4T Score Calculator | HIT

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Anna Szczepanek, PhD
Last updated: Jun 05, 2023

The 4T score calculator checks for signs typical of heparin-induced thrombocytopenia (HIT) and the probability of its diagnosis. 💉

We won't be just calculating; we'll also talk about the elements of 4T, HIT criteria, as well as platelet nadir and other definitions useful in HIT medical evaluation & HIT diagnosis.

Let's start the ride!

What is HIT?

HIT, or heparin-induced thrombocytopenia, is a rare complication of treatment with heparin, a blood-thinning medicine. HIT leads to thrombocytopenia (lack of blood platelets) and thrombosis (blood clots).

We can recognize two major HIT types:

  1. HIT type 1 — non-immunological. More common, with mild thrombocytopenia and a good prognosis.
  2. HIT type 2 — immunological. Rare, with severe thrombocytopenia and relatively worse outcome.

Once we know what the 4 Ts of HIT score are, it's time to explain how to calculate the 4 T score.

What are the 4 Ts of HIT?

The 4 Ts score is a short art of memory technique used to easily memorize the four most important parts of HIT diagnosis. Let's enumerate them one by one:

  1. Magnitude of Thrombocytopenia.
    • Typically for HIT, the platelets fall exceeds 50%, but the lowest point doesn't go below 20 × 10⁹/ L.
  2. Timing.
    • HIT usually starts 5-10 days after the start of heparin treatment.
  3. Thrombosis.
    • Newly diagnosed thrombosis.
  4. OTher causes of thrombocytopenia.
    • There is no other probable cause.

Thrombosis, which is the presence of blood clots in your bloodstream, is a common HIT complication.
When you're all done playing with the HIT calculator, you might also like:

How do I calculate the 4T score?

2 pts

1 pts

0 pts


Platelets count fall >50% AND platelet nadir ≥20×10⁹/L

Platelet count fall 30-50% OR nadir 10-19.9×10⁹/L

Platelet count <30% OR nadir <10×10⁹/L


• Clear onset 5–10 days

• Non-clean onset 5–10 days

Onset <4 days, no heparin exposure within 100 days

• Onset 1st day, with exposure 5-30 days before

• Onset >10 days

• Onset 1st day, with exposure within 31-100 days


New confirmed thrombosis:

• Recurrent/ unconfirmed thrombosis


• Skin necrosis

• Injection-related rash

• Anaphylaxis

• Adrenal hemorrhage

Other possible causes




📍 Once you take a look at the table, it all gets pretty effortless - count all the points, according to your patient's state and their blood tests results. If you don't want to do it yourself, don't worry — that's what our 4T score calculator is here for! 😉

❗ The 4T score calculator cannot be used as a substitute for clinical knowledge and/or consultation with a medical specialist. Always consult your doctor.


What are other possible causes of thrombocytopenia?

Other causes of thrombocytopenia are the fourth part of the 4 T score. Let's enumerate them one by one!

  • Sepsis without a documented source of infection;
  • Ventilation-related thrombocytopenia / ICU-related thrombocytopenia;
  • 72h after the surgery;
  • Bloodstream infections (bacteremia / fungemia);
  • Chemotherapy / radiation in the last 20 days;
  • DIC (Disseminated intravascular coagulation — plenty of blood clots & risk of bleeding combined);
  • Transfusion-related thrombocytopenia;
  • Drug-induced thrombocytopenia; and
  • Immune thrombocytopenia.

How do I find platelet nadir?

Platelet nadir is the lowest point of a platelet count fall, chosen among continuous results of blood tests taken during a selected number of days.

To put things simply — to find nadir, find the lowest platelet count your patient had during the studied period. 📅

How do I calculate the platelet count fall in percent?

Platelet count fall is an important part of the HIT score, as featured in Omni's 4T score calculator (HIT calculator). Here's the instruction on how to calculate it manually:

  1. Analyze all the platelet counts that are available for a given period.

  2. Find the greatest and the smallest value detected.

  3. Use the equation:

    Platelet count change(%) = (Smallest value × 100) / Greatest value

    Yay! You got this. 🥳

Łucja Zaborowska, MD, PhD candidate
Platelet count fall
Onset <4 days, no heparin exposure
Platelet nadir
<10 × 10⁹/ L
Other possible causes
4Ts score
✅ The probability of HIT (Heparin-Induced Thrombocytopenia) is low (~5%).
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