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A Practical Guide to Haemostasis


Estimating the Risk of Heparin-induced Thrombocytopaenia [HIT]: The 4T Score


Introduction:

The '4T' scoring system is widely used to predict the risk that an individual with Thrombocytopenia may have HIT. 
The 4Ts in the 4T score are:
  - Thrombocytopaenia
  - Timing
  - Thrombosis
  - Other causes of Thrombocytopaenia are not evident.

The 4T Score:

Select Criteria:

Platelet Count
>50% fall in platelet count or a platelet nadir of 20-100 x 109/L
2 Points
30-50% in platelet count or a platelet nadir of 10-19 x 109/L
1 Point
<30% fall in platelet count or a platelet nadir of <10 x 109/L
0 Points
Timing
Onset with 5-10 days of exposure to heparin
or
<1 day if previous exposure to heparin within 100 days
2 Points
Unclear -
Platelet count falls after 10 days
1 Point
Platelet count falls too early and without recent exposure to heparin
0 Points
Thrombosis
New thrombosis
Skin necrosis
2 Points
Progressive or recurrent thrombosis
Some skin lesions e.g. erythema
1 Point
None
0 Points
Other causes of Thrombocytopaenia not evident
No other cause identified
2 points
Possible other cause
1 point
Other cause clearly identifiable
0 points


Score
[Max score 8]

4T Score
 0-3 Low Risk
 4-5 Intermediate Risk 
 6-8 High Risk


Click HERE to Reset

Comments

Additional information derived from combining the 4T score and the PF4-Heparin ELISA Assay has been developed to identify patients with HIT and to exclude patients with suspected HIT or to replace the Serotonin-Release Assay [SRA] in the diagnosis of HIT - see References.

Zheng et al [2018] combined the 4T score and the PF4-Heparin ELISA Assay  to replace the Serotonin-Release Assay [SRA and were able to show that this had high diagnostic accuracy and potentially could eliminate the need for SRA testing in at least 90% patients with suspected HIT.




4T Score + PF4-Heparin ELISA OD
4T Score
PF4-Heparin
ELISA OD
Sensitivity & Specificity
8   OD ≥1.0 Sensitivity 75%

Specificity 98%
6 - 7
4 - 5   OD ≥2.0