(P-TS-13) Assessment of the Rates of Positive HLA Class I Antibody Testing in Relation to Clinical and Laboratory Parameters in Patients with Platelet Refractoriness

Background/Case Studies: Platelet transfusion refractoriness is commonly encountered in patients with thrombocytopenia. Platelet refractoriness can be due to alloimmunization but is more frequently secondary to non-immune factors. Our institution has a large cohort of platelet refractory patients. We observed that HLA class I antibody testing sent for patients with suboptimal responses to platelet transfusions was often negative. We performed a study to evaluate the rate of HLA class I antibody positivity related to clinical and laboratory parameters.

Study

Design/Methods: This retrospective cohort study examined HLA class I antibody testing performed on patients with suspected immune-mediated platelet refractoriness at a tertiary cancer center from 01/2023-04/2023. Patient records were evaluated for primary diagnosis, sex, and the number of suboptimal 1–4-hour post-transfusion platelet counts (corrected count increments [CCI] < 7500) obtained prior to approval for testing. Non-immune causes of platelet refractoriness (e.g., splenomegaly, fevers, bleeding, medications) and number of platelet transfusions prior to testing were also assessed. The rate (%) of positivity for several parameters was determined.

Results/Findings: HLA class I antibody testing was performed on 66 patients during the study period. Most patients had a primary diagnosis of a hematologic malignancy (59 patients with hematologic malignancies and 7 patients with solid organ malignancies). The overall rate of positive HLA class I antibody testing was 29% (19/66 patients tested). While there were more males than females on whom testing was performed (number of males = 36; number of females = 30), 50% of females had positive testing compared to 11% of males. Neither the number of previous platelet transfusions nor the presence of non-immune factors showed significant correlation with antibody positivity rates. The number of suboptimal CCIs showed a trend toward increased HLA class I antibody positivity rates (no CCI = 0% positive [5 patients], 1 CCI = 9% positive [22 patients], 2 CCIs = 43% positive [37 patients]).

Conclusions: At our center, HLA class I alloimmunization is more common in females with platelet refractoriness. Rates of HLA alloimmunization were not higher with increased numbers of platelet transfusions, suggesting that pregnancy may be a more important sensitizing event. The percentage of patients with positive HLA class I antibody testing was higher in patients with 2 or more CCIs < 7500 on 1–4-hour post-transfusion platelet measurements. We conclude that testing for HLA alloimmunization is most likely to be revealing in females with at least 2 accurate CCIs < 7500.

Importance of research: Our study assessed a large number of clinical and laboratory parameters in a cohort of platelet refractory patients. We showed that female sex and the presence of two CCIs < 7500 were associated with increased likelihood of positive HLA class I antibody testing, while non-immune factors and transfusion history were not as predictive of positive testing. This will help guide practitioners in deciding whether to send testing for platelet alloimmunization.