Seventeen RCTs (n=645) were included, eleven of which were included in the meta-analysis comparing LMWH and UFH. Eleven trials used a crossover design and six were parallel trials.
Most of the studies were rated as poor quality. Losses to follow-up were considerable (14.8% overall studies). The majority of the studies did not state whether the analyses were based on intention-to-treat or not.
Bleeding.
Most bleeding events across the studies were minor; few major events were reported.
LMWH compared with UFH: there was no statistically significant difference between LMWH and UFH in the number of major and minor bleeding events (6 RCTs; RR 0.96, 95% CI: 0.27, 3.43) or the vascular access compression time (5 RCTs; WMD -0.87, 95% CI: -2.75, 1.02). There was evidence of statistically significant heterogeneity in the pooling of bleeding events (I2 62.8%, P=0.03). The sensitivity analysis suggested similar results; significant heterogeneity was found for the analysis of studies with more than 1 month of follow-up (P=0.01), but studies using therapeutic doses were statistically homogeneous (P=0.24).
LMWH compared with other anticoagulants (3 RCTs): there were no difference between tinzaparin compared with dalteparin, reviparin compared with nadroparin, and dalteparin compared with danaparoid.
Extracorporeal thrombosis.
LMWH compared with UFH (5 RCTs): there was no statistically significant difference between LMWH and UFH in extracorporeal thrombosis (RR 1.15, 95% CI: 0.70, 1.91). There was evidence of statistically significant heterogeneity (I2 57.3%). The sensitivity analysis suggested similar results; significant heterogeneity was found for the analysis of studies with more than 1 month of follow-up (P=0.01), but studies using therapeutic doses were statistically homogeneous (P=0.72). LMWH compared with other anticoagulants (6 RCTs): no differences were found in the 3 RCTs comparing two LMWHs and 1 RCT comparing nadroparin with citrate. There was a trend towards lower filter clotting scores with citrate in comparison with dalteparin in 2 RCTs.
Anti-Xa levels (14 RCTs): the anti-Xa levels were reported for individual studies, although an overall synthesis was not reported.