4Ts score: how the four parts add up

The 4Ts score is a pretest score for heparin-induced thrombocytopenia (HIT). It adds 0, 1 or 2 points for each of four parts: Thrombocytopenia (how far and how low the platelet count fell), Timing of the fall, Thrombosis or other new events, and oTher causes of a low platelet count. The total runs from 0 to 8.

Not a medical device. Not a diagnosis or dosing recommendation.

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Hematology calculators
Lo GK et al. · 2006
The 4Ts formula
Worked example
Frequently asked questions

What is the 4Ts score?

The 4Ts score comes from a 2006 paper by Lo, Juhl, Warkentin and colleagues in the Journal of Thrombosis and Haemostasis. Magent's calculator adds four numbers that the user assigns from the Lo 2006 table. It doesn't grade the platelet count or work out the day of onset itself.

This page explains the arithmetic. It doesn't group the total into probability bands, and it isn't a HIT antibody result.

The 4Ts formula

4Ts = thrombocytopenia (0–2)
    + timing (0–2)
    + thrombosis (0–2)
    + other causes (0–2)
maximum = 8

Exact expression the calculator uses:

4Ts = thrombocytopenia + timing + thrombosis + other_causes; each 0-2 caller-assigned from Lo 2006 table; pretest probability low 0-3, intermediate 4-5, high 6-8; max 8

What each part means

The wording below is from the parameter descriptions in Magent's record.

Part	What earns points	Points
Thrombocytopenia	Platelet fall of more than 50% and a nadir of 20 or more	2
	Fall of 30–50%, or a nadir of 10–19	1
	Fall of less than 30%, or a nadir under 10	0
Timing	Clear onset on days 5–10, or within 1 day with heparin in the past 30 days	2
	Consistent with days 5–10 but not clear; onset after day 10; or within 1 day with heparin 30–100 days ago	1
	Fall too early, without recent heparin	0
Thrombosis	New thrombosis, skin necrosis, or an acute systemic reaction after a heparin bolus	2
	Progressive or recurrent thrombosis, silent thrombosis, or red skin lesions	1
	None	0
Other causes	No other apparent cause of the low platelet count	2
	A possible other cause	1
	A definite other cause	0

How the score works

Thrombocytopenia. Assign 0, 1 or 2 from the size of the platelet fall and the lowest count.
Timing. Assign 0, 1 or 2 from when the fall started relative to heparin.
Thrombosis. Assign 0, 1 or 2 for new clots, skin changes or a reaction after a heparin bolus.
Other causes. Assign 2 if there's no other apparent cause, 1 if there's a possible one, and 0 if there's a definite one. More points means the picture looks more like HIT.
Add them up. The total is 0 to 8.

Inputs come from the user

The record says each part is an integer of 0, 1 or 2 assigned by the person using the calculator from the published Lo 2006 table. Magent's calculator doesn't take platelet counts or dates. It returns the total, the maximum of 8 and a line for each part.

Worked example

This is the worked example in Magent's record for the calculator.

Example 1: every part 0

Thrombocytopenia 0. Timing 0.
Thrombosis 0. Other causes 0.
Score = 0 out of 8.

Values along the range

All are Magent's expected values for the calculator.

Thrombocytopenia 1, timing 1, thrombosis 1, other causes 0 → 3.
All four parts at 1 → 4.
Thrombocytopenia 2 and the other three at 1 → 5.
Thrombocytopenia 2, timing 2, thrombosis 2, other causes 0 → 6.
All four parts at 2 → 8.
Any one part at 2 and the rest at 0 → 2.

History of the 4Ts score

The score comes from Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P and Greinacher A, Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings, Journal of Thrombosis and Haemostasis, 2006 (volume 4, issue 4, pages 759–765).

When it's used

The paper's title describes it as a pretest clinical score for heparin-induced thrombocytopenia.

Where it's used

Each part comes from the platelet count history, the heparin history and the clinical picture, before a HIT antibody result.

Who uses it

The person using the calculator assigns each part from the published table. The calculator does the adding.

Limits of the calculation

It adds four numbers. It doesn't confirm or rule out HIT, and it isn't a HIT antibody result.
It's the 4Ts score (Lo 2006), not the HEP score.
The calculator doesn't grade platelet counts or work out the day of onset; the user assigns each part.
This page doesn't group scores into bands or say what to do with a score.

Clinical decisions belong to a qualified clinician who knows the patient.

Absolute neutrophil count is a separate blood-count calculation. It works out a cell count rather than adding points.

Glossary

Heparin-induced thrombocytopenia (HIT): a drop in platelets caused by an immune reaction to heparin.
Thrombocytopenia: a low platelet count.
Nadir: the lowest value reached.
Thrombosis: a blood clot inside a blood vessel.
Heparin bolus: a single injection of heparin given at once.

Common mistakes

Entering a platelet count. Each part is an integer of 0, 1 or 2 that the user assigns.
Scoring other causes backwards. No other apparent cause scores 2; a definite other cause scores 0.
Using a part value above 2. Each part is capped at 2, so the total can't pass 8.
Mixing up 4Ts and HEP. They're different scores.

Source

Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost. 2006;4(4):759-765. DOI: 10.1111/j.1538-7836.2006.01787.x

Frequently asked questions

How do you calculate the 4Ts score?

Give 0, 1 or 2 points for each of thrombocytopenia, timing, thrombosis and other causes, using the Lo 2006 table, then add them. The total is 0 to 8.

What does a 4Ts score of 4 mean?

This page doesn't group scores into bands or give a probability by score. It adds up the points; what a score means depends on the person and the clinical context.

What are the 4 T's?

Thrombocytopenia, Timing, Thrombosis, and oTher causes of a low platelet count.

What is the highest 4Ts score?

Is the 4Ts score a HIT test result?

No. It's a pretest score built from the clinical picture, not an antibody result.