4TS Score - HIT Pretest Probability Tally

The 4TS Score is a bedside pretest probability tool for heparin-induced thrombocytopenia, an immune reaction that drops the platelet count after heparin exposure. It groups clinical and laboratory clues into four T categories and returns a low, intermediate, or high probability band to help clinicians decide whether to stop heparin, send antibody testing, or continue anticoagulation.

• • Bedside HIT triage: An inpatient on heparin whose platelet count has fallen and needs a quick structured review.
• • Pre-test for PF4 antibody testing: A nurse, pharmacist, or trainee preparing to order ELISA or functional assay testing.
• • Teaching rounds: Trainees learning the difference between type 1 non-immune platelet drop and antibody-mediated type 2 reaction.

Heparin-induced thrombocytopenia is rare but serious. Once platelet-activating antibodies form against the heparin-PF4 complex, the platelet count can fall by half and new clots can form. The 2012 Cuker meta-analysis confirmed that a low score reliably rules the diagnosis out.

The calculator mirrors the published 4T layout: each T is scored 0 to 2 points, the four sub-scores are added, and the total is read against the 0 to 3, 4 to 5, and 6 to 8 bands. The result is a record-organizing aid, not a lab replacement.

When the T4 (other causes) review uncovers a coagulopathy or disseminated intravascular coagulation (DIC) picture, the Fresh Frozen Plasma Dose Calculator supports the volume and bag estimate that often runs in parallel to the platelet-count workup.

The calculator walks through the four T categories, picks the highest-matching option in each, and sums the numbers into a 0 to 8 total read against the published probability band.

• T1 - Thrombocytopenia: Compares the percent platelet fall and the lowest count to give 0, 1, or 2 points.
• T2 - Timing: Captures how the platelet fall aligns with the heparin exposure window. Two patterns earn 2 points: a clear day 5 to 10 onset, and a rapid day 1 fall on re-exposure when the prior heparin dose was within the last 30 days.
• T3 - Thrombosis: Distinguishes a new confirmed clot or skin necrosis from progressive or recurrent events and from no thrombosis at all.
• T4 - Other causes: Reflects the clinical judgment that another cause is definite, possible, or not apparent.

The numeric answer is a structured summary, not a treatment instruction. Each T uses the highest-matching option, so a 2 can dominate.

According to Cuker et al. 2012 meta-analysis in Blood, a low-probability 4TS Score has a 0.998 negative predictive value for HIT, while intermediate and high scores have positive predictive values of 0.14 and 0.64.

When a T3 (thrombosis) sub-score of 2 reflects a suspected new DVT or PE that needs a parallel rule-out workup, the Age-Adjusted D-Dimer Calculator helps interpret the d-dimer that is often ordered alongside the 4TS review.

The four T categories each capture a different angle on a possible HIT reaction.

The 4T layout is read together. A high platelet fall without a clear 5 to 10 day onset or a day 1 re-exposure with heparin in the last 30 days is less convincing, and a clear onset without a clean alternative-cause assessment is also weaker.

The 0 to 3, 4 to 5, and 6 to 8 bands are the thresholds Lo et al. (2006) validated. A low band supports continuing heparin, the intermediate band triggers PF4 antibody testing, and the high band calls for stopping heparin and starting a non-heparin anticoagulant.

Because argatroban, danaparoid, and fondaparinux dosing all depend on renal function, the GFR Calculator supports the kidney-function review that the team usually runs before picking and dosing the alternative anticoagulant.

Treat the calculator as a bedside-review checklist. Work through the four T categories in any order, but record the sub-scores and total so the next reviewer can challenge the inputs.

1. 1 Gather the platelet history: Pull every platelet count since heparin started, find the highest and lowest, and compute the percent fall. Pick the highest-matching thrombocytopenia option.
2. 2 Map the timing: Compare the day the fall was first noticed with the day heparin began. A clear 5 to 10 day onset, or a day 1 re-exposure with prior heparin in the last 30 days, both earn 2 points. A day 1 fall with prior heparin 31 to 100 days ago, a plausible 5 to 10 day window without clear documentation, or a fall after day 10 each earn 1 point.
3. 3 Check the thrombosis and skin/systemic findings: Review imaging and bedside notes for a new clot, skin necrosis, or acute systemic reaction. Progressive or recurrent events still earn 1 point.
4. 4 Rule out other causes: Look for sepsis, bacteremia, recent major surgery, chemotherapy, DIC, or a platelet-lowering drug. Score 2 only when those causes are not apparent.
5. 5 Read the probability band: Add the four sub-scores, look at the band, and record the PPV or NPV context in the chart note. The result is a structured review aid, not an order to stop or continue heparin.

A practical use: a 68-year-old on heparin whose platelet count drops from 220 to 95 on day 8. T1=2, T2=2, T3=0, T4=1. Total=5 (intermediate band).

In severe or refractory HIT where the team is adding IVIG to lower circulating platelet-activating antibodies, the IVIG Dose Calculator supports the dosing-weight calculation that needs to be done before the infusion is ordered.

A 4TS review can be done in the chart with a pen, but a calculator makes the tally consistent, traceable, and easier to defend.

• • Standardised review across providers: Pharmacists, nurses, and physicians use the same four T categories, so the discussion is less dependent on memory of the 4T table.
• • Transparent record-keeping: Each sub-score and the total can be quoted in the chart note, so a later reviewer can challenge the inputs.
• • Quick link to the published PPV and NPV: The calculator ties the band to the Cuker 2012 numbers, so the user does not re-look up the predictive value.

The 4T scoring tool was designed to make the pretest probability of HIT easier to discuss. The calculator keeps that goal front and centre but does not diagnose HIT, prescribe an anticoagulant, or replace lab testing.

The result is most useful when paired with the same reasoning the bedside team would use without the calculator.

Several things can move the score up or down.

• • The tool is a pretest probability calculator, not a diagnostic test. A low score rules HIT out with a high NPV, but a high score still needs PF4 ELISA and a confirmatory serotonin release assay before the diagnosis is final. Inter-rater agreement is moderate; two clinicians can arrive at totals that differ by 1 to 2 points, which is why the band is the safer thing to document.

Bleeding risk, kidney function, pregnancy status, and the overall anticoagulation plan matter for what to do with the result, but those are not part of the scoring tool. The calculator intentionally stops before that step so the result is a record-organizing aid, not a treatment order.

The four T categories (Thrombocytopenia, Timing, Thrombosis, oTher causes) each score 0 to 2 points for a maximum of 8. The 0 to 3, 4 to 5, and 6 to 8 bands drive the PF4 antibody testing decision.

The ASH 2018 HIT guidelines outline when to stop heparin, which non-heparin anticoagulants to consider (argatroban, danaparoid, fondaparinux, or a direct oral anticoagulant), and when to restart standard anticoagulation.