Should a Ventilation/Perfusion (V/Q) scan be ordered versus Computed Tomography Angiography (CTA) to rule out Pulmonary Embolism (PE) in a patient with impaired renal function, as indicated by elevated creatinine levels?

From the Guidelines

For patients with elevated creatinine, a V/Q scan is recommended over CTA to rule out PE due to the risk of contrast-induced nephropathy associated with CTA, as indicated in the NCCN Guidelines for Cancer-Associated Venous Thromboembolic Disease, Version 2.2024 1. When considering the diagnosis of pulmonary embolism (PE) in patients with elevated creatinine, it's crucial to weigh the risks and benefits of different imaging modalities. The primary concern with computed tomography angiography (CTA) is the use of intravenous contrast, which can exacerbate renal impairment.

Key Considerations

• The NCCN Guidelines highlight the advantages of CTA, including accurate imaging of mediastinal and parenchymal structures and the ability to detect signs of right ventricular enlargement 1.
• However, the guidelines also note the disadvantages of CTA, such as radiation exposure and the need for large amounts of intravenous contrast, which is particularly concerning in patients with renal insufficiency 1.
• Ventilation-perfusion (V/Q) scans are presented as an alternative when CTA is contraindicated, such as in cases of renal insufficiency or contrast allergy 1.
• A randomized, single-blind, noninferiority study mentioned in the guidelines found that V/Q scans identified significantly fewer PE than CTA, but there was no difference in symptomatic VTE within 3 months in patients where PE was considered excluded 1.

Clinical Decision Making

• The decision between V/Q scan and CTA should be based on the patient's renal function, with V/Q scans being preferred in cases of moderate to severe renal dysfunction.
• Clinical context, including the pre-test probability for PE and the presence of underlying lung disease, should also guide the choice of imaging modality.
• Alternative diagnostic approaches, such as lower extremity ultrasound or MR angiography, may be considered in specific cases, taking into account their own limitations in PE diagnosis.
• Ultimately, the choice of imaging should prioritize minimizing further kidney injury while accurately diagnosing or ruling out PE, with V/Q scans offering a safer alternative in terms of renal risk for patients with elevated creatinine 1.

From the Research

Considerations for Imaging Tests

When a patient has an elevated creatinine level, it is crucial to carefully consider the choice of imaging test to rule out pulmonary embolism (PE), weighing the risks and benefits of different diagnostic approaches.

• CTA vs V/Q Scan: The decision between ordering a Computed Tomography Angiography (CTA) and a Ventilation-Perfusion (V/Q) scan depends on several factors, including the patient's renal function, the clinical suspicion of PE, and the availability of imaging modalities.
• Risk of Contrast-Induced Nephropathy: Studies have shown that the use of contrast media in CTA can lead to contrast-induced nephropathy (CIN), particularly in patients with pre-existing renal impairment 2, 3. The incidence of CIN varies, but it is a significant concern in patients with elevated creatinine levels.

Evidence on CIN Risk

Research indicates that:

• The incidence of severe renal failure after CTA for PE is low, but the incidence of laboratory-defined contrast nephropathy is higher than expected 2.
• CIN is at least as common as the diagnosis of PE after CTA, and its development is associated with an increased risk of severe renal failure and death within 45 days 3.
• Using ultralow contrast medium doses at CT can be a feasible approach to diagnose PE in patients with moderate to severe renal impairment, reducing the risk of CIN 4.

Alternative Imaging Approaches

• V/Q Scan: A V/Q scan does not require the use of contrast media and may be a safer alternative for patients with significantly elevated creatinine levels or those at high risk of CIN.
• Considerations for V/Q Scan: However, the diagnostic accuracy of V/Q scans can be lower than that of CTA, and they may not be as readily available or widely used for diagnosing PE.

Clinical Decision-Making

The choice between CTA and V/Q scan should be based on individual patient factors, including the severity of renal impairment, the clinical likelihood of PE, and the potential risks and benefits of each imaging modality. Clinicians should consider discussing the risks of CIN with patients and weighing these against the need for accurate diagnosis of PE 3.