Fifty-one cohort studies with a total of 10,311 participants were included.
Apart from the studies that augmented an ultrasonography reference standard with further testing based on clinical probability, in most studies the reference standard was applied independently of the results of the clinical assessment. The reporting of blinding of the clinical assessment and the reference standard was generally poor.
Clinical features of DVT.
The positive and negative LRs were, respectively: for calf pain, 1.08 (95% confidence interval, CI: 0.96, 1.20) and 0.90 (95% CI: 0.78, 1.03) (12 studies); for calf swelling, 1.45 (95% CI: 1.25, 1.69) and 0.67 (95% CI: 0.58, 0.78) (16 studies); for history of DVT, 2.25 (95% CI: 1.57, 3.23) and 0.90 (95% CI: 0.85, 0.95) (11 studies); for malignant disease, 2.71 (95% CI: 2.16, 3.39) and 0.89 (95% CI: 0.85, 0.93) (20 studies); for recent immobilisation 1.98 (95% CI: 1.70, 2.30) and 0.90 (95% CI: 0.85, 0.94) (17 studies); for recent surgery, 1.76 (95% CI: 1.40, 2.20) and 0.94 (95% CI: 0.91, 0.97) (17 studies); for obesity, 0.85 (95% CI: 0.59, 1.23) and 1.04 (95% CI: 0.96, 1.13) (5 studies); for difference in calf diameter, 1.80 (95% CI: 1.48, 2.19) and 0.57 (95% CI: 0.44, 0.72) (8 studies); for Homan sign, 1.40 (95% CI: 1.18, 1.66) and 0.87 (95% CI: 0.79, 0.96) (11 studies); for warmth, 1.29 (95% CI: 1.07, 1.54) and 0.97 (95% CI: 0.78, 0.98) (12 studies); for tenderness, 1.27 (5% CI: 1.11, 1.45) and 0.80 (5% CI: 0.72, 0.89) (14 studies); for erythema, 1.30 (95% CI: 1.02, 1.67) and 0.88 (95% CI: 0.80, 0.98) (6 studies); and for oedema, 1.35 (95% CI: 1.05, 1.74) and 0.86 (95% CI: 0.79, 0.94).
Based on an LR of greater than 2 being useful for ruling in DVT and a ratio of less than 0.5 being useful for ruling out DVT, then only a history of DVT and malignancy are useful for ruling in DVT. No clinical features were useful for ruling out DVT. Recent immobilisation, recent surgery, or a difference in calf diameter were of borderline value for ruling in DVT. The absence of calf swelling or a difference in calf diameter were of borderline significance for ruling out DVT.
Wells clinical probability score (22 studies).
A high Wells score increased the probability of DVT (LR=5.2), while a low Wells score decreased the probability of DVT (LR=0.25).
Physicians' empirical judgements (8 studies).
Four studies categorised patients into low, intermediate or high risk for DVT, while four dichotomised assessment as low or high risk or dichotomised DVT as present or absent. For the 4 studies that categorised patients into low, intermediate or high risk, the LRs for the empirical judgements of high and low risk were similar to those of the Wells scores. However, there was evidence of significant heterogeneity across these studies. The meta-analysis of the dichotomised empirical judgements gave a sensitivity of 86.6% (95% CI: 80.7, 91.2), a specificity of 69.3% (95% CI: 64.4, 73.9), a positive LR of 6.2 (95% CI: 1.0, 40.0) and a negative LR of 0.18 (955 CI: 0.13, 0.26). Again, there was evidence of significant heterogeneity across these studies.