When should D-dimer (D-dimer assay) levels be checked in suspected pulmonary embolism?

When to Check D-Dimer in Suspected Pulmonary Embolism

D-dimer should be checked only in patients with low or intermediate pretest probability of PE who do not meet all PERC criteria, and should never be ordered in high pretest probability patients. 1

Step 1: Establish Pretest Probability

Before considering D-dimer testing, you must first stratify patients using validated clinical prediction rules such as Wells score or Geneva score, or use experienced clinical gestalt. 1 This categorizes patients into:

• Low probability: ~3-13% PE prevalence 1, 2
• Intermediate probability: ~16-26% PE prevalence 1, 3
• High probability: ~36-50% PE prevalence 1, 3

Step 2: Apply PERC Criteria (Low Probability Patients Only)

For patients with low pretest probability, apply the Pulmonary Embolism Rule-Out Criteria (PERC) before ordering D-dimer. 1 If all 8 PERC criteria are met (age <50, heart rate <100, oxygen saturation ≥95%, no unilateral leg swelling, no hemoptysis, no recent trauma/surgery, no prior VTE, no hormone use), do not order D-dimer or any imaging—PE is safely excluded. 1, 2

The risk of PE when PERC-negative is lower than the risks of testing itself. 1, 4

Step 3: When to Order D-Dimer

Order high-sensitivity D-dimer in these specific scenarios:

• Low pretest probability patients who do not meet all PERC criteria 1
• All intermediate pretest probability patients 1

Never order D-dimer in:

• Low probability patients who are PERC-negative 1
• High pretest probability patients (proceed directly to imaging) 1

Step 4: Interpret D-Dimer Results with Age-Adjusted Thresholds

The interpretation of D-dimer depends critically on patient age:

For patients ≤50 years old:

• Use standard cutoff of <500 ng/mL 1, 2
• Negative result safely excludes PE 1

For patients >50 years old:

• Use age-adjusted cutoff: age × 10 ng/mL 1, 2
• This maintains sensitivity >97% while significantly improving specificity 1, 2
• Standard 500 ng/mL cutoff has poor specificity in elderly (only 10% in patients >80 years) 2

The age-adjusted approach increases specificity from 14.7% to 35.2% in patients >80 years, and from 24.5% to 44.2% in patients aged 71-80. 2

Step 5: Act on D-Dimer Results

If D-dimer is below the appropriate threshold (standard or age-adjusted):

• Do not obtain imaging—PE is safely excluded 1
• Negative predictive value is 99.5% when combined with low clinical probability 5

If D-dimer is elevated:

• Proceed immediately to CT pulmonary angiography (CTPA) 1, 4
• Any D-dimer above threshold requires imaging, regardless of how minimally elevated 4

Critical Pitfalls to Avoid

Common errors that lead to missed diagnoses or unnecessary testing:

• Never apply PERC to patients >50 years old—age <50 is one of the required criteria 2
• Never order D-dimer in high pretest probability patients—even a negative result will not obviate the need for imaging 1
• Never use standard 500 ng/mL cutoff in elderly patients—this leads to excessive false positives and unnecessary imaging 1, 2
• Never skip imaging when D-dimer is elevated—any elevation above threshold mandates CTPA 4
• Never use point-of-care D-dimer assays when laboratory-based tests are available—they have lower sensitivity (88% vs 95%) 2

Assay Type Matters

Only high-sensitivity D-dimer assays (ELISA or turbidimetric) should be used for ruling out PE in low and intermediate probability patients. 1 These have sensitivity of 97% and negative likelihood ratio of 0.07. 1 Older latex agglutination assays have inadequate sensitivity (70%) and should not be used. 1

Special Considerations for Inpatients

D-dimer specificity is lower in hospitalized patients due to comorbidities, but testing remains appropriate as sensitivity stays high. 2 The same pretest probability stratification and age-adjusted thresholds apply. 2