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Good Positive and Negative Likelihood Ratios

Heparin Induced Thrombocytopenia: Likelihood Ratios

Introduction: Also known as HITT (heparin induced thrombocytopenic thrombosis) because of its predilection for causing thromboses. Note that these can be arterial or venous, though DVT is most common.

Occurs 5-14 days after exposure, unless patient has been exposed to heparin in the preceeding 100 days (in which case it can present earlier than 4 days). Platelet nadir is ~50k, i.e. not as low as for drug-induced immune related thrombocytopenias.

Must treat with a DTI in addition to stopping heparin. Do NOT use coumadin as this can lead to increased thrombosis.

A pretest probability score-based system has been validated (see table in UpToDate: http://www.utdol.com/online/content/topic.do?topicKey=coagulat/8950 or article PMID 16634744).

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Prevalence

Population / CalculatorPrevalence Comments / Study / Link
Patients taking heparin 2% Blood 2005 Oct 15;106(8):2710-5. PMID 15985543
Heparin type 0% bovine greater risk than porcine. UFH greater risk than LMWH; fondaparinux is thought to be safest, though there are 2 case reports.
Clinical context 0% surgical patients have higher risk than medical patients.
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The likelihood ratios of findings for Heparin Induced Thrombocytopenia are listed below. See the left navigation bar to change the display.

Good Positive and Negative Likelihood Ratios

Finding +LR-LR Comments, Study
Anti-Heparin-PF4 ELISA Edit 4.90

false positives are caused by IgM antibodies that do not cause HIT but do react positively on the ELISA.

Study: N Engl J Med. 2006 Aug 24;355(8):809-17. PMID: 16928996

14C-Serotonin Release Assay Edit 190.1

"gold standard"

Study: http://www.utdol.com/online/content/topic.do?topicKey=coagulat/8950

Platelet Aggregation Test Edit 90.1

generally thought of as more specific than sensitive, though different numbers are reported depending on the study.

Study: N Engl J Med. 2006 Aug 24;355(8):809-17. PMID: 16928996

and Thromb Haemost 1993 Apr 1;69(4):344-50. PMID 8497847