Heparin‐induced thrombocytopenia (HIT) is a clinicopathologic process that is the end result of antibodies directed at platelet factor 4–heparin complexes. The clinical findings can be subtle or dramatic, and serologic confirmation is problematic. Current screening immunoassays, although sensitive, have poor specificit,y resulting in an overdiagnosis of HIT. Because of this poor specificity, all positive immunoassay screening tests require a confirmatory test, the serotonin releasing assay (SRA). However, this confirmatory test is only available at limited reference laboratories in the United States. Therefore, a clinical scoring system, the 4T's, was developed and has been promoted as a validated tool to risk‐stratify patients into low, intermediate, and high risk of HIT. A total score of 0–8 is possible based on the following 4 variables: (1) degree of thrombocytopenia, (2) time since onset of heparin initiation, (3) presence of thrombosis, and (4) other potential explanations for thrombocytopenia. However, for a clinical scoring system to have external validity, it must have high interrater reliability. The goal of this study was to determine the interrater reliability of the 4T scoring system.
A retrospective study of hospitalized patients with possible HIT at a community hospital from July 1, 2010, through December 31, 2010, was performed. Two physicians (H.M., R.D.) independently abstracted the components of the 4T scoring system in all patients with a positive screening test (PIFA Heparin/Platelet Factor 4 Rapid Assay). The summed 4T score and risk‐stratification category assigned by the score were then analyzed for interrater reliability.
PIFA antibody screening was performed in 155 patients during the study period. Fortytests were positive. Sixteen cases were eliminated because of incomplete data, outpatient testing, or repeat studies. Of the 24 scores, 15 matched exactly for the summed 4T score (tau‐b, 0.681; Spearman rho, 0.741; Pearson correlation, 83%; Cohen's kappa, 0.80). Twenty‐three of 24 scores matched exactly for the risk level assigned by the 4T score (tau‐b, 0.7977; Spearman rho, 0.7977; Pearson correlation, 79.77%; Cohen's kappa, 0.78).
Our results showed that there is a high degree of interrater agreement for the 4T scoring system. These results are based on our patient population, the majority of whom are at low or moderate risk for HIT. Further studies should be done to include patients at high risk in tertiary‐care centers to determine the generalizability of this risk stratifying scoring system.
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Comparison between 2 scorers of the 4T score and risk of 4T score for heparin‐induced thrombocytopenia.