In this episode, I’ll discuss why it is important to check the 4T score before further testing for heparin induced thrombocytopenia.
This episode assumes that you know what the 4T score is and how to calculate it. If you don’t, go here for a 4T score calculator and link to the evidence behind the scoring system.
When encountering a patient who develops thrombocytopenia while receiving heparin or low molecular weight heparin (LMWH), there are 3 general causes to consider:
1. Heparin associated thrombocytopenia also known as Heparin Induced Thrombocytopenia (HIT) Type 1; this is a modest and harmless decrease in platelet count.
2. Heparin Induced Thrombocytopenia Type 2 which is the result of an immune response to heparin. In this type of HIT platelet factor 4 antibodies form and place the patient at a significant risk of thrombosis.
3. Thrombocytopenia from an alternative cause such as sepsis or infection, drug induced thrombocytopenia, nutrient deficiency, ethanol use, or hypersplenism due to chronic liver disease.
It is relatively easy to tell the difference between Type 1 and Type 2 HIT. Type 1 HIT occurs after 1-4 days of heparin use and is mild in nature. Type 2 HIT occurs after 5-10 days of heparin use or within 1 day if there was heparin exposure in the previous 100 days. The drop in platelets may be greater than 50% with Type 2 HIT.
But telling the difference between Type 2 HIT and thrombocytopenia of another cause can be very challenging.
The treatment for Type 2 HIT is full intensity anticoagulation with a non-heparin anticoagulant such as argatroban. If this treatment is given to a patient who doesn’t have HIT and instead has thrombocytopenia from another cause, their bleeding risk will be greatly increased.
The most rapid test for Type 2 HIT is the platelet factor 4 (PF4) antibody assay. If the PF4 result is <0.4, this is generally negative for HIT. If the PF4 is >1 (or 2 depending on your lab/institution), this is generally positive for HIT.
But there is a large range from 0.4 to 1 (or 2) that is indeterminate. When this occurs, further testing with the serotonin release assay is done. Unfortunately, this test takes a while to come back at most laboratories. This can leave clinicians in a tough position, where they may need to place the patient on full intensity anticoagulation only to find out days later that the patient did not have HIT.
By using the 4T score before testing for PF4 antibodies, the risk of unnecessary anticoagulation is greatly reduced.
The performance of any test is dependent on the prevalence of the disease in the population being tested. The application of the 4T score is intended to identify a population of patients that are likely to have HIT.
The 4T scoring system for HIT involves checking the degree of thrombocytopenia, the timing of the platelet drop, findings of new thrombosis, and presence of other causes of thrombocytopenia. Depending on the score the patient may be judged to be at a low, medium or high risk of having HIT.
The PF4 antibody assay has a much higher positive predictive value if used in patients with a high risk of HIT as judged by the 4T score. But the test has a poor positive predictive value if used in a population with a low risk of HIT.
So by applying the 4T score before deciding to check a PF4 assay, you are greatly reducing the number of patients that are subjected needlessly to the risk of full intensity anticoagulation.
If you like this post, check out my book – A Pharmacist’s Guide to Inpatient Medical Emergencies: How to respond to code blue, rapid response calls, and other medical emergencies.
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