The 4T Scoring System Explained Simply
The 4T score is a point-based checklist that helps doctors figure out how likely it is that a patient's low platelet count is caused by heparin (a blood thinner), rather than something else. 1, 2
How the Scoring Works
The system looks at four main things (the "4 T's"), and gives 0,1, or 2 points for each one: 3
1st T: Thrombocytopenia (How Low Are the Platelets?)
- 2 points: Your platelet count dropped by more than half from where it started, but still stays above 20,000 3, 2
- 1 point: Your platelet count dropped by 30-50%, or the lowest count is between 10,000-19,000 2
- 0 points: Your platelet count dropped by less than 30%, or fell below 10,000 (this is usually not heparin-induced thrombocytopenia) 3
2nd T: Timing (When Did the Platelets Drop?)
- 2 points: Your platelets dropped 5-10 days after starting heparin (this is the classic pattern) 3
- 1 point: Your platelets dropped earlier than 5 days, but you had heparin within the past 3 months 3
- 0 points: The timing doesn't fit either pattern 3
3rd T: Thrombosis (Do You Have Blood Clots?)
- 2 points: You developed new blood clots (in veins or arteries) while your platelets were dropping 3
- 1 point: You have suspicious symptoms that might be clots, or your clots are getting worse 3
- 0 points: No blood clots at all 3
4th T: Other Causes (Could Something Else Explain This?)
- 2 points: There's no other obvious reason for your low platelets 3
- 1 point: There might be another possible cause 3
- 0 points: There's definitely another clear reason (like sepsis, recent surgery, chemotherapy, or other medications) 3
What Your Total Score Means
Add up all the points (0-8 total possible): 1, 2
- 0-3 points (Low probability): It's very unlikely to be heparin causing this—doctors typically don't need to do special testing or stop heparin 2, 4
- 4-5 points (Intermediate probability): It might be heparin—doctors should stop all heparin immediately, start a different blood thinner, and do lab tests 1, 2
- 6-8 points (High probability): It's very likely heparin—doctors must stop all heparin immediately, start a different blood thinner at full strength, and do lab tests 1, 2
Important Caveats
The score works extremely well at ruling out heparin-induced thrombocytopenia when it's low (negative predictive value of 99.8%), meaning if you score 0-3, you almost certainly don't have it. 4, 5 However, even high scores need confirmation with blood tests because only about 64% of high-scoring patients actually have the condition. 4
The standard 4T score doesn't work well after heart surgery because platelets naturally drop after these operations, creating a "biphasic" pattern that's more predictive than the score itself. 3, 1, 2 Similarly, in intensive care patients with multiple medical problems, the score can be harder to interpret accurately. 3, 6
Why This Matters
Untreated heparin-induced thrombocytopenia kills 5-10% of patients through dangerous blood clots, and 17-55% develop serious clotting complications if not treated. 1 On the flip side, unnecessarily treating patients who don't have it leads to serious bleeding complications—in one study, 71% of low-risk patients incorrectly treated had major bleeding requiring transfusions. 5