Confirmation of the Utility of the 4T Scoring System for HIT
Authors: Trisel Zachary , Maddox Mark , Safa Ahmed , Bemis Thomas , Ward Kristine , Styler Michael 
Published: October 2019

Abstract
Background: Heparin-induced thrombocytopenia (HIT) is a complication of heparin-based anticoagulation (AC) resulting in thrombocytopenia and thrombosis. Laboratory testing can often be avoided as the 4T score (4TS) has a negative predictive value (NPV) of 0.998 for low risk patients. Despite this scoring system, which has been validated since 2006, physicians continue to send inappropriate studies despite a low probability of HIT. We sought to evaluate our academic institution's compliance, perform a cost analysis and determine if appropriate AC was initiated. By analyzing our data, we sought to educate our staff and implement measures to improve cost efficiency and quality of care.
Methods: We performed a retrospective chart review of patients admitted to Hahnemann University Hospital (HUH) between November 1, 2016 and April 30, 2017 who had HIT antibodies (HITAb) and serotonin release assay (SRA) studies. These laboratory tests were performed at Quest Diagnostics. This data was compiled from the EMR at HUH. According to the 4TS, patients were assigned a score of 0-8: 0-3 for low, 4-5 for intermediate, 6-8 for high probability respectively. Laboratory results of HITAb and SRA were then compared to the calculated 4TS. We then investigated whether appropriate AC was initiated. Data on the cost associated with the inappropriate management of suspected HIT was compiled.
Results: 72 patients had HITAb sent during the interval studied. Table 1 shows the 4TS and results of HITAb and SRA testing. Table 2 lists the AC used based on the 4TS.
The NPV of not having HIT in the low probability group was 100%. The positive predictive value (PPV) of having HIT in the high probability group was 100%. At our institution, HITAb with reflex SRA costs $503. Expenditure due to inappropriate testing was estimated to be around $23,000 dollars over the study's time course. Inappropriately switching to argatroban cost up to $1,000 per day or fondaparinux $500 per day of overspending on anticoagulation per patient.
Discussion: We found the majority of HITAb and SRA testing was unnecessary based on the 4TS. Our data showed a low 4TS had a very high NPV confirming the scoring system's utility. HIT testing was often overutilized as part of a general workup for thrombocytopenic patients who were often septic, on marrow suppressive medications and had multiple comorbidities such as hepatitis and HIV infections which confounded their clinical picture. Furthermore, this scoring system had a very high PPV in the high probability group. This study confirmed that HIT laboratory studies rarely change patient management in these scenarios. With the turnaround time of laboratory studies taking up to 4 days, there is a significant increase to the cost of patient care when solely relying on HITAb and SRA due to the use of expensive anticoagulants. In contrast, it remains unknown if HITAb and SRA could be useful in patients with an intermediate 4TS as our data is limited with no SRA results for patients with intermediate scores and a positive HITAb.
To prevent unnecessary testing in the future and to improve the management of HIT, we propose to implement the following at our institution: 1. create a hard stop in our EMR which would prevent studies from being sent off inappropriately; 2. add a 4TS to the calculator section of the EMR and encourage collaboration with the hematology department if additional questions remain after calculating a 4TS; 4. start resident based educational sessions on the importance of calculating a 4TS and its significance prior to sending laboratory studies. In conclusion, the 4TS remains a useful tool to prevent unnecessary diagnostic testing and use of expensive therapeutic anticoagulants in patients with suspected HIT.

Disclosures
No relevant conflicts of interest to declare.