Figure 1.
Algorithm for the diagnosis and initial management of patients with suspected HIT. Numbered recommendations are listed in the corresponding portion of the algorithm. Actions are in dark gray boxes; test results are in light gray boxes. aMissing or inaccurate information may lead to a faulty 4Ts score and inappropriate management decisions. Every effort should be made to obtain accurate and complete information necessary to calculate the 4Ts score. If key information is missing, it may be prudent to err on the side of a higher 4Ts score. HIT laboratory testing may be appropriate for patients with a low-probability 4Ts score if there is uncertainty about the 4Ts score (eg, because of missing data). Patients should be reassessed frequently. If there is a change in the clinical picture, the 4Ts score should be recalculated. bIf the patient has an intermediate-probability 4Ts score, has no other indication for therapeutic-intensity anticoagulation, and is judged to be at high risk for bleeding, the panel suggests treatment with a non-heparin anticoagulant at prophylactic intensity rather than therapeutic intensity. If the patient has an intermediate-probability 4Ts score and is not judged to be at high risk for bleeding or has another indication for therapeutic-intensity anticoagulation, the panel suggests treatment with a non-heparin anticoagulant at therapeutic intensity rather than prophylactic intensity. In a patient with a high-probability 4Ts score, the panel recommends treatment with a non-heparin anticoagulant at therapeutic intensity. cDifferent immunoassays are available. The choice of assay may be influenced by accuracy, availability, cost, feasibility, and turnaround time. If an enzyme-linked immunoassay is used, a lower threshold is preferred over a high threshold. dFor all patients with a positive immunoassay, including those who were receiving prophylactic-intensity treatment with a non-heparin anticoagulant before the availability of the immunoassay result, the panel recommends treatment with a non-heparin anticoagulant at therapeutic intensity. eDifferent functional assays are available. The choice of assay may be influenced by accuracy, availability, cost, feasibility, and turnaround time. In some settings, a functional assay may not be available, and decisions may need to be made on the basis of the results of the 4Ts score and immunoassay. A functional assay may not be necessary in patients with a high 4Ts score and a strongly positive immunoassay. fMost patients with a negative functional assay do not have HIT and may be managed accordingly. However, depending on the type of functional assay and the technical expertise of the laboratory, false-negative results are possible. Therefore, a presumptive diagnosis of HIT may be considered for some patients with a negative functional assay, especially if there is a high-probability 4Ts score and a strongly positive immunoassay (represented in the figure by a dashed line).

Algorithm for the diagnosis and initial management of patients with suspected HIT. Numbered recommendations are listed in the corresponding portion of the algorithm. Actions are in dark gray boxes; test results are in light gray boxes. aMissing or inaccurate information may lead to a faulty 4Ts score and inappropriate management decisions. Every effort should be made to obtain accurate and complete information necessary to calculate the 4Ts score. If key information is missing, it may be prudent to err on the side of a higher 4Ts score. HIT laboratory testing may be appropriate for patients with a low-probability 4Ts score if there is uncertainty about the 4Ts score (eg, because of missing data). Patients should be reassessed frequently. If there is a change in the clinical picture, the 4Ts score should be recalculated. bIf the patient has an intermediate-probability 4Ts score, has no other indication for therapeutic-intensity anticoagulation, and is judged to be at high risk for bleeding, the panel suggests treatment with a non-heparin anticoagulant at prophylactic intensity rather than therapeutic intensity. If the patient has an intermediate-probability 4Ts score and is not judged to be at high risk for bleeding or has another indication for therapeutic-intensity anticoagulation, the panel suggests treatment with a non-heparin anticoagulant at therapeutic intensity rather than prophylactic intensity. In a patient with a high-probability 4Ts score, the panel recommends treatment with a non-heparin anticoagulant at therapeutic intensity. cDifferent immunoassays are available. The choice of assay may be influenced by accuracy, availability, cost, feasibility, and turnaround time. If an enzyme-linked immunoassay is used, a lower threshold is preferred over a high threshold. dFor all patients with a positive immunoassay, including those who were receiving prophylactic-intensity treatment with a non-heparin anticoagulant before the availability of the immunoassay result, the panel recommends treatment with a non-heparin anticoagulant at therapeutic intensity. eDifferent functional assays are available. The choice of assay may be influenced by accuracy, availability, cost, feasibility, and turnaround time. In some settings, a functional assay may not be available, and decisions may need to be made on the basis of the results of the 4Ts score and immunoassay. A functional assay may not be necessary in patients with a high 4Ts score and a strongly positive immunoassay. fMost patients with a negative functional assay do not have HIT and may be managed accordingly. However, depending on the type of functional assay and the technical expertise of the laboratory, false-negative results are possible. Therefore, a presumptive diagnosis of HIT may be considered for some patients with a negative functional assay, especially if there is a high-probability 4Ts score and a strongly positive immunoassay (represented in the figure by a dashed line).

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