To estimate the sensitivity and specificity of the various types of D-dimer test in the diagnosis of deep vein thrombosis (DVT) of the lower limb in symptomatic outpatients with a clinical prediction rule score. Patients' clinical prediction rule scores will be used to assess D-dimer test accuracy in different risk groups.

To investigate the following as potential sources of heterogeneity: age, sex, cancer, previous venous thromboembolism (VTE), prolonged immobilisation, anticoagulant treatment, time lapse between onset of symptoms and testing, and type of reference standard. However, we recognise that all of these listed items, except type of reference standard, are patient-specific rather than study-specific, and so study reports may lack the necessary level of detail to enable an informative analysis.