V/Q Scan Test: Purpose, Test Details & Results

Purpose of V/Q Scan Test

A ventilation/perfusion (V/Q) scan is a specialized nuclear medicine test that evaluates how well air and blood flow move through your lungs. This test is a cornerstone in diagnosing certain lung conditions, particularly pulmonary embolism (PE) and chronic thromboembolic pulmonary hypertension (CTEPH). Understanding its purpose helps you appreciate why your healthcare provider might recommend this test and what conditions it can help detect or rule out.

Table 1: Main Clinical Purposes of V/Q Scan

Diagnosing Pulmonary Embolism (PE)

Pulmonary embolism is a life-threatening condition caused by blood clots blocking lung arteries. The V/Q scan is a leading test for diagnosing PE, especially when other tests, like CT angiography, are not suitable (for example, in people with kidney problems or allergies to contrast dye) 1, 2. By comparing air flow (ventilation) and blood flow (perfusion) in the lungs, the test can spot areas where blood flow is blocked but air flow is normal—classic signs of PE.

Investigating Pulmonary Hypertension (CTEPH)

Chronic thromboembolic pulmonary hypertension (CTEPH) is a type of high blood pressure in the lungs caused by old, unresolved blood clots. The V/Q scan is especially valuable here. Research shows it is even more sensitive than some advanced CT scans for picking up CTEPH and can help identify patients eligible for potentially curative surgery 3, 4, 6.

Use in Patients with Underlying Disease

A common concern is whether the scan remains accurate in patients with other lung or heart problems. Studies confirm the V/Q scan maintains its diagnostic value, even in people with pre-existing lung or cardiac disease or those with chronic obstructive pulmonary disease (COPD) 5, 7. This makes it a flexible and safe choice in a wide range of clinical situations.

V/Q Scan Test Details

The V/Q scan is a straightforward, minimally invasive procedure performed in a hospital or specialized imaging center. Here, we’ll walk you through how the test works, what you can expect, and some technical details that make it so effective.

Table 2: V/Q Scan Process Overview

How the V/Q Scan Works

The V/Q scan actually involves two parts:

• Ventilation Scan: You’ll breathe in a small amount of a radioactive gas or mist, such as xenon or technetium-labeled aerosol. This part shows how well air moves throughout your lungs 1, 2, 8.
• Perfusion Scan: Next, a different radioactive tracer is injected into a vein. This tracer travels to the lungs, highlighting how well blood flows through the lung tissues 1, 2.

Both parts are painless and typically completed within an hour.

Imaging and Technology

A special camera called a gamma camera takes images as the tracers move through your lungs. These images are then compared, allowing doctors to spot areas where air is reaching but blood is not—suggesting a blockage 8, 9. Increasingly, V/Q scans are performed with advanced techniques like SPECT (single-photon emission computed tomography), which produces 3D images for even greater accuracy 8, 9.

Safety and Patient Suitability

• No Contrast Dye: Unlike CT angiography, the V/Q scan does not use contrast dye, making it safer for people with allergies or kidney issues 5, 7.
• Low Radiation: The amount of radiation used is minimal and considered safe for most patients, including those with other lung or heart diseases 5, 7.
• Noninvasive & Well-Tolerated: The procedure is noninvasive, involves no incisions, and is generally very well tolerated.

When Is the V/Q Scan Preferred?

• When CT Angiography Is Contraindicated: Kidney dysfunction, allergy to contrast
• Pregnancy: Lower radiation dose to the fetus compared to CT in some protocols
• Pre-existing Lung Disease: Remains accurate in COPD and other chronic conditions 5, 7

V/Q Scan Test Results & Follow-Up

The results of a V/Q scan provide crucial information about the likelihood of pulmonary embolism or chronic pulmonary hypertension. Understanding what these results mean—and what happens next—can help you and your healthcare team make informed decisions.

Table 3: V/Q Scan Result Categories and Clinical Actions

How Are Results Interpreted?

V/Q scan results are categorized into probability levels:

• Normal/Very Low Probability: Indicates no significant PE. In studies, a normal V/Q scan virtually excludes clinically significant PE 2, 8.
• Low Probability: PE unlikely, but not completely ruled out, especially if clinical suspicion is high 2, 7, 8.
• Intermediate Probability: The scan is indeterminate; further diagnostic testing, such as CT pulmonary angiography or conventional angiography, is often required 1, 2, 7.
• High Probability: Strongly suggests PE or CTEPH, particularly when clinical suspicion is also high. In these cases, treatment can often be started promptly 1, 2, 3, 6, 8.

Accuracy and Predictive Value

• Sensitivity & Specificity: High-probability scans have a sensitivity of around 77–97% and specificity up to 98% for PE 1, 8, 9. For CTEPH, V/Q scans are highly sensitive, often outperforming CT angiography 3, 4, 6.
• Negative Predictive Value: A normal V/Q scan effectively rules out PE 2, 8.
• Effect of Underlying Diseases: The accuracy of V/Q scans remains robust even in people with chronic heart or lung diseases 5, 7.

Combining Results with Clinical Assessment

The value of the V/Q scan is maximized when combined with clinical assessment tools, such as the Wells score. A high-probability scan in a patient with high clinical suspicion virtually confirms the diagnosis, while a normal scan in a low-risk patient excludes it 2, 8.

What Happens After the Test?

• High/Intermediate Probability: You may be started on anticoagulant medication or referred for further imaging (such as pulmonary angiography) to confirm the diagnosis, especially if surgery is being considered for CTEPH 1, 3, 4, 6.
• Low/Normal Probability: Usually means PE can be safely excluded, and no further testing or treatment is needed unless clinical suspicion remains high 2, 8.

Conclusion

The V/Q scan is a powerful, safe, and versatile tool in the diagnosis of pulmonary embolism and chronic pulmonary hypertension. It offers high accuracy, even in patients with complex medical backgrounds, and is especially valuable when other imaging options are unsuitable.

Key Takeaways:

• Purpose: The V/Q scan is primarily used to diagnose or rule out pulmonary embolism and chronic thromboembolic pulmonary hypertension, remaining accurate even in patients with other lung or heart diseases 1, 2, 3, 4, 5, 6, 7.
• Test Details: The procedure is noninvasive, involves both inhalation and injection of tracers, and produces detailed images of lung ventilation and perfusion without using contrast dye 1, 2, 8, 9.
• Results & Follow-Up: Results are categorized by probability, guiding next steps from reassurance to further testing or immediate treatment. The scan is highly sensitive and specific, especially when combined with clinical assessment 1, 2, 3, 6, 8, 9.

If you or a loved one is scheduled for a V/Q scan, rest assured that this test is a cornerstone of modern pulmonary medicine, offering reliable answers when they matter most.