**Question:** Investigation of choice for pulmonary embolism

**Core Concept:** Pulmonary embolism (PE) is a life-threatening condition characterized by the presence of a blood clot in one of the pulmonary arteries in the lungs. Diagnosis of PE relies on identifying the presence of clots and assessing their size, location, and extent.

**Why the Correct Answer is Right:**

The correct answer is D: **D-dimer test**. A D-dimer test is a blood test that helps in ruling out deep vein thrombosis (DVT) and PE. D-dimers are fragments of fibrin (a protein in blood responsible for clot formation and dissolution) that are elevated in the presence of active clot formation or clot breakdown.

D-dimer testing is a non-invasive, rapid, and cost-effective method for excluding PE when combined with clinical assessment. A negative D-dimer test result, especially when coupled with a low clinical probability score, significantly reduces the need for further testing and invasive procedures.

**Why Each Wrong Option is Incorrect:**

A. **CT pulmonary angiography (CTPA):** CTPA is a contrast-enhanced CT scan that visualizes the pulmonary vessels and is highly sensitive and specific for detecting PE. However, it is not suitable for ruling out PE, as it requires exposure to ionizing radiation, contrast media, and is resource-intensive, making it less suitable for a high-throughput diagnostic setting.

B. **Venography:** Venography is an invasive procedure that involves injecting a contrast agent into a vein to visualize blood flow within it. It is a rarely used test due to advancements in non-invasive imaging techniques like CTPA or MRV (magnetic resonance venography).

C. **Venous duplex ultrasound:** Duplex ultrasound is a non-invasive imaging technique used to evaluate deep vein thrombosis (DVT) and is commonly used in clinical practice. However, it is not a definitive test for PE diagnosis as PE can occur in the absence of DVT.

D-dimer testing is a non-invasive, rapid, and cost-effective method for excluding PE when combined with clinical assessment. A negative D-dimer test result, especially when coupled with a low clinical probability score, significantly reduces the need for further testing and invasive procedures.

**Clinical Pearl:** The Wells score or the Geneva score is a clinical probability assessment tool that can be used to determine the likelihood of PE based on patient symptoms and risk factors. Combining D-dimer testing with clinical probability scoring can aid in making a more accurate and cost-effective diagnosis of PE.