Our results and study design are similar but not identical to that of Zhou et al, who found that a campaign to increase VTE prophylaxis resulted in 71% increase of UFH use over 5 years but no increase in number of HIT assays ordered or in the distribution of HIT assay results-both surrogate endpoints [5].But not all analyses of heparin order interventions show similar results. A recent study of a heparin avoidance program in a Canadian tertiary care hospital showed a reduction of 79% and 91% in adjudicated cases of HIT and HITT respectively [4].Moreover, hospital-related expenditures for HIT decreased by nearly $267,000 (Canadian dollars) per year though the additional acquisition costs of LMWH were not stated.A small retrospective heparin avoidance protocol among orthopedic surgery patients showed a reduction of HIT incidence from 5.2% with UFH to 0% with LMWH after universal substitution of LMWH for UFH [8].A recent systematic review identified only 3 prospective studies involving over 1398 postoperative surgical patients that measured HIT and HITT as outcomes [9].The review authors, in pooled analysis, found a lower incidence of HIT and HITT with LMWH postoperatively but downgraded the evidence to “low quality” due to methodologic issues and concerns over bias.A nested case-control study of adult medical patients found that HIT was 6 times more common with UFH than with LMWH and the cost of admissions associated with HIT was 3.5 times higher than for those without HIT, though this increase in costs are not necessarily due to the HIT diagnosis itself but may be markers of patients with more severe illness [10].The duration of heparin therapy was not stated.
There are several potential reasons that our data differs from some of the previous reports described above. We used a strict definition of HIT, requiring the serotonin release assay to be positive in the appropriate clinical setting and did not rely solely upon antibody tests to make the diagnosis, a less rigorous standard found in some studies. Furthermore, our results may differ from previously reports because of differences in patient risk and duration of therapy. Our institution does not perform cardiac surgery and the very large orthopedic surgery programs do not generally use heparin. Another potentially important difference in our study from prior studies is that many of the patients treated at this institution did not receive heparin long enough to be considered at risk; only a quarter were treated for longer than 5 days, generally considered a minumum [11].This is less than half of the duration of the patients in the studies included in the meta-analysis of HIT incidence [2].
We do not contend that UFH is as safe as LMWH with regard to HIT for all populatons, but rather that the increased risk is not manifest in all patient populations and settings and so the increased cost may not be justified in low-risk patients. Indeed while variability in HIT risk among patients is well documented [3,12], the guidelines for prophylaxis do not generally take this into account when recommending particular VTE prophylaxis strategies.Clinical practice guidelines do recommend different degrees of monitoring the platelet count based on risk of HIT however.