What are the limitations of the 4ts score hit?

While the 4ts score hit is highly validated, it should not replace clinical judgment. OpiCalc provides the latest evidence-based limitations for the 4ts score hit to ensure safe bedside use and optimal diagnostic accuracy.

How does the 4ts score hit compare to other Hematology scores?

The 4ts score hit is a gold-standard diagnostic tool. OpiCalc provides side-by-side clinical guidance to help clinicians choose the most specific evidence-based tool for their patient's presentation.

Is the 4ts score hit free to use?

Yes. OpiCalc provides the 4ts score hit completely ad-free and optimized for mobile bedside use, ensuring you can calculate medical risk swiftly and without distraction.

OpiCalc

927 Clinical Tools

OpiCalc

4Ts Score (HIT)aaIPI (Age-adjusted IPI)ABC Score (Massive Transfusion)Absolute Neutrophil Count Age-adjusted D-dimer Threshold aGvHD Grading (Glucksberg/MAGIC)AL Amyloidosis Staging (Mayo)Anti-Xa Monitoring Interpreter Antiphospholipid Syndrome Classification Binet / Rai Staging (CML)Blood Volume Calculator Caprini VTE Risk CISNE Score CLL-IPI Coombs-Negative Haemolysis Checklist Corrected Reticulocyte Count CRS Grading (ASTCT 2019)DAT Interpretation Algorithm Deauville Score Interpreter DIC Score (ISTH)DIPSS (MF)DIPSS-Plus (MF)DOAC Dose Selector DOAC Renal Dose Adjustment DRI (Disease Risk Index)Durie-Salmon Staging (historical)EBMT Risk Score ELN 2022 AML Risk Classification ELTS Score (CML)England & Fraser Index EUTOS Score (CML)Expected Hb Rise per RBC Unit Factor Inhibitor Bethesda Titre Interpreter Febrile Neutropaenia Risk (MASCC)Fibrinogen Replacement FLIPI (Follicular Lymphoma IPI)FLIPI-2 French TTP Score GAPSS (Global APS Score)Haemophilia Severity Classification HbS Percentage Target Calculator HCT-CI (Comorbidity Index)HEP Score Heparin Nomogram (Weight-based)Hepatic Iron Index HERDOO2 Rule HFE Haemochromatosis Risk Estimator ICANS Grading (ASTCT 2019)IMWG Response Criteria (Myeloma)IPI (International Prognostic Index)IPSID / Lugano Classification IPSS-M (Molecular IPSS)IPSS-R (Revised IPSS for MDS)Iron Deficit Calculator (Ganzoni Formula)ISS (Myeloma)ISTH Bleeding Assessment Tool (BAT)ISTH Non-Overt DIC Score ITP Bleeding Score iwCLL Response Criteria (2018)IWG ITP Response Criteria JAK2V617F Allele Burden Interpreter Karnofsky Performance Scale Leucostasis Risk Threshold Lupus Anticoagulant (LAC) Reporting Framework MAGIC aGvHD Biomarker Score MCV-based Anaemia Algorithm Mentzer Index MIPI (MCL IPI)MIPSS70+ v2.0 (MF)Mixing Study Interpretation MRD Assessment Threshold Interpreter mSMART 3.0 (Myeloma)MTP Activation Criteria NCCN-IPI NIH Chronic GvHD Scoring NTDT Iron Burden Estimator Padua RAM PBT (Paediatric BAT)PCNSL MSKCC Score PERC Rule Perioperative Anticoagulation Bridging PESI Score (PE Mortality)PLASMIC Score Platelet Increment Calculator (CCI)PNH Clone Size Interpreter R-IPI (Revised IPI)R-ISS (Revised ISS)Reticulocyte Index (RPI)Revised Geneva Score (PE)Revised Response Criteria (NHL)Revised Zubrod / ECOG Performance Shine & Lal Index Sickle Cell Disease Risk (TCD)SLiM-CRAB Criteria SOKAL Score (CML)Sokal/EUTOS/ELTS Comparison sPESI (Simplified PESI)TACO Risk Score Thalassaemia Transfusion Threshold Thrombocytopaenia Differential Thrombophilia Screening Eligibility Algorithm Time in Therapeutic Range (TTR)TLS Risk Classification Transferrin Saturation Calculator Vienna Prediction Model von Willebrand Factor Activity Interpreter Warfarin Dose Calculator Wells Score (DVT)Wells Score (PE)WHO 2022 AML Classification Mapper WHO Bleeding Scale YEARS Algorithm

4Ts Score (HIT)

4Ts Score: Used to assess the pre-test probability of Heparin-Induced Thrombocytopenia (HIT). Clinical score should be calculated before ordering HIT antibody tests.

1. Thrombocytopenia

2. Timing of Platelet Fall

3. Thrombosis or Sequelae

4. oTher Causes for Platelet Fall

Guidelines & Evidence

Clinical Details

Section 1

When to Use

Clinical Use

Assessing the pre-test probability of Heparin-Induced Thrombocytopenia (HIT).

Deciding whether to discontinue heparin and start an alternative anticoagulant (e.g., argatroban, danaparoid).

Determining if HIT antibody testing (PF4/heparin EIA) or functional assays (SRA) are indicated.

Section 2

Formula & Logic

Criteria

Category	2 Points	1 Point	0 Points
Thrombocytopenia	>50% fall and nadir ≥20k	30-50% fall or nadir 10-19k	<30% fall or nadir <10k
Timing of fall	Days 5-10 (or <1d if recent heparin)	After Day 10 (or unclear)	≤4 days (no prior heparin)
Thrombosis	New thrombosis, skin necrosis	Progressive/recurrent thrombosis	None
oTher causes	None apparent	Possible	Definite

Section 3

Pearls/Pitfalls

Interpretation

Score	Probability	Management
0-3	Low (< 1%)	Continue heparin; do not test for HIT.
4-5	Intermediate (~10%)	Stop heparin; start alternative; test for HIT.
6-8	High (~50%)	Stop heparin; start alternative; test for HIT.

Section 4

Evidence Appraisal

Key Reference

Evaluation of pretest clinical score (4 Ts) for the diagnosis of heparin-induced thrombocytopenia

Lo GK et al. • Journal of Thrombosis and Haemostasis. 2006;4(4):759-65

Last Comprehensive Review: 2026

Related Hematology Tools

Absolute Neutrophil Count

MCV-based Anaemia Algorithm

Mentzer Index

England & Fraser Index

Shine & Lal Index

Corrected Reticulocyte Count

Have feedback about this calculator?Let us know.