severity of peripheral arterial disease?

## Investigation of Choice for Peripheral Arterial Disease Confirmation and Severity Assessment **Key Point:** The Ankle-Brachial Index (ABI) is the **first-line investigation of choice** to confirm the diagnosis of peripheral arterial disease (PAD) and assess its severity. It is non-invasive, inexpensive, reproducible, and provides both diagnostic and prognostic information. ### Why Ankle-Brachial Index (ABI)? The ABI is calculated as: > **ABI = Ankle systolic pressure (higher of DP or PT) ÷ Brachial systolic pressure (higher of both arms)** | ABI Value | Interpretation | |---|---| | **> 1.3** | Non-compressible vessels (calcified, e.g., diabetes) | | **0.91 – 1.30** | Normal | | **0.71 – 0.90** | Mild PAD | | **0.41 – 0.70** | Moderate PAD (claudication) | | **≤ 0.40** | Severe PAD / critical limb ischaemia | In this patient with claudication at 200 metres and diminished pedal pulses, an ABI of 0.41–0.70 would be expected, **confirming the diagnosis and quantifying severity** — exactly what the stem asks for. ### Role of Other Investigations | Investigation | Role | Limitation | |---|---|---| | **ABI** | ✅ First-line: confirms diagnosis, grades severity | Poor in calcified vessels (ABI >1.3 falsely elevated) | | **Duplex Ultrasonography** | Second-line: anatomic localisation of stenosis | Operator-dependent; used after ABI for pre-intervention planning | | **CT Angiography** | Pre-intervention anatomic roadmap | Radiation, contrast nephropathy risk | | **MR Angiography** | Alternative to CTA; no radiation | Expensive; contraindicated with metallic implants | ### Why Not Duplex Ultrasonography? Duplex ultrasonography is an excellent **imaging** modality for localising and characterising stenoses, but it is used **after** ABI has established the diagnosis and when anatomic detail is needed for intervention planning. It does not replace ABI as the initial confirmatory investigation. **Clinical Pearl (Harrison's Principles of Internal Medicine):** ABI ≤ 0.90 is diagnostic of PAD with sensitivity ~95% and specificity ~99%. It is the recommended first-line test by ACC/AHA guidelines for suspected PAD. **High-Yield:** The stem asks to "confirm the diagnosis AND assess severity" — ABI achieves both simultaneously with a single, non-invasive, bedside measurement. Duplex and angiography are reserved for anatomic localisation prior to revascularisation. **Mnemonic: ABI FIRST** — **A**nkle-**B**rachial **I**ndex is the **F**irst **I**nvestigation to **R**ule-in PAD, **S**tage severity, and **T**riage for imaging.