Things We Do For No Reason: HIT Testing in Low Probability Patients

Laboratory testing for HIT is appropriate when the pretest probability for HIT is intermediate or high based on the 4T’s score.^14-16 Studies assessing the application of the 4T’s score have shown that a moderate or high pretest probability carries a probability of having true HIT in 14% and 64% of the cases respectively.¹⁴ However, due to the subjective nature of the 4T’s score components, it is important to recognize that in nonexpert hands, the 4T’s scoring system can suffer from a lack of interrater reliability.¹⁶

As discussed above, a negative ELISA (OD < 0.4) helps to rule out HIT and allow heparin to be safely reintroduced without any further testing. If ELISA is positive (OD ≥ 0.4) confirmation testing with SRA should be performed.⁵ However, studies suggest that the magnitude of the OD is associated with increased likelihood for true HIT, with an OD of greater than 2.00 associated with a positive SRA approximately 90% of the time.²¹ This suggests that if OD values are strongly positive (≥2.00), SRA can be deferred.⁵

Due to the SRA limited availability, confirmatory testing is not always possible or in some situations, SRA results may be negative despite a positive OD. In both these cases, discussion with the Hematology service is recommended.

When presented with a case of thrombocytopenia, it is important for clinicians to consider a broad approach in their differential diagnosis. Hospitalists should investigate common etiologies, consider the coagulation parameters, liver enzymes, nutritional status, peripheral blood smear, and a detailed history and physical exam to identify other common potential cause such as sepsis.

The 4T’s score should be applied in patients who have had recent heparin exposure. A score of ≤3 indicates a low pretest probability; therefore, HIT is unlikely and further testing is not needed. A score of ≥4 indicates an intermediate or high pretest probability and should prompt clinicians to consider further HIT testing with ELISA. In these situations, heparin should be held, and nonheparin agents should be initiated to prevent thromboembolic complications. In their study of ICU patients, Pierce et al. found that 17% of patients did not have a concurrent cessation of heparin and initiation of alternative agents despite a high clinical suspicion for HIT.¹ Lastly, if hospitalists have concerns regarding HIT testing or management, expert consultation with the Hematology service is recommended.

• Consider a broad differential diagnosis when presented with a hospitalized patient with new thrombocytopenia given the low incidence of HIT (<5%).
• Apply the 4T’s score in those who have thrombocytopenia and recent heparin exposure. A low scores 4T’s score (≤3) predicts a low pretest probability and further testing is not required.
• Patients with moderate or high 4T’s score (≥4) should have the ELISA test. During this time, heparin should be discontinued and nonheparin agents initiated while waiting for test results.
• Confirmatory testing with SRA should be performed for all positive ELISAs; however, they can be deferred in patients with strongly positive OD (≥2.00) on ELISA.