Multi-Year Study of Unfractionated Heparin Serotonin Release Testing at a Large U.S. Reference Laboratory

Introduction: The serotonin release assay (SRA) is a functional assay for the detection of heparin-induced thrombocytopenia (HIT). Laboratory practice suggests that it is important to test several low doses of unfractionated heparin (UFH), followed by suppression at a high dose (100 U/mL) of UFH to demonstrate the specificity for HIT. However, little is known of the demographic associations with UFH SRA testing at a large reference laboratory.

Purpose: To characterize the population characteristics of UFH SRA testing at a large US reference laboratory

Methods: We performed a multi-year retrospective review of positivity rates among adults (n=63,651) and children (<18 yrs, n=233) who underwent SRA testing; we examined positivity rates at 0.1 and/or 0.5 U/mL of UFH. A positive SRA test was defined by ≥20% release at either low dose (0.1 or 0.5 U/mL) of UFH and >50% suppression of release at high dose (100 U/mL) of UFH. Indeterminant results were defined by ≥20% release at either low dose of UFH, but without >50% suppression at the high dose of UFH. A negative result was defined as <20% release at both low doses of UFH. All testing was performed at Quest Diagnostics, Nichols Institute in Chantilly, VA. Demographic and laboratory information collected included age, gender, date of collection, and SRA results at 0.1 and 0.5 U/mL UFH. Descriptive statistics were performed as well as non-parametric statistical analyses using the Mann-Whitney U test (two-tailed) with significance defined at p <0.05.

Results: Over the study period, SRA testing included 63,051 adults and 233 children who encompassed 99.5% and 0.5% of the total population tested, respectively. Over the years studied, the positivity rate increased steadily from 4.08% to 7.50%, which was seen in adults but not in children. For positive SRA results, the ratio of male to female was nearly identical to the overall male to female ratio (55:45) in the tested population. In additional, no seasonal variation was noted. Yearly median percent positive, indeterminant, and negative SRA results were 5.42%, 3.32%, and 89.73%, respectively, for adults, and 1.96%, 6.52%, and 93.48%, respectively, for children, with children having significantly lower SRA positivity than adults (p<0.037).

Among positive tests in adults, we examined the results by level of low dose UFH: the median positivity rates were 3.46% for 0.1 U/mL only, 1.03%, for 0.5 U/mL only, and 95.89% for both 0.1 and 0.5 U/mL UFH. In contrast, children demonstrated positive tests only when both 0.1 and 0.5 U/mL UFH results were positive. These children (n=6, median age 14 yrs with range 3 to 15 yrs) demonstrated median serotonin release of 81% and 77% at 0.1 and 0.5 U/mL of UFH, respectively, which was not significantly different from each other and similar to the corresponding adult positive (both 0.1 and 0.5 U/mL UFH) SRA population.

Conclusions: Adults have a higher prevalence of SRA testing compared to children and have median positivity rates that are significantly higher (2-3 fold).The consistent increase in yearly percentage of positive tests, suggests improved clinical screening and/or diagnostic pathways for HIT. Testing was not influenced by gender or seasonal variation. To our knowledge, these findings provide, for the first time, a detailed picture of SRA testing from a large US reference laboratory.