Twenty-one studies (4,472 participants) were included. However, not all of the studies were included in the meta-analyses.
Clot reduction (by venography).
The unadjusted overall improvement was greater in the LMWH group than in the UFH group: 443 out of 707 participants (62.7%) and 394 out of 716 participants (55%), respectively. Four studies showed a considerable improvement in clot reduction in favour of LMWH: the OR (fixed-effect model) was 0.73 (95% CI: 0.59, 0.90, p=0.004). The number that needed to switch from UFH to LMWH to get one case improvement by venography was 13 (95% CI: 8, 40).
Incidence of recurrent VTE.
There was a near to statistically significant reduction in the recurrence of VTE in favour of LMWH: the OR was 0.78 (95% CI: 0.59, 1.04, p=0.103).
Total mortality.
There was a 33% reduction in total mortality rate in favour of LMWH: the OR was 0.68 (95% CI: 0.50, 0.91, p=0.012).
Safety and haemorrhagic events.
There was a significant reduction in the risk of major haemorrhage in the LMWH group: the OR was 0.65 (95% CI: 0.43, 0.98, p=0.047). The number that needed to switch from UFH to LMWH to prevent one episode of severe bleeding was 106 (95% CI: 55, 1,294).
Comparison between LMWH administered twice daily and once daily (as compared with UFH).
The twice-daily regimen appeared to be more effective than a single dose for reducing clot size: the OR was 0.56 (95% CI: 0.42, 0.74) for two daily doses, compared with 1.08 (95% CI: 0.77, 1.51) for a single daily dose. However, the single daily dose appeared safer in terms of major bleeds: the OR was 0.07 (95% CI: 0.01, 0.54) for the once-daily dose and 0.79 (95% CI: 0.47, 1.32)) for the twice-daily dose.
There was no statistically-significant difference in the two regimens for the outcomes of mortality or recurrent thromboembolic events.