Below is a calculator for the Wells clinical decision rule for use in patients with suspected DVT. 

DVT probabilities are based on a large IPD meta-analysis. D-dimer testing is usually added to the score result. Given that D-dimer tests have a high sensitivity for ruling-out DVT, a negative D-dimer test can be used to exclude DVT safely in patients with a pre-test probability of DVT of up to ~20% (i.e. a Wells-rule score of 1). Post-test probability after a negative D-dimer test is then below 2%, which is commonly deemed as safe.