A Meta-Analysis to Determine the Risk of Heparin Induced Thrombocytopenia (HIT) and Isolated Thrombocytopenia in Prophylaxis Studies Comparing Unfractioneted Heparin (UFH) and Low Molecular Weight Heparin (LMWH).

HIT is an uncommon but potentially devastating complication of anticoagulation with UFH or LMWH. The absolute risk of HIT and thrombocytopenia are not clearly defined and no summary data to provide odds ratio is available. We conducted a meta-analysis to determine and compare the incidences of HIT in surgical or medical patients receiving thromboprophylaxis with either UFH or LMWH. We searched MEDLINE-OVID and MEDLINE-PubMed using and combining the following terms: heparin induced thrombocytopenia, low molecular weight heparin, prophylaxis, randomized controlled trials, prospective studies. The function Explode was used. Search was limited to humans from 1984 to 2004. Over 400 abstracts were reviewed and then 91 articles were independently reviewed by two authors, without any restriction of article language. Included studies were those comparing prophylactic UFH and LMWH and measuring HIT (defined as platelets drop > 50% or < 100 X 109/L AND positive laboratory HIT assay) or thrombocytopenia (defined as platelets drop > 50% or < 100 X 109/L) as outcomes. Studies defining thrombocytopenia with lower thresholds were excluded because cases could have been missed. Extracted data included patient characteristics, drug regimens, HIT, thrombocytopenia and venous thromboembolism rates. Disagreements were resolved by consensus. Eligible studies were included into the meta-analysis using a random-effects model to determine the odds ratio for the incidences of HIT and thrombocytopenia between UFH and LMWH. Funnel plots were made to assess possible publication bias. 17 articles were eligible with a total of 8500 patients: 2 RCTs measuring HIT; 10 RCTs measuring thrombocytopenia, and 5 prospective non-randomized studies with comparison groups measuring HIT. Three analysis were performed and all favoured the use of LMWH: 1) 2 RCTs measuring HIT showed an OR of 0.10 (95% confidence interval [CI] 0.01–0.77; p=0.03); 2) all 7 studies measuring HIT showed an OR of 0.11 (95%CI= 0.05–0.26; p< 0.00001); 3) 12 RCTs measuring thrombocytopenia showed an OR of 0.45 (95% CI= 0.26–0.80; p=0.006). Comparing the rates in the 7 studies measuring HIT UFH resulted in HIT in 3.4% (95%CI=2.6% to 4.3%) of cases and LMWH resulted in HIT in 0.2% (95% CI=0.1% to 0.6%), a statistically significant difference (p<0.0001). This meta-analysis confirms the lower incidences of HIT and thrombocytopenia with LMWH prophylaxis compared to UFH. Absolute rates of HIT with LMWH are very low. The HIT rates should be considered when determining the drug of choice for thromboprophylaxis in surgical and medical patients.