From the Guidelines
A negative D-dimer test combined with a low Wells score (≤4 points, which includes 1.5) has a sensitivity of approximately 97-99% for ruling out pulmonary embolism (PE) 1. This means that when both the D-dimer is negative and the Wells score is low, the probability of missing a PE is only about 1-3%. The high sensitivity of this combination makes it a reliable tool for excluding PE in low-risk patients, allowing clinicians to safely avoid unnecessary imaging studies like CT pulmonary angiography 1. Some key points to consider when using this approach include:
- Ensuring the D-dimer assay is a high-sensitivity type
- Correct calculation of the Wells score, as different institutions may use slightly different cutoffs for "PE unlikely" classifications
- Being aware that D-dimer specificity is relatively low (around 40-50%), meaning positive results often require further investigation
- Understanding the potential harms associated with overuse of CT pulmonary angiography, including radiation exposure, contrast administration risks, and the potential for overdiagnosis and overtreatment of clinically insignificant PE 1. The negative predictive value of a negative D-dimer and low Wells score is particularly strong because D-dimer detects fibrin degradation products that are almost always present with thromboembolism. Therefore, in patients with a Wells score of 1.5 and a negative D-dimer, the risk of PE is low, and further imaging may not be necessary, reducing the risk of unnecessary radiation exposure and other potential harms 1.
From the Research
Sensitivity of a Negative D-dimer for Pulmonary Embolism
- The sensitivity of a negative D-dimer for pulmonary embolism (PE) is high, with estimates ranging from 80% to 100% 2.
- A study found that a negative D-dimer result had a negative predictive value of 93.3% (95% CI = 76.5%-98.8%) for excluding PE 3.
- Another study reported a negative predictive value of 99.5% (CI, 99.1% to 100%) for a combined strategy of using a clinical model with D-dimer testing in patients with low clinical probability 4.
- The sensitivity of D-dimer in predicting PE was found to be 1.0, with a specificity of 0.2, in a study that categorized D-dimer levels into four groups based on CTPA findings 5.
Wells' Score and D-dimer
- A study found that combining Wells' items with the D-dimer test resulted in a simplified decision rule, which reduces the need for CT scanning in patients with suspected PE 6.
- The study identified two groups based on the Wells' score: (i) none of the three items positive (41%); (ii) one or more of these items positive (59%) 6.
- However, the studies do not provide specific data on the sensitivity of a negative D-dimer for PE with a Wells' score of 1.5.
Limitations and Variability
- The studies note that D-dimer assays demonstrate high sensitivity but with high levels of false-positive results, especially among those over the age of 65 years 2.
- The utility of the D-dimer test may be less in older populations, but no empirical evidence is available to support an increase in the diagnostic threshold of interpretation of D-dimer results for those over the age of 65 years 2.