The Wells score is a validated clinical prediction tool for estimating the pre-test probability of deep vein thrombosis (DVT) or pulmonary embolism (PE). It uses a structured set of clinical criteria — each weighted by evidence — to produce a score that guides the next diagnostic step.
The DVT version uses the 2003 modified Wells criteria, scoring from −2 to +9. A score below 2 puts DVT in the "unlikely" category — D-dimer testing is the recommended next step to rule it out. A score of 2 or above puts DVT in the "likely" category — proximal leg ultrasound is recommended without waiting for D-dimer. The single negative criterion ("alternative diagnosis at least as likely") deducts 2 points and is the most clinically significant item on the list — if another diagnosis is equally probable, the score shifts meaningfully toward unlikely.
The PE version uses the original 2001 Wells criteria, scoring 0 to 12.5 points. A score of 4 or below indicates PE is unlikely — high-sensitivity D-dimer testing can be used to exclude PE without imaging. A score above 4 indicates PE is likely — CT pulmonary angiography (CTPA) is the recommended next step. The two highest-weighted items (clinical signs of DVT, and PE being the most likely diagnosis) each add 3 points and require clinical judgement to score accurately.
The Wells score is a pre-test probability tool, not a diagnostic test. A low Wells score combined with a negative D-dimer effectively excludes DVT or PE in the vast majority of patients without requiring imaging. A high Wells score bypasses D-dimer and goes straight to imaging. This two-step approach — Wells score followed by D-dimer or imaging depending on the score — is embedded in NICE guidelines (NG158 for DVT, NG115 for PE) and reduces unnecessary investigations while ensuring high-risk patients are imaged promptly.
The Wells score is a decision-support tool. It was validated in specific patient populations and performs best when used as intended — as part of a structured diagnostic pathway, not in isolation. It does not replace clinical judgement, particularly in patients with atypical presentations, significant comorbidities, or where the pre-test probability feels discordant with the score. The modified Wells DVT score has known limitations in patients with bilateral symptoms, recent surgery, or active cancer. Always interpret the score in the context of the full clinical picture.