Wells Score Selects Low-Risk Patients for D-dimer Testing
Low clinical suspicion patients are screened before any blood draw.
- ≤4: Wells score cutoff (low/moderate PE probability)
- ~10%: Low-risk prevalence (actual PE in this group)
- Only if low risk: Test appropriateness (skips high-risk patients)
- Avoid unneeded CT: Goal (in low-probability patients)
Wells criteria overview
Clinical signs, symptoms, and risk factors scored briefly.
Why gate testing by risk
D-dimer only useful when pretest probability is low.
D-dimer Blood Test Quantifies Fibrin Degradation Products
A quantitative ELISA or immunoturbidimetric assay reports ng/mL level.
- ELISA / turbidimetric: Assay type (quantitative D-dimer)
- ng/mL FEU: Units (fibrinogen equivalent units)
- ~1 hour: Turnaround (rapid lab result)
- >95%: Sensitivity (high negative predictive value)
What D-dimer measures
Cross-linked fibrin breakdown fragment from clot turnover.
Confounders
Age, pregnancy, inflammation, cancer raise baseline levels.
Age-Adjusted Cutoff Replaces the Fixed 500 ng/mL Rule
Patients over 50 use age×10 ng/mL instead of a flat 500.
- 500 ng/mL: Standard cutoff (used under age 50)
- age × 10: Age-adjusted formula (ng/mL, for age >50)
- 800 ng/mL: Example at 80 (vs. flat 500 threshold)
- ADJUST-PE, 2014: Validation study (JAMA multicenter trial)
Why D-dimer rises with age
Baseline coagulation activity increases naturally over decades.
Formula in practice
Multiply age by ten once patient passes fifty.
Below Threshold Rules Out PE; Above Threshold Triggers CTPA
A single comparison decides imaging versus discharge without a scan.
- PE ruled out: Below threshold (no further imaging needed)
- CTPA ordered: Above threshold (CT pulmonary angiography)
- <1%: Missed PE risk (when rule-out criteria met)
- Single comparison: Decision basis (level vs. threshold line)
Ruling out pathway
Below-threshold patients are discharged without CT scan.
Imaging pathway
Above-threshold patients proceed to CT angiography confirmation.
Age Adjustment Reduces Unnecessary CT Scans in Older Patients
Fewer older patients get scanned, without increasing missed PE cases.
- ~14%: CT scans avoided (additional in older cohorts)
- <1%: Missed PE at 3 months (safety endpoint unchanged)
- Yes: Radiation avoided (per unnecessary CT skipped)
- Yes: Contrast risk avoided (nephropathy, allergy exposure)
Population-level benefit
Older cohorts see the largest drop in scan rate.
Safety maintained
Outcome studies show no rise in missed embolism.