A total of 17 studies were included. Sixteen studies were included in the main meta-analysis: 7 examining heparin versus control, and 9 comparing LMWH with UFH. A further study was considered as part of a separate analysis of heparin compared with control. Four studies examining heparin versus control and 6 comparing LMWH and UFH were included in an analysis restricted to 'best-study subgroup'. The 16 trials in the meta-analysis incorporated 19,764 patients: 15,095 patients in trials comparing heparin with control, and 4,669 patients in trials comparing LMWH and UFH.
Heparin (LMWH or UFH) versus control (n=7).
The pooled RR for deep-vein thrombosis was O.44 (95% confidence interval, CI: O.29, 0.64, p<0.001; heterogeneity p=0.31).
The pooled RR for clinical pulmonary embolism was 0.48 (95% CI: 0.34, 0.68, p<0.01; heterogeneity p=0.63).
The pooled RR for major bleeding was 1.87 (95% CI: 0.94, 3.75, p=0.08; heterogeneity p=0.53).
The pooled RR for death was 0.95 (95% CI: 0.84, 1.07, p=0.40; heterogeneity p=0.39).
The pooled risk estimates were reduced when the analysis was restricted to 4 studies in the 'best-study subgroup': the RR was 0.50 (95% CI: 0.33, 0.76) for deep-vein thrombosis, 0.57 (95% CI: 0.27, 1.20) for clinical pulmonary embolism, 1.20 (95% CI: 0.41, 3.56) for major bleeding, and 0.98 (95% CI: 0.79, 1.21) for death.
LMWH versus UFH (n=9).
The pooled RR For deep-vein thrombosis was 0.83 (95% CI: 0.56, 1.24, p=0.37; heterogeneity p=0.93).
The pooled RR for clinical pulmonary embolism was 0.74 (95% CI: 0.29, 1.88, p=0.52; heterogeneity p=0.84).
The pooled RR for major bleeding was 0.48 (95% CI: 0.23, 1.00, p=0.049; heterogeneity p=0.65).
The pooled RR for death was 1.07 (95% CI: 0.79, 1.45, p=0.66; heterogeneity p=0.47).
After restricting the analysis to 6 trials in the 'best-study subgroup', the pooled risk estimates were 0.87 (95% CI: 0.49, 1.59) for deep-vein thrombosis, 0.76 (95% CI: 0.27, 2.19) for clinical pulmonary embolism, 0.51 (95% CI: 0.23, 1.12) for major bleeding, and 1.26 (95% CI: 0.82, 1.93) for death.