D-Dimer is the Screening Test Used with Clinical Signs to Detect DVT and PE
The correct answer is A. D-dimer, which serves as the primary screening test used in conjunction with clinical probability assessment to help detect or exclude deep vein thrombosis (DVT) and pulmonary embolism (PE). 1
Role of D-Dimer in VTE Diagnosis
D-dimer is a fibrin degradation product that measures ongoing fibrinolysis, providing evidence of active clot formation and breakdown. 2 The test functions as an excellent "rule-out" tool rather than a "rule-in" test, with the following performance characteristics:
- Sensitivity: 96-100% for venous thromboembolism (VTE) when using highly sensitive ELISA-based assays 2, 3
- Specificity: Only 35-47%, making it unsuitable for confirming thrombosis 2, 3
- Negative predictive value: 99-100% when combined with low clinical probability 1, 2
Proper Diagnostic Algorithm
D-dimer must always be used in conjunction with clinical probability assessment using validated scoring systems (Wells score or revised Geneva score), never as a standalone test. 1, 2
For Suspected DVT:
- Low clinical probability (≤10%) + negative D-dimer (<500 ng/mL): DVT safely excluded, no imaging needed 2, 3
- Intermediate/high probability: Proceed directly to compression ultrasonography regardless of D-dimer 2
For Suspected PE:
- Low/intermediate probability + negative D-dimer: PE safely excluded without imaging 1
- High clinical probability: Proceed directly to CT pulmonary angiography; D-dimer should not be ordered as normal results do not safely exclude PE 1
Age-Adjusted Interpretation
For patients over 50 years old, use the age-adjusted cutoff (age × 10 ng/mL) rather than the standard 500 ng/mL threshold. 1, 4 This maintains sensitivity >97% while improving specificity, which drops to only 10% in patients over 80 using standard cutoffs. 4
Critical Limitations
D-dimer has severely limited utility in specific populations due to high false-positive rates: 2, 3
- Hospitalized patients
- Post-surgical patients
- Pregnant women
- Active cancer patients
- Patients with infection or sepsis
Why Other Options Are Incorrect
PT (Prothrombin Time) and PTT (Partial Thromboplastin Time) measure coagulation factor function and are used to monitor anticoagulation therapy, not to screen for thrombosis. [@General Medicine Knowledge]
Fibrinogen is an acute phase reactant that may be elevated in thrombotic states but lacks the sensitivity and specificity for VTE screening. [@General Medicine Knowledge]
Essential Pitfall to Avoid
Never use a positive D-dimer alone to diagnose DVT or PE—confirmation with imaging is mandatory before initiating anticoagulation, as the positive predictive value is only 35-50%. 2, 3 A positive D-dimer simply indicates the need for further diagnostic imaging, not the presence of thrombosis.