Objective To investigate the diagnosis rate of heparin-induced thrombocytopenia (HIT), the incidence of new thrombosis, the dynamic evolution pattern of platelets, and the clinical characteristics in patients with suspected HIT after cardiac surgery; and to evaluate the diagnostic value of HIT antibodies (HIT-Ab), 4T's score, and Lillo-Le Louet (LLL) score for HIT.

Methods A retrospective cohort study was conducted. A total of 307 patients with suspected HIT after cardiac surgery at Fuwai Hospital of the Chinese Academy of Medical Sciences from April 2023 to December 2024 were included. Clinical and laboratory data were collected. All patients underwent 4T's score, LLL score, and HIT-Ab testing. Patients were divided into the non-HIT group (n = 269) and the HIT group (n = 38) based on the final clinical diagnosis. The platelet evolution patterns and other clinical characteristics of the two groups were compared. HIT-Ab concentration was divided into four groups: negative (< 1.0 U/mL, n = 257), weakly positive (1.0-4.9 U/mL, n = 34), moderately positive (5.0-9.9 U/mL, n = 11), and strongly positive (≥ 10 U/mL, n = 5). Differences in the incidence of new thrombosis and the diagnosis rate of HIT among the groups were analyzed, and the diagnostic efficacy of HIT-Ab concentration, 4T's score, and LLL score for HIT was evaluated using receiver operating characteristic (ROC) curves.

Results The diagnosis rate of HIT was 12.38% (38/307), and the incidence of new thrombosis in the HIT group was 63.16%, higher than the 34.57% in the non-HIT group (P < 0.001). Among HIT patients, 76.32% showed a biphasic decline pattern of platelets (pattern A), and their risk of HIT was significantly higher than that of patients with pattern B (odds ratio OR = 10.32, 95% confidence interval CI: 4.64-22.95). The incidence of new thrombosis increased stepwise with higher HIT-Ab concentration (32.7% in the negative group to 100% in the strongly positive group, P < 0.001), and the diagnosis rate of HIT increased from 0 in the negative group to 64.71% in the weakly positive group, reaching 100% in the moderate/strong positive group. The areas under the ROC curves for HIT-Ab concentration, 4T's score, and LLL score for diagnosing HIT were 0.996 (95% CI: 0.991-1.000), 0.799 (95% CI: 0.727-0.870), and 0.860 (95% CI: 0.811-0.908), respectively. When the HIT-Ab concentration was 1 U/mL, the diagnostic sensitivity and negative predictive value (NPV) were both 100%. Compared with the traditional diagnostic criteria for high-risk HIT populations (4T's score cutoff of 4, LLL score cutoff of 2), the optimal cutoff value of 4T's score selected by the maximum Youden index was 5, and the optimal cutoff value of LLL score was 3.

Conclusion The incidence of new thrombosis in patients with suspected HIT after cardiac surgery increases stepwise with higher HIT-Ab levels. The diagnostic value of the LLL score for HIT is superior to that of the 4T's score. HIT-Ab testing has high sensitivity and negative predictive value, making it a reliable tool for guiding early discontinuation of heparin and excluding HIT. Combining HIT-Ab concentration, platelet evolution pattern, and clinical scores helps more accurately identify HIT risk, initiate alternative anticoagulation therapy, and optimize anticoagulation management strategies.