Heparin-induced thrombocytopenia testing requires accurate pretest clinical probability analysis

Mohamad A. Younes, MD, and colleagues at Henry Ford Hospital in Detroit, Michigan report that it’s important to follow established protocols and have an accurate pretest clinical probability for diagnosing heparin-induced thrombocytopenia (HIT); further, there is a need to improve the current clinical scoring system to avoid unneeded anticoagulation or, conversely, withholding needed anticoagulation.

The researchers evaluated 1173 HIT antibody (Ab) tests performed between January 2008 and June 2010 at their hospital. They found that 826 (70.4%) of patients had low clinical score. Of these, 764 (92.4%) had negative HIT Ab test and four had serotonin release assay (SRA) checked and all were negative. They found that 62 (7.6%) patients had positive HIT Ab test, 11 had SRA checked, and nine (81.8%) were negative and two (18.2%) were positive. 

There were 347 (29.6%) patients with intermediate/high clinical score. Of these, 260 (74.9%) had negative HIT Ab, six had SRA checked (all were negative). Further, 87 (24.1%) had positive HIT Ab, 18 had SRA checked, and four (22.2%) were positive and 14 (77.8%) were negative.

“This study confirms the importance of following the established protocols of ordering the HIT score only when there is at least an intermediate or high clinical suspicion for diagnosis,” the authors concluded. This is because only 7.6% of patients with low clinical score had a positive HIT Ab test, and even in these patients 81.8% of SRA tests sent were negative, compared to 24.1% positive HIT Ab in patients with intermediate/high clinical score. However, it also shows that the current pretest clinical scores should be improved.”