Cardiac Imaging

What Is the Right Imaging for Suspected PE with a Positive D-dimer and Low Pretest Probability?

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What Is the Right Imaging for Suspected PE with a Positive D-dimer and Low Pretest Probability?

It’s 10 p.m. in the emergency department, and you’re evaluating a 45-year-old patient with pleuritic chest pain and mild tachycardia. Their Wells score is 2, placing them in the low-to-intermediate pretest probability category for pulmonary embolism. Following the PERC rule and subsequent D-dimer testing, the lab calls with a positive result. The initial strategy to rule out pulmonary embolism (PE) without imaging is no longer an option. The next decision is critical: which imaging study provides a definitive answer with the best balance of accuracy, speed, and safety? This article details the American College of Radiology (ACR) guided workflow for this exact scenario. For a patient with low or intermediate pretest probability and a positive D-dimer, the ACR rates CTA pulmonary arteries with IV contrast as Usually Appropriate.

Who Fits This Clinical Scenario?

This guidance applies to a specific and common clinical crossroads: the patient in whom you suspect a pulmonary embolism, but whose clinical risk score is not high. This includes patients who meet the following criteria:

  • Low or Intermediate Pretest Probability: The patient’s clinical picture does not strongly suggest a PE. This is typically quantified using a validated clinical decision rule like the Wells’ Criteria (score ≤4) or the simplified revised Geneva Score (score ≤2).
  • Positive D-dimer Test: A laboratory test was performed to rule out thromboembolic disease, and the result came back positive (above the laboratory or age-adjusted threshold). This positive result necessitates further investigation, as PE can no longer be reasonably excluded.

It is crucial to distinguish this patient from those in neighboring scenarios where the imaging algorithm differs significantly. This guidance does not apply if:

  • The D-dimer is negative: In a low or intermediate risk patient, a negative D-dimer effectively rules out PE, and no imaging is warranted.
  • Pretest probability is high: For patients with a high Wells’ score (>4), D-dimer testing is not recommended. These patients should proceed directly to definitive imaging, as a negative D-dimer is insufficient to rule out PE in this context.
  • The patient is pregnant: Suspected PE in pregnancy involves a distinct diagnostic algorithm that prioritizes minimizing fetal radiation exposure and requires specialized imaging protocols.

What Diagnoses Are You Working Up in This Scenario?

When a patient presents with symptoms like dyspnea, tachycardia, or pleuritic chest pain, and a D-dimer is positive, the differential diagnosis broadens, but the primary concern remains thromboembolic disease. The imaging choice is designed to confirm or exclude the most life-threatening possibilities.

Pulmonary Embolism (PE) is the principal diagnosis to exclude. The positive D-dimer, while not specific, significantly increases the post-test probability of PE, making it the central focus of the workup. The goal of imaging is to directly visualize thrombus within the pulmonary arterial tree.

Aortic Dissection, while less common, is a catastrophic mimic of PE that can present with chest pain and an elevated D-dimer. While a standard PE-protocol CT is not optimized for the aorta, large central filling defects and secondary signs can sometimes be visualized, and a “triple rule-out” protocol exists for ambiguous high-risk cases.

Pneumonia or Pleuritis can cause pleuritic chest pain and systemic inflammation, which may lead to a non-specific D-dimer elevation. A Computed Tomography Angiography (CTA) of the chest will clearly visualize the lung parenchyma, readily identifying consolidation, infiltrates, or pleural effusions that would suggest an infectious or inflammatory cause.

Pericarditis or Myocarditis can also present with chest pain. While CTA is not the primary modality for these conditions, it can reveal secondary signs like pericardial effusions. More importantly, by ruling out PE, the CTA allows the clinical team to confidently pivot the workup toward cardiac causes, often involving echocardiography or cardiac MRI.

Why Is CTA Pulmonary Arteries with IV Contrast the Recommended Study for This Presentation?

For a patient with low or intermediate pretest probability and a positive D-dimer, the ACR designates CTA pulmonary arteries with IV contrast as Usually Appropriate. This recommendation is based on its high diagnostic accuracy, speed, and ability to provide alternative diagnoses.

The primary strength of a dedicated PE-protocol CTA is its excellent sensitivity and specificity for detecting thrombi in the main, lobar, and segmental pulmonary arteries. The use of a precisely timed intravenous contrast bolus allows for dense opacification of the pulmonary vasculature, making even small clots visible as filling defects. This rapid and definitive result is crucial for initiating or withholding anticoagulation therapy.

Other imaging modalities are rated lower for this specific scenario for several reasons:

  • V/Q Scan (Ventilation/Perfusion Scan): This nuclear medicine study is also rated Usually Appropriate and is a strong alternative, particularly in patients with severe contrast allergy or renal insufficiency. However, it is often less specific than CTA, especially in patients with underlying lung disease (e.g., COPD), which can lead to indeterminate results. Its primary limitation is that it does not visualize other structures and cannot provide an alternative diagnosis like pneumonia or aortic pathology.
  • US Duplex Doppler Lower Extremity: This study is rated Usually Not Appropriate as the initial imaging test for suspected PE. While it can confirm a deep vein thrombosis (DVT), a negative ultrasound of the legs does not rule out PE, as the clot may have already embolized or originated from a different location (e.g., pelvic veins, upper extremities). It is a test for DVT, not a primary test for PE.

