A Cochrane review included 2 studies with a total of 147 subjects (10 deep vein thrombosis event). The earlier study comparing calcium heparin with placebo recruited only 49 out of a planned 150 patients and was terminated due to very high incidence of major bleeding (8/24 vs. 1/25) following heparin prophylaxis (OR 12.0, 95% CI 1.4 to 105.4). At the point when the study was terminated, 24% of the control group and 8% of the heparin group had developed deep vein thrombosis (DVT), but this difference did not reach statistical significance (OR 0.29, 95% CI 0.05 to 1.60). All DVT cases were in calf veins and only one case extended above the calf. The only completed study (n=98) compared unfractionated heparin + calf length compression device with no prophylaxis at all. It reported a very low incidence of DVT (below-knee DVT not addressed) of 2% in both control and intervention group (only 1 patient in each group developed DVT) with no major bleeding or mortality. In the included studies there was only one non-fatal pulmonary embolism.
Comment: The quality of evidence is downgraded by study quality (unclear allocation concealment), by inconsistency (variability in results across studies) and by imprecise results (few patients and wide confidence intervals).