From a safety perspective, CTA involves ionizing radiation (ACR Relative Radiation Level ☢☢☢, corresponding to 1-10 mSv) and requires iodinated intravenous contrast, which carries risks of allergy and contrast-induced nephropathy. These risks must be weighed against the benefit of a rapid, definitive diagnosis. When ordering, it is critical to specify “CTA for Pulmonary Embolism” to ensure the radiology department uses the correct contrast timing protocol, which is distinct from a routine “CT Chest with contrast.”

What’s Next After CTA Pulmonary Arteries? Downstream Workflow

The results of the CTA will guide your immediate next steps, branching the clinical pathway toward treatment, discharge, or further investigation.

If the study is positive for pulmonary embolism: The diagnosis is confirmed. The next steps involve risk stratification using a validated score (like the PESI or sPESI score) to determine the severity and risk of adverse outcomes. Based on this risk, a decision is made regarding initiation of anticoagulation and the appropriate disposition—admission to the hospital for higher-risk patients or outpatient management for select low-risk individuals.

If the study is negative for pulmonary embolism: PE is effectively ruled out in this low-to-intermediate risk population. Anticoagulation should not be started. The clinical focus must pivot back to the other potential causes on the differential diagnosis. Review the lung windows and soft tissues on the CT scan for alternative explanations like pneumonia, pleurisy, or a hiatal hernia. Further workup may include cardiac evaluation or treatment for a musculoskeletal cause.

If the study is indeterminate or technically limited: This is an unwelcome but possible outcome, often due to patient motion or a suboptimal contrast bolus. The report will specify the limitation. In this situation, the next step depends on the ongoing clinical suspicion. You may need to repeat the CTA or consider an alternative test like a V/Q scan. A consultation with the radiologist is essential to determine the best path forward.

Pitfalls to Avoid (and When to Get Help)

Navigating this workup requires careful attention to detail to avoid common errors that can delay diagnosis or expose patients to unnecessary risk.

  • Ordering the Wrong Test: Requesting a “CT chest with contrast” instead of a “CTA for PE” can result in a non-diagnostic study due to improper contrast timing. Be specific in your order.
  • Ignoring Contraindications: Failing to screen for severe renal insufficiency (check eGFR) or a history of severe anaphylactic reaction to iodinated contrast can lead to preventable harm. Have a clear plan for these patients, which often involves a V/Q scan or MRA.
  • Misinterpreting the Pretest Probability: Applying this algorithm to a high-risk patient is an error. High-risk patients should bypass D-dimer testing and proceed directly to CTA, as their risk of PE is high enough to warrant imaging regardless of the lab result.
  • Anchoring on the Positive D-dimer: Remember that a positive D-dimer is not specific for PE. If the CTA is negative, do not continue to “chase” a PE diagnosis; actively pursue alternative causes for the patient’s symptoms.

If the patient is hemodynamically unstable (hypotensive, tachycardic with signs of shock), this workflow does not apply. This constitutes a suspected massive PE, a medical emergency requiring immediate escalation, often involving a bedside echocardiogram and consultation with critical care or a PE response team (PERT).

Related ACR Topics and Tools

This article covers one specific branch of the suspected PE workup. For a comprehensive overview of all clinical variants, including high-risk and pregnant patients, and for tools to help with study selection and patient communication, the following resources are essential.

Frequently Asked Questions

What if my patient has a severe allergy to IV contrast?

If a patient has a history of a severe allergic reaction to iodinated contrast, a CTA is relatively contraindicated. In this case, a V/Q scan is the best alternative and is also rated ‘Usually Appropriate’ by the ACR for this scenario. If a V/Q scan is unavailable or likely to be indeterminate, a contrast-enhanced MRA of the pulmonary arteries may be considered.

What should I do if the patient has chronic kidney disease (CKD)?

For patients with severe renal insufficiency (typically an eGFR < 30 mL/min/1.73m²), the risk of contrast-induced nephropathy from a CTA is a significant concern. The V/Q scan is the preferred imaging modality as it does not use iodinated contrast. If a V/Q scan is not an option, the decision to proceed with CTA should be made after a careful risk-benefit discussion with the patient and consultation with radiology and nephrology.

Is a V/Q scan as good as a CTA in this scenario?

Both CTA and V/Q scans are rated ‘Usually Appropriate.’ A V/Q scan is excellent for ruling out PE if the result is ‘normal’ or ‘very low probability.’ However, it is more likely than CTA to yield an ‘intermediate’ or ‘indeterminate’ result, especially in patients with pre-existing lung conditions like COPD or pneumonia. CTA is generally preferred when available and not contraindicated because it is faster, more specific, and can identify alternative diagnoses.

Why not just get a lower extremity ultrasound first to look for DVT?

An ultrasound is ‘Usually Not Appropriate’ as the first test for suspected PE. A positive ultrasound for DVT confirms the need for anticoagulation but doesn’t assess the clot burden in the lungs. More importantly, a negative ultrasound does not rule out PE, as the clot may have already embolized completely from the legs or originated elsewhere. Therefore, it is not a sufficient rule-out test for a patient with respiratory symptoms.

What defines ‘low or intermediate pretest probability’?

This is determined using a validated clinical decision rule. The most common is the Wells’ Criteria for PE. A score of 0-4 is considered ‘PE unlikely,’ which corresponds to low or intermediate probability. Another tool is the simplified revised Geneva Score, where a score of 0-2 is considered low risk. These scores incorporate clinical signs, symptoms, and risk factors to standardize risk assessment before testing.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 29, 2